LTC2415-1_15 [Linear]
24-Bit No Latency ADCs with Differential Input and Differential Reference;
| 型号： | LTC2415-1_15 |
| 厂家： | 
Linear |
| 描述： | 24-Bit No Latency ADCs with Differential Input and Differential Reference |
| 文件： | 总40页 (文件大小：601K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC2415/LTC2415-1
24-Bit No Latency ∆ΣTM
ADCs with Differential Input and
Differential Reference
U
DESCRIPTIO
FEATURES
The LTC®2415/2415-1 are micropower 24-bit differential
∆Σ analog to digital converters with integrated oscillator,
2ppmINL, 0.23ppmRMSnoiseanda2.7Vto5.5Vsupply
range. They use delta-sigma technology and provide
single cycle settling time for multiplexed applications.
Through a single pin, the LTC2415 can be configured for
better than 110dB input differential mode rejection at
50Hz or 60Hz ±2%, or it can be driven by an external
oscillator for a user defined rejection frequency. The
LTC2415-1 can be configured for better than 87dB input
differential mode rejection over the range of 49Hz to
61.2Hz (50Hz and 60Hz ±2% simultaneously). The inter-
nal oscillator requires no external frequency setting com-
ponents.
■
2× Speed Up Version of the LTC2410/LTC2413:
15Hz Output Rate, 50Hz or 60Hz Notch—LTC2415;
13.75Hz Output Rate, Simultaneous 50Hz/60Hz
Notch—LTC2415-1
Differential Input and Differential Reference with
GND to VCC Common Mode Range
2ppm INL, No Missing Codes
2.5ppm Gain Error
0.23ppm Noise
Single Conversion Settling Time for Multiplexed
Applications
Internal Oscillator—No External Components
Required
■
■
■
■
■
■
■
24-Bit ADC in Narrow SSOP-16 Package
(SO-8 Footprint)
Single Supply 2.7V to 5.5V Operation
The converters accept any external differential reference
voltage from 0.1V to VCC for flexible ratiometric and
remote sensing measurement configurations. The full-
■
■
Low Supply CurrUent (200µA) and Auto Shutdown
scale differential input range is from –0.5VREF to 0.5VREF
.
APPLICATIO S
The reference common mode voltage, VREFCM, and the
input common mode voltage, VINCM, may be indepen-
dently set anywhere within the GND to VCC range of the
LTC2415/LTC2415-1. The DC common mode input rejec-
tion is better than 140dB.
■
Direct Sensor Digitizer
Weight Scales
Direct Temperature Measurement
Gas Analyzers
Strain Gage Transducers
Instrumentation
Data Acquisition
Industrial Process Control
■
■
■
■
The LTC2415/LTC2415-1 communicate through a flexible
3-wire digital interface which is compatible with SPI and
MICROWIRETM protocols.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No Latency ∆Σ is a trademark of Linear Technology Corporation.
MICROWIRE is a trademark of National Semiconductor Corporation.
■
■
■
■
6-Digit DVMs
U
TYPICAL APPLICATIO S
V
CC
1µF
2.7V TO 5.5V
V
CC
1µF
= INTERNAL OSC/50Hz REJECTION (LTC2415)
2
14
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/60Hz REJECTION (LTC2415)
= INTERNAL 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
LTC2415/
O
2
3
+
REF
V
12 SDO
13 SCK
11 CS
CC
LTC2415-1
BRIDGE
IMPEDANCE
100Ω TO 10k
5
+
3
4
IN
IN
3-WIRE
SPI INTERFACE
LTC2415/
+
13
REFERENCE
VOLTAGE
REF
REF
6
–
SCK
LTC2415-1
–
0.1V TO V
CC
–
4
3-WIRE
SPI INTERFACE
REF
GND
F
O
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
1, 7, 8
9, 10,
15, 16
14
REF
REF
–
IN
1, 7, 8, 9, 10, 15, 16
2415 TA02
GND
2415 TA01
sn2415 24151fs
1
LTC2415/LTC2415-1
W W U W
U
W
U
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Notes 1, 2)
TOP VIEW
ORDER PART NUMBER
Supply Voltage (VCC) to GND.......................–0.3V to 7V
Analog Input Pins Voltage
to GND.................................... –0.3V to (VCC + 0.3V)
Reference Input Pins Voltage
to GND.................................... –0.3V to (VCC + 0.3V)
Digital Input Voltage to GND........ –0.3V to (VCC + 0.3V)
Digital Output Voltage to GND ..... –0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC2415C/LTC2415-1C........................... 0°C to 70°C
LTC2415I/LTC2415-1I ........................ –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
GND
GND
LTC2415CGN
LTC2415IGN
LTC2415-1CGN
LTC2415-1IGN
V
CC
+
F
O
REF
REF
IN
–
+
–
SCK
SDO
CS
IN
GN PART MARKING
GND
GND
GND
GND
2415
2415I
24151
24151I
GN PACKAGE
16-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 95°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Resolution (No Missing Codes)
Integral Nonlinearity
0.1V ≤ V ≤ V , –0.5 • V ≤ V ≤ 0.5 • V , (Note 5)
●
●
●
24
Bits
REF
CC
REF
IN
–
REF
+
5V ≤ V ≤ 5.5V, REF = 2.5V, REF = GND, V
= 1.25V, (Note 6)
= 2.5V, (Note 6)
1
2
5
ppm of V
ppm of V
ppm of V
CC
INCM
REF
REF
REF
+
–
5V ≤ V ≤ 5.5V, REF = 5V, REF = GND, V
14
2
CC
INCM
+
–
REF = 2.5V, REF = GND, V
= 1.25V, (Note 6)
INCM
+
–
Offset Error
2.5V ≤ REF ≤ V , REF = GND,
0.5
mV
CC
+
–
GND ≤ IN = IN ≤ V , (Note 14)
CC
+
–
Offset Error Drift
Positive Gain Error
Positive Gain Error Drift
Negative Gain Error
Negative Gain Error Drift
Output Noise
2.5V ≤ REF ≤ V , REF = GND,
20
nV/°C
CC
+
–
GND ≤ IN = IN ≤ V
CC
+
–
2.5V ≤ REF ≤ V , REF = GND,
●
●
2.5
12
12
ppm of V
REF
CC
+
+
–
+
IN = 0.75REF , IN = 0.25 • REF
+
–
2.5V ≤ REF ≤ V , REF = GND,
0.03
2.5
ppm of V /°C
REF
CC
+
+
–
+
IN = 0.75REF , IN = 0.25 • REF
+
–
2.5V ≤ REF ≤ V , REF = GND,
ppm of V
REF
CC
+
+
–
+
+
IN = 0.25 • REF , IN = 0.75 • REF
+
–
2.5V ≤ REF ≤ V , REF = GND,
0.03
1.1
ppm of V /°C
REF
CC
+
+
–
IN = 0.25 • REF , IN = 0.75 • REF
+
–
5V ≤ V ≤ 5.5V, REF = 5V, REF = GND,
GND ≤ IN = IN ≤ V , (Note 13)
µV
RMS
CC
–
+
CC
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CO VERTER CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
+
–
Input Common Mode Rejection DC 2.5V ≤ REF ≤ V , REF = GND,
●
130
140
dB
CC
–
+
GND ≤ IN = IN ≤ V
CC
sn2415 24151fs
2
LTC2415/LTC2415-1
U
CO VERTER CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
+
–
Input Common Mode Rejection
60Hz ±2% (LTC2415)
2.5V ≤ REF ≤ V , REF = GND,
●
●
●
●
●
●
●
140
dB
CC
–
+
GND ≤ IN = IN ≤ V , (Note 7)
CC
+
–
Input Common Mode Rejection
50Hz ±2% (LTC2415)
2.5V ≤ REF ≤ V , REF = GND,
140
110
110
140
87
dB
dB
dB
dB
dB
dB
CC
–
+
GND ≤ IN = IN ≤ V , (Note 8)
CC
Input Normal Mode Rejection
60Hz ±2% (LTC2415)
(Note 7)
(Note 8)
140
140
Input Normal Mode Rejection
50Hz ±2% (LTC2415)
+
–
Input Common Mode Rejection
49Hz to 61.2Hz (LTC2415-1)
2.5V ≤ REF ≤ V , REF = GND,
CC
–
+
GND ≤ IN = IN ≤ V , (Note 7)
CC
Input Normal Mode Rejection
49Hz to 61.2Hz (LTC2415-1)
F = GND
O
Input Normal Mode Rejection
External Oscillator
87
External Clock f
/2560 ±14%
EOSC
(LTC2415-1)
Input Normal Mode Rejection
External Oscillator
●
●
110
130
140
140
dB
dB
External Clock f
(LTC2415-1)
/2560 ±4%
EOSC
+
–
Reference Common Mode
Rejection DC
2.5V ≤ REF ≤ V , GND ≤ REF ≤ 2.5V,
CC
–
+
V
= 2.5V, IN = IN = GND
REF
+
–
–
+
Power Supply Rejection, DC
REF = V , REF = GND, IN = IN = GND
100
120
120
dB
dB
dB
CC
+
–
–
+
Power Supply Rejection, 60Hz ±2% REF = 2.5V, REF = GND, IN = IN = GND, (Note 7)
+
–
–
+
Power Supply Rejection, 50Hz ±2% REF = 2.5V, REF = GND, IN = IN = GND, (Note 8)
U
U
U
U
A ALOG I PUT A D REFERE CE The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
+
+
IN
Absolute/Common Mode IN Voltage
●
●
●
GND – 0.3V
GND – 0.3V
V
V
+ 0.3V
V
V
V
CC
CC
–
–
IN
Absolute/Common Mode IN Voltage
+ 0.3V
/2
V
Input Differential Voltage Range
–V /2
REF
V
IN
REF
+
–
(IN – IN )
+
–
+
REF
REF
Absolute/Common Mode REF Voltage
●
●
●
0.1
GND
0.1
V
V
V
V
CC
–
Absolute/Common Mode REF Voltage
V
– 0.1V
CC
V
Reference Differential Voltage Range
V
CC
REF
+
–
(REF – REF )
+
+
C (IN )
IN Sampling Capacitance
18
18
18
18
1
pF
pF
pF
pF
nA
nA
nA
S
–
–
C (IN )
IN Sampling Capacitance
S
+
+
C (REF )
REF Sampling Capacitance
S
–
–
C (REF )
REF Sampling Capacitance
S
+
+
+
I
I
I
I
(IN )
IN DC Leakage Current
CS = V , IN = GND
●
●
●
●
–10
–10
–10
–10
10
DC_LEAK
DC_LEAK
DC_LEAK
DC_LEAK
CC
–
–
–
(IN )
IN DC Leakage Current
CS = V , IN = GND
1
10
10
10
CC
+
+
+
(REF )
REF DC Leakage Current
CS = V , REF = 5V
1
CC
–
–
–
(REF )
REF DC Leakage Current
CS = V , REF = GND
1
nA
CC
sn2415 24151fs
3
LTC2415/LTC2415-1
U
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DIGITAL I PUTS A D DIGITAL OUTPUTS
The ● denotes specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
2.7V ≤ V ≤ 5.5V
MIN
TYP
MAX
UNITS
V
IH
V
IL
V
IH
V
IL
High Level Input Voltage
●
●
●
●
●
●
2.5
2.0
V
V
CC
CS, F
2.7V ≤ V ≤ 3.3V
O
CC
Low Level Input Voltage
CS, F
4.5V ≤ V ≤ 5.5V
0.8
0.6
V
V
CC
2.7V ≤ V ≤ 5.5V
O
CC
High Level Input Voltage
SCK
2.7V ≤ V ≤ 5.5V (Note 9)
2.5
2.0
V
V
CC
2.7V ≤ V ≤ 3.3V (Note 9)
CC
Low Level Input Voltage
SCK
4.5V ≤ V ≤ 5.5V (Note 9)
0.8
0.6
V
V
CC
2.7V ≤ V ≤ 5.5V (Note 9)
CC
I
I
Digital Input Current
0V ≤ V ≤ V
CC
–10
–10
10
µA
µA
pF
pF
V
IN
IN
CS, F
O
Digital Input Current
SCK
0V ≤ V ≤ V (Note 9)
10
IN
IN
CC
C
C
V
V
V
V
Digital Input Capacitance
10
10
IN
CS, F
O
Digital Input Capacitance
SCK
(Note 9)
IN
High Level Output Voltage
SDO
I = –800µA
O
●
●
●
●
●
V
V
– 0.5
OH
OL
OH
OL
CC
CC
Low Level Output Voltage
SDO
I = 1.6mA
O
0.4
V
High Level Output Voltage
SCK
I = –800µA (Note 10)
O
– 0.5
V
Low Level Output Voltage
SCK
I = 1.6mA (Note 10)
O
0.4
10
V
I
Hi-Z Output Leakage
SDO
–10
µA
OZ
W U
POWER REQUIRE E TS
The ● denotes specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
Supply Voltage
Supply Current
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
●
2.7
5.5
V
I
CC
Conversion Mode
Sleep Mode
CS = 0V (Note 12)
●
●
200
20
300
30
µA
µA
CS = V (Note 12)
CC
sn2415 24151fs
4
LTC2415/LTC2415-1
W U
TI I G CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
2.56
0.25
0.25
TYP
MAX
2000
390
UNITS
kHz
µs
f
t
t
t
External Oscillator Frequency Range
External Oscillator High Period
External Oscillator Low Period
Conversion Time (LTC2415)
●
●
●
EOSC
HEO
390
µs
LEO
F = 0V
●
●
●
65.43
78.52
66.77
80.12
EOSC
72.8
EOSC
68.1
81.72
(in kHz)
ms
ms
ms
CONV
O
F = V
O
CC
External Oscillator (Note 11)
10278/f
Conversion Time (LTC2415-1)
Internal SCK Frequency
F = 0V
●
●
71.3
45
74.3
(in kHz)
ms
ms
O
External Oscillator (Note 11)
10278/f
f
Internal Oscillator (Note 10), LTC2415
Internal Oscillator (Note 10), LTC2415-1
External Oscillator (Notes 10, 11)
19.2
17.5
kHz
kHz
kHz
ISCK
f
/8
EOSC
D
ISCK
Internal SCK Duty Cycle
(Note 10)
(Note 9)
(Note 9)
(Note 9)
●
●
●
●
55
%
kHz
ns
f
t
t
t
External SCK Frequency Range
External SCK Low Period
External SCK High Period
2000
ESCK
250
250
LESCK
ns
HESCK
DOUT_ISCK
Internal SCK 32-Bit Data Output Time Internal Oscillator (Notes 10, 12), LTC2415
Internal Oscillator (Notes 10, 12), LTC2415-1
●
●
●
1.64
1.80
1.67
1.83
1.70
1.86
ms
ms
ms
External Oscillator (Notes 10, 11)
256/f
(in kHz)
EOSC
t
t
External SCK 32-Bit Data Output Time (Note 9)
CS ↓ to SDO Low Z
●
●
●
●
●
●
●
●
●
32/f
(in kHz)
ms
ns
ns
ns
ns
ns
ns
ns
ns
DOUT_ESCK
1
ESCK
0
0
200
200
200
t2
t3
t4
CS ↑ to SDO High Z
CS ↓ to SCK ↓
(Note 10)
(Note 9)
0
CS ↓ to SCK ↑
50
t
t
SCK ↓ to SDO Valid
SDO Hold After SCK ↓
SCK Set-Up Before CS ↓
SCK Hold After CS ↓
220
50
KQMAX
KQMIN
(Note 5)
15
50
t
t
5
6
Note 1: Absolute Maximum Ratings are those values beyond which the
life of the device may be impaired.
Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz ±2%
(external oscillator).
Note 2: All voltage values are with respect to GND.
Note 9: The converter is in external SCK mode of operation such that
the SCK pin is used as digital input. The frequency of the clock signal
driving SCK during the data output is fESCK and is expressed in kHz.
Note 10: The converter is in internal SCK mode of operation such that
the SCK pin is used as digital output. In this mode of operation the
SCK pin has a total equivalent load capacitance CLOAD = 20pF.
Note 3: VCC = 2.7 to 5.5V unless otherwise specified.
V
REF = REF+ – REF–, VREFCM = (REF+ + REF–)/2;
VIN = IN+ – IN–, VINCM = (IN+ + IN–)/2.
Note 4: FO pin tied to GND or to VCC or to external conversion clock
source with fEOSC = 153600Hz unless otherwise specified.
Note 11: The external oscillator is connected to the FO pin. The external
oscillator frequency, fEOSC, is expressed in kHz.
Note 12: The converter uses the internal oscillator.
Note 5: Guaranteed by design, not subject to test.
Note 6: Integral nonlinearity is defined as the deviation of a code from
a straight line passing through the actual endpoints of the transfer
curve. The deviation is measured from the center of the quantization
band.
FO = 0V or FO = VCC
.
Note 13: The output noise includes the contribution of the internal
calibration operations.
Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz ±2%
(external oscillator).
Note 14: Refer to Offset Accuracy and Drift in the Applications
Information section.
sn2415 24151fs
5
LTC2415/LTC2415-1
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TYPICAL PERFOR A CE CHARACTERISTICS
Total Unadjusted Error Over
Temperature (VCC = 5V,
Total Unadjusted Error Over
Temperature (VCC = 5V,
Total Unadjusted Error Over
Temperature (VCC = 2.7V,
VREF = 2.5V)
VREF = 5V)
VREF = 2.5V)
106.5
106.0
105.5
105.0
104.5
104.0
103.5
215
213
211
209
207
205
125
121
117
113
109
105
V
V
V
= 5V
= 5V
+
–
CC
REF
T
= 90°C
V
V
V
= 2.7V
REF = 2.5V
A
CC
= 2.5V
REF = GND
T = 90°C
A
REF
= 2.5V
INCM
= 1.25V
F = GND
+
INCM
O
REF = 5V
–
T
= 90°C
A
REF = GND
F
= GND
O
T
= 25°C
A
V
V
V
= 5V
CC
T
T
= –45°C
T
= 25°C
A
A
A
= 2.5V
REF
= 1.25V
INCM
+
= 25°C
REF = 2.5V
T
= –45°C
A
–
T
= –45°C
A
REF = GND
F
= GND
O
–2.5 –2 –1.5 –1 –0.5
V
0
(V)
0.5
1
1.5
2
2.5
–1.25 –0.75 –0.25
V
0.25
(V)
0.75
1.25
–1.25 –0.75 –0.25
0.25
(V)
IN
0.75
1.25
V
IN
IN
2415 G01
2415 G02
2415 G03
Integral Nonlinearity Over
Temperature (VCC = 5V,
VREF = 5V)
Integral Nonlinearity Over
Temperature (VCC = 5V,
VREF = 2.5V)
Integral Nonlinearity Over
Temperature (VCC = 2.7V,
VREF = 2.5V)
10
8
1.5
1.0
2.5
2.0
+
–
V
V
V
= 5V
= 5V
V
V
V
= 5V
REF = 2.5V
CC
REF
CC
T
= 90°C
A
T
T
= –45°C
= 25°C
= 2.5V
REF = GND
A
A
REF
= 2.5V
= 1.25V
F = GND
INCM
INCM
O
6
1.5
+
REF = 5V
T
= 25°C
A
–
REF = GND
4
1.0
0.5
F
= GND
O
2
0.5
0
0
0
T
T
= 25°C
A
A
T
= 90°C
–2
–4
–6
–8
–10
A
–0.5
–1.0
–1.5
–2.0
–2.5
–0.5
–1.0
–1.5
= –45°C
T
= 90°C
A
+
–
V
V
V
= 2.7V
REF = 2.5V
CC
T
= –45°C
A
= 2.5V
REF = GND
REF
INCM
= 1.25V
F = GND
O
–1.25 –0.75 –0.25
V
0.25
(V)
0.75
1.25
–2.5 –2 –1.5 –1 –0.5
V
0
0.5
1
1.5
2
2.5
–1.25 –0.75 –0.25
V
0.25
(V)
0.75
1.25
(V)
IN
IN
IN
2415 G05
2415 G04
2415 G05
Noise Histogram
(Output Rate = 15Hz,
VCC = 5V, VREF = 5V)
Noise Histogram
(Output Rate = 45Hz,
VCC = 5V, VREF = 5V)
Noise Histogram
(Output Rate = 105Hz,
VCC = 5V, VREF = 5V)
12
10
8
12
10
8
10
8
GAUSSIAN
10,000 CONSECUTIVE
READINGS
GAUSSIAN
10,000 CONSECUTIVE
READINGS
GAUSSIAN
DISTRIBUTION
m = –199.0ppm
σ = 0.9ppm
DISTRIBUTION
m = –103.5ppm
σ = 0.27ppm
DISTRIBUTION
m = –104.0ppm
σ = 0.25ppm
V
V
V
= 5V
V
V
V
= 5V
CC
REF
IN
CC
REF
= 5V
= 5V
= 0V
= 0V
IN
+
+
–
10,000 CONSECUTIVE
READINGS
REF = 5V
REF = 5V
–
6
REF = GND
REF = GND
+
–
+
–
V
REF
V
= 5V
= 5V
= 0V
CC
IN = 2.5V
IN = 2.5V
6
6
V
IN = 2.5V
IN = 2.5V
IN
F
= GND
= 25°C
F = 460800Hz
O
A
4
O
+
REF = 5V
T
T
A
= 25°C
4
4
–
REF = GND
+
IN = 2.5V
–
2
IN = 2.5V
2
2
F
O
= 1075200Hz
T
A
= 25°C
0
0
0
–202
–199.5
–197
–194.5
–192
–105.5
–104.8
–104
–103.3
–102.5
–105
–104.5
–104
–103.5
–103
OUTPUT CODE (ppm OF V
)
OUTPUT CODE (ppm OF V
)
OUTPUT CODE (ppm OF V
)
REF
REF
REF
2415 G09
2415 G07
2415 G08
sn2415 24151fs
6
LTC2415/LTC2415-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Noise Histogram
Noise Histogram
Noise Histogram
(Output Rate = 15Hz,
(Output Rate = 45Hz,
(Output Rate = 105Hz,
VCC = 5V, VREF = 2.5V)
V
CC = 5V, VREF = 2.5V)
VCC = 5V, VREF = 2.5V)
12
10
8
12
10
8
15
12
9
10,000 CONSECUTIVE
READINGS
GAUSSIAN
10,000 CONSECUTIVE GAUSSIAN
GAUSSIAN
DISTRIBUTION
m = –209.3ppm
σ = 0.49ppm
READINGS
DISTRIBUTION
DISTRIBUTION
m = –206.5ppm
σ = 1.07ppm
V
V
V
= 5V
V
V
V
= 5V
m = –209.2ppm
σ = 0.56ppm
CC
REF
IN
CC
= 2.5V
= 2.5V
REF
= 0V
= 0V
IN
+
+
10,000 CONSECUTIVE
REF = 2.5V
REF = 2.5V
–
–
READINGS
REF = GND
REF = GND
+
–
+
–
V
= 5V
CC
REF
IN = 1.25V
IN = 1.25V
6
6
V
= 2.5V
IN = 1.25V
IN = 1.25V
V
= 0V
IN
F
= 460800Hz
= 25°C
F
= GND
= 25°C
O
A
O
A
6
+
REF = 2.5V
T
T
4
4
–
REF = GND
+
–
IN = 1.25V
3
IN = 1.25V
2
2
F
O
= 1075200Hz
T
= 25°C
A
0
0
0
–211.5
–210.5
–209.5
–208.5
–207.5
–212
–210.5
–209
–207.5
–206
–210
–207
–204
–201
–198
OUTPUT CODE (ppm OF V
)
OUTPUT CODE (ppm OF V
)
OUTPUT CODE (ppm OF V
)
REF
REF
REF
2415 G11
2415 G10
2415 G12
Noise Histogram
Noise Histogram
(Output Rate = 45Hz,
VCC = 2.7V, VREF = 2.5V)
Noise Histogram
(Output Rate = 15Hz,
(Output Rate = 105Hz,
VCC = 2.7V, VREF = 2.5V)
VCC = 2.7V, VREF = 2.5V)
10
8
10
8
12
10
8
10,000 CONSECUTIVE
READINGS
GAUSSIAN
10,000 CONSECUTIVE
READINGS
GAUSSIAN
10,000 CONSECUTIVE
READINGS
GAUSSIAN
DISTRIBUTION
m = –109.8ppm
σ = 0.50ppm
DISTRIBUTION
m = –20.5ppm
σ = 1.90ppm
DISTRIBUTION
m = –113.1ppm
σ = 0.59ppm
V
V
V
= 2.7V
= 2.5V
V
V
V
= 2.7V
= 2.5V
V
V
V
= 2.7V
= 2.5V
CC
REF
IN
CC
REF
IN
CC
REF
IN
= 0V
= 0V
= 0V
+
+
+
REF = 2.5V
REF = 2.5V
REF = 2.5V
–
–
–
6
6
REF = GND
REF = GND
REF = GND
+
–
+
–
+
–
IN = 1.25V
IN = 1.25V
IN = 1.25V
6
IN = 1.25V
IN = 1.25V
IN = 1.25V
F = 460800Hz
F = 1075200Hz
F
= GND
= 25°C
4
O
4
O
O
T = 25°C
A
T
= 25°C
T
A
A
4
2
2
2
0
0
0
–116
–114.5
–113
–111.5
–110
–112
–110.9
–109.8
–108.6
–107.5
–30
–25.5
–21
–16.5
–12
OUTPUT CODE (ppm OF V
)
OUTPUT CODE (ppm OF V
)
REF
OUTPUT CODE (ppm OF V
)
REF
REF
2415 G13
2415 G14
2415 G15
Long-Term Histogram
(60Hrs)
Consecutive ADC Readings vs
Time
RMS Noise vs Input Differential
Voltage
12
10
8
–101.0
–101.5
–102.0
–102.5
–103.0
–103.5
–104.0
–104.5
–105.0
–105.5
0.5
0.4
0.3
0.2
0.1
0
V
V
V
= 5V
= 5V
GAUSSIAN
CC
REF
IN
+
V
V
V
= 5V
IN = 2.5V
CC
DISTRIBUTION
m = –103.9ppm
σ = 0.27ppm
–
= 5V
IN = 2.5V
REF
= 0V
+
= 0V
F
T
= GND
= 25°C
IN
O
A
REF = 5V
+
–
REF = 5V
REF = GND
–
+
–
REF = GND
IN = 2.5V
IN = 2.5V
F
= GND
= 25°C
O
A
6
T
V
V
V
= 5V
CC
= 5V
REF
= 2.5V
4
INCM
+
REF = 5V
–
REF = GND
2
F
= GND
= 25°C
O
A
T
0
–103
–103.5
OUTPUT CODE (ppm OF V
–104
–104.5
–105
0
5
10 15 20 25 30 35 40 45 50 55 60
TIME (HRS)
–2.5 –2 –1.5 –1 –0.5
0
0.5
1
1.5
2
2.5
)
INPUT DIFFERENTIAL VOLTAGE (V)
REF
2415 G16
2415 G17
2415 G18
sn2415 24151fs
7
LTC2415/LTC2415-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
RMS Noise vs VINCM
RMS Noise vs Temperature (TA)
RMS Noise vs VCC
1560
1520
1480
1440
1400
1360
1320
1280
1800
1600
1400
1200
1000
1400
1250
1100
950
+
V
V
V
= 5V
IN = V
CC
INCM
INCM
–
= 5V
IN = V
REF
= 0V
F = GND
O
IN
+
REF = 5V
T = 25°C
A
–
REF = GND
V
= 2.5V
V
V
= 5V
REF
CC
IN
+
REF = 2.5V
= 0V
–
+
REF = GND
REF = 5V
+
–
–
IN = GND
REF = GND
+
–
IN = GND
IN = 2.5V
F
= GND
= 25°C
IN = 2.5V
O
A
T
F
= GND
O
800
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
(V)
–0.5 0 0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
–50
–25
0
25
50
75
100
V
CC
V
(V)
TEMPERATURE (°C)
INCM
2415 G21
2415 G19
2415 G20
RMS Noise vs VREF
Offset Error vs VINCM
Offset Error vs Temperature (TA)
1600
1400
1200
1000
800
–103.0
–103.4
–103.8
–104.2
–104.6
–105.0
–103.8
–104.0
–104.2
–104.4
–104.6
V
V
V
= 5V
CC
= 5V
REF
= 0V
IN
+
REF = 5V
–
REF = GND
IN = V
IN = V
+
–
INCM
INCM
= GND
= 25°C
F
O
A
T
V
V
= 5V
CC
IN
= 0V
V
= 5V
CC
+
–
REF = 5V
REF = GND
–
+
–
REF = GND
IN = GND
IN = GND
+
–
IN = 2.5V
IN = 2.5V
F
= GND
= 25°C
O
A
F
= GND
O
T
0
0.5
1
1.5
2
V
2.5
(V)
3
3.5
4
4.5
5
–0.5 0 0.5
1
1.5
2
2.5
3
(V)
3.5 4 4.5 5 5.5
–50
–25
0
25
50
75
100
V
INCM
TEMPERATURE (°C)
REF
2415 G22
2415 G23
2415 G24
+Full-Scale Error vs
Temperature (TA)
Offset Error vs VCC
Offset Error vs VCC and VREF
–110
–130
–150
–170
–190
–210
–230
–103.2
3
2
V
= 2.5V
REF
+
REF = 2.5V
–
REF = GND
–103.6
–104.0
–104.4
–104.8
–105.2
+
–
IN = GND
IN = GND
1
F
= GND
= 25°C
O
A
T
0
V
= 5V
CC
+
–1
–2
–3
REF = 5V
–
REF = GND
+
–
IN = 2.5V
IN = GND
F
O
= GND
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
(V)
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
AND V (V)
–45 –30 –15
0
15 30 45 60 75 90
V
CC
V
CC
TEMPERATURE (°C)
REF
2415 G25
2415 G26
2415 G27
sn2415 24151fs
8
LTC2415/LTC2415-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
–Full-Scale Error vs
Temperature (TA)
+Full-Scale Error vs VCC
+Full-Scale Error vs VREF
5
4
3
2
1
0
0
–1
–2
–3
8
4
V
= 2.5V
REF
+
REF = 2.5V
–
REF = GND
+
–
IN = 1.25V
IN = GND
F
= GND
= 25°C
O
A
T
0
V
= 5V
CC
+
REF = V
V
= 5V
CC
REF
–
+
–4
–5
–6
REF = GND
REF = 5V
+
–
+
–
IN = 0.5 • REF
REF = GND
–4
–8
+
–
IN = GND
IN = GND
F
= GND
= 25°C
IN = 2.5V
O
A
T
F = GND
O
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
(V)
–45 –30 –15
0
15 30 45 60 75 90
0.5
1
1.5
2
2.5
V
3
(V)
3.5
4
4.5
5
V
TEMPERATURE (°C)
CC
REF
2415 G28
2415 G30
2415 G29
–Full-Scale Error vs VCC
–Full-Scale Error vs VREF
PSRR vs Frequency at VCC
0
–1
–2
–3
–4
–5
8
0
–20
V
= 4.1V DC + 1.4V AC
CC
+
REF = 2.5V
–
4
0
REF = GND
IN = IN = GND
+
–
F
= GND
= 25°C
O
A
–40
T
–60
V
= 2.5V
V
= 5V
REF
CC
+
+
–4
–8
–12
REF = 2.5V
REF = V
REF
–
–
REF = GND
REF = GND
–80
+
–
+
–
IN = GND
IN = GND
+
IN = 1.25V
IN = 0.5 • REF
–100
–120
F
= GND
= 25°C
F
= GND
= 25°C
O
A
O
A
T
T
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
(V)
0
50
100
150
200
250
0.5
1
1.5
2
2.5
V
3
(V)
3.5
4
4.5
5
V
FREQUENCY AT V (Hz)
CC
CC
REF
2415 G31
2415 G33
2415 G32
Conversion Current vs
Temperature (TA)
PSRR vs Frequency at VCC
PSRR vs Frequency at VCC
0
–20
0
220
210
200
190
180
170
160
150
140
+
–
+
–
V
V
V
= V
CC
REF = 2.5V
V
= 4.1V DC + 0.7V AC
CC
REF
+
= GND
REF = GND
REF = 2.5V
REF
–20
+
–
+
–
–
= V = GND
IN = IN = GND
REF = GND
IN
IN
V
= 5.5V
+
–
CC
F
= GND
= 25°C
IN = IN = GND
O
A
T
F = GND
–40
–40
–60
O
F
= GND
CS = GND
SCK = SDO = N/C
T
= 25°C
O
A
V
V
= 4.1V
= 2.7V
CC
CC
–60
–80
–80
–100
–120
–100
–120
1
100
10000
1000000
15200
15300
15400
15500
–45 –30 –15
0
15 30 45 60 75 90
FREQUENCY AT V (Hz)
FREQUENCY AT V (Hz)
TEMPERATURE (°C)
CC
CC
2415 G34
2415 G35
2415 G36
sn2415 24151fs
9
LTC2415/LTC2415-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Conversion Current vs Output
Data Rate
Sleep Current vs
Temperature (TA)
1000
25
24
23
22
21
20
19
18
17
16
15
+
–
V
= 5V
CC
V
V
V
= V
CC
REF
900
800
700
600
500
400
300
200
100
0
+
REF = 5V
= GND
REF
–
+
–
REF = GND
= V = GND
= GND
CS = V
SCK = SDO = N/C
IN
IN
+
–
IN = GND
F
O
IN = GND
CC
F
O
= EXT OSC
CS = GND
SCK =N/C
SDO = N/C
V
V
= 5.5V
= 2.7V
CC
CC
V
= 4.1V
CC
0
10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
–45 –30 –15
0
15 30 45 60 75 90
TEMPERATURE (°C)
2415 G37
2415 G38
U
U
U
PI FU CTIO S
GND (Pins 1, 7, 8, 9, 10, 15, 16): Ground. Multiple ground
pins internally connected for optimum ground current flow
and VCC decoupling. Connect each one of these pins to a
groundplanethroughalowimpedanceconnection.Allseven
pins must be connected to ground for proper operation.
CS (Pin 11): Active LOW Digital Input. A LOW on this pin
enables the SDO digital output and wakes up the ADC.
Following each conversion, the ADC automatically enters
the Sleep mode and remains in this low power state as
long as CS is HIGH. A LOW-to-HIGH transition on CS
during the Data Output transfer aborts the data transfer
and starts a new conversion.
VCC (Pin 2): Positive Supply Voltage. Bypass to GND
(Pin 1) with a 10µF tantalum capacitor in parallel with
0.1µF ceramic capacitor as close to the part as possible.
REF+ (Pin 3), REF– (Pin 4): Differential Reference Input.
ThevoltageonthesepinscanhaveanyvaluebetweenGND
and VCC as long as the reference positive input, REF+, is
maintained more positive than the reference negative
input, REF –, by at least 0.1V.
SDO (Pin 12): Three-State Digital Output. During the Data
Output period, this pin is used as serial data output. When
the chip select CS is HIGH (CS = VCC) the SDO pin is in a
high impedance state. During the Conversion and Sleep
periods, this pin is used as the conversion status output.
TheconversionstatuscanbeobservedbypullingCSLOW.
SCK (Pin 13): Bidirectional Digital Clock Pin. In Internal
Serial Clock Operation mode, SCK is used as digital output
for the internal serial interface clock during the Data
Output period. In External Serial Clock Operation mode,
SCK is used as digital input for the external serial interface
clock during the Data Output period. A weak internal pull-
up is automatically activated in Internal Serial Clock Op-
eration mode. The Serial Clock Operation mode is deter-
mined by the logic level applied to the SCK pin at power up
IN+ (Pin 5), IN– (Pin 6): Differential Analog Input. The
voltage on these pins can have any value between
GND – 0.3V and VCC + 0.3V. Within these limits the
converter bipolar input range (VIN = IN+ – IN–) extends
from –0.5•(VREF)to0.5•(VREF). Outsidethisinputrange
the converter produces unique overrange and underrange
output codes.
or during the most recent falling edge of CS.
sn2415 24151fs
10
LTC2415/LTC2415-1
U
U
U
PI FU CTIO S
FO (Pin 14): Frequency Control Pin. Digital input that
controls the ADC’s notch frequencies and conversion
time. WhentheFO pinisconnectedtoVCC (LTC2415only),
the converter uses its internal oscillator and the digital
filter first null is located at 50Hz. When the FO pin is
connectedtoGND(FO =OV),theconverterusesitsinternal
oscillator and the digital filter first null is located at 60Hz
(LTC2415) or simultaneous 50Hz/60Hz (LTC2415-1).
When FO is driven by an external clock signal with a
frequency fEOSC, the converter uses this signal as its
system clock and the digital filter first null is located at a
frequency fEOSC/2560.
U
U
W
FU CTIO AL BLOCK DIAGRA
INTERNAL
OSCILLATOR
V
CC
GND
AUTOCALIBRATION
AND CONTROL
F
O
(INT/EXT)
+
IN
IN
+
–
–
∫
∫
∫
SDO
SERIAL
INTERFACE
∑
ADC
SCK
CS
+
REF
REF
–
DECIMATING FIR
–
+
DAC
2415 FD
Figure 1. Functional Block Diagram
V
CC
TEST CIRCUITS
1.69k
SDO
SDO
C
LOAD
= 20pF
1.69k
C
LOAD
= 20pF
Hi-Z TO V
OL
OL
Hi-Z TO V
OH
OH
V
V
TO V
OH
OL
V
OL
V
OH
TO V
TO Hi-Z
2415 TA04
TO Hi-Z
2415 TA03
sn2415 24151fs
11
LTC2415/LTC2415-1
U
W
U U
APPLICATIO S I FOR ATIO
CONVERTER OPERATION
is updated on the falling edge of SCK allowing the user to
reliably latch data on the rising edge of SCK (see Figure 3).
The data output state is concluded once 32 bits are read
out of the ADC or when CS is brought HIGH. The device
automatically initiates a new conversion and the cycle
repeats.
Converter Operation Cycle
TheLTC2415/LTC2415-1arelowpower,delta-sigmaana-
log-to-digital converters with an easy to use 3-wire serial
interface (see Figure 1). Their operation is made up of
threestates. Theconverteroperatingcyclebeginswiththe
conversion, followed by the sleep state and ends with the
dataoutput(seeFigure2). The3-wireinterfaceconsistsof
serialdataoutput(SDO),serialclock(SCK)andchipselect
(CS).
Through timing control of the CS and SCK pins, the
LTC2415/LTC2415-1 offer several flexible modes of op-
eration (internal or external SCK and free-running conver-
sion modes). These various modes do not require pro-
gramming configuration registers; moreover, they do not
disturbthecyclicoperationdescribedabove.Thesemodes
of operation are described in detail in the Serial Interface
Timing Modes section.
CONVERT
SLEEP
Conversion Clock
A major advantage the delta-sigma converter offers over
conventional type converters is an on-chip digital filter
(commonly implemented as a Sinc or Comb filter). For
high resolution, low frequency applications, this filter is
typically designed to reject line frequencies of 50Hz or
60Hz plus their harmonics. The filter rejection perfor-
mance is directly related to the accuracy of the converter
system clock. The LTC2415/LTC2415-1 incorporate a
highly accurate on-chip oscillator. This eliminates the
need for external frequency setting components such as
crystals or oscillators. Clocked by the on-chip oscillator,
the LTC2415 achieves a minimum of 110dB rejection at
the line frequency (50Hz or 60Hz ±2%), while the
LTC2415-1 achieves a minimum of 87db rejection at 50Hz
±2% and 60Hz ±2% simultaneously.
FALSE
CS = LOW
AND
SCK
TRUE
DATA OUTPUT
2415 F02
Figure 2. LTC2415 State Transition Diagram
Initially, the LTC2415/LTC2415-1 perform a conversion.
Once the conversion is complete, the device enters the
sleep state. While in this sleep state, power consumption
isreducedbyanorderofmagnitudeifCSisHIGH. Thepart
remains in the sleep state as long as CS is HIGH. The
conversion result is held indefinitely in a static shift
register while the converter is in the sleep state.
Ease of Use
The LTC2415/LTC2415-1 data output has no latency,
filter settling delay or redundant data associated with the
conversion cycle. There is a one-to-one correspondence
between the conversion and the output data. Therefore,
multiplexing multiple analog voltages is easy.
Once CS is pulled LOW, the device begins outputting the
conversion result. There is no latency in the conversion
result. The data output corresponds to the conversion just
performed. This result is shifted out on the serial data out
pin (SDO) under the control of the serial clock (SCK). Data
sn2415 24151fs
12
LTC2415/LTC2415-1
U
W U U
APPLICATIO S I FOR ATIO
The LTC2415/LTC2415-1 perform a full-scale calibration
every conversion cycle. This calibration is transparent to
the user and has no effect on the cyclic operation de-
scribed above. The advantage of continuous calibration is
extremestabilityoffull-scalereadingswithrespecttotime,
supply voltage change and temperature drift.
The LTC2415/LTC2415-1 can accept a differential refer-
ence voltage from 0.1V to VCC. The converter output noise
is determined by the thermal noise of the front-end cir-
cuits, and as such, its value in nanovolts is nearly constant
withreferencevoltage.Adecreaseinreferencevoltagewill
not significantly improve the converter’s effective resolu-
tion. On the other hand, a reduced reference voltage will
improve the converter’s overall INL performance. A re-
duced reference voltage will also improve the converter
performance when operated with an external conversion
clock (external FO signal) at substantially higher output
data rates (see the Output Data Rate section).
Unlike the LTC2410 and LTC2413, the LTC2415 and
LTC2415-1 do not perform an offset calibration every
conversion cycle. This enables the LTC2415/LTC2415-1
todoubletheiroutputratewhilemaintaininglinefrequency
rejection. The initial offset of the LTC2415/LTC2415-1 is
within 2mV independent of VREF. Based on the LTC2415/
LTC2415-1 new modulator architecture, the temperature
drift of the offset is less then 0.01ppm/°C. More informa-
tion on the LTC2415/LTC2415-1 offset is described in the
Offset Accuracy and Drift section of this data sheet.
Input Voltage Range
The analog input is truly differential with an absolute/
common mode range for the IN+ and IN– input pins
extending from GND – 0.3V to VCC + 0.3V. Outside
these limits, the ESD protection devices begin to turn on
and the errors due to input leakage current increase
rapidly. Withintheselimits, theLTC2415/LTC2415-1con-
vert the bipolar differential input signal, VIN = IN+ – IN–,
Power-Up Sequence
The LTC2415/LTC2415-1 automatically enter an internal
reset state when the power supply voltage VCC drops
below approximately 2.2V. This feature guarantees the
integrity of the conversion result and of the serial interface
mode selection. (See the 2-wire I/O sections in the Serial
Interface Timing Modes section.)
from –FS = –0.5 • VREF to +FS = 0.5 • VREF where VREF
=
REF+ – REF–. Outside this range, the converters indicate
the overrange or the underrange condition using distinct
output codes.
When the VCC voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with a duration of approximately 0.5ms. The POR
signal clears all internal registers. Following the POR
signal,theLTC2415/LTC2415-1startanormalconversion
cycleandfollowthesuccessionofstatesdescribedabove.
The first conversion result following POR is accurate
within the specifications of the device if the power supply
voltage is restored within the operating range (2.7V to
5.5V) before the end of the POR time interval.
Input signals applied to IN+ and IN– pins may extend by
300mV below ground and above VCC. In order to limit any
fault current, resistors of up to 5k may be added in series
with the IN+ and IN– pins without affecting the perfor-
mance of the device. In the physical layout, it is important
to maintain the parasitic capacitance of the connection
betweentheseseriesresistorsandthecorrespondingpins
as low as possible; therefore, the resistors should be
located as close as practical to the pins. The effect of the
series resistance on the converter accuracy can be evalu-
ated from the curves presented in the Input Current/
Reference Current sections. In addition, series resistors
will introduce a temperature dependent offset error due to
the input leakage current. A 1nA input leakage current will
develop a 1ppm offset error on a 5k resistor if VREF = 5V.
This error has a very strong temperature dependency.
Reference Voltage Range
These converters accept a truly differential external refer-
ence voltage. The absolute/common mode voltage speci-
ficationfortheREF+ andREF– pinscoverstheentirerange
from GND to VCC. For correct converter operation, the
REF+ pin must always be more positive than the REF– pin.
sn2415 24151fs
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Output Data Format
DataisshiftedoutoftheSDOpinundercontroloftheserial
clock (SCK), see Figure 3. Whenever CS is HIGH, SDO
remains high impedance and any externally generated
SCK clock pulses are ignored by the internal data out shift
register.
The LTC2415/LTC2415-1 serial output data stream is 32
bits long. The first 3 bits represent status information
indicating the sign and conversion state. The next 24 bits
are the conversion result, MSB first. The remaining 5 bits
are sub LSBs beyond the 24-bit level that may be included
in averaging or discarded without loss of resolution. The
third and fourth bit together are also used to indicate an
underrange condition (the differential input voltage is
below –FS) or an overrange condition (the differential
input voltage is above +FS).
In order to shift the conversion result out of the device, CS
mustfirstbedrivenLOW. EOCisseenattheSDOpinofthe
deviceonceCSispulledLOW.EOCchangesrealtimefrom
HIGH to LOW at the completion of a conversion. This
signal may be used as an interrupt for an external
microcontroller. Bit 31 (EOC) can be captured on the first
risingedgeofSCK. Bit30isshiftedoutofthedeviceonthe
first falling edge of SCK. The final data bit (Bit 0) is shifted
out on the falling edge of the 31st SCK and may be latched
on the rising edge of the 32nd SCK pulse. On the falling
edgeofthe32ndSCKpulse,SDOgoesHIGHindicatingthe
initiation of a new conversion cycle. This bit serves as EOC
(Bit 31) for the next conversion cycle. Table 2 summarizes
the output data format.
Bit 31 (first output bit) is the end of conversion (EOC)
indicator. This bit is available at the SDO pin during the
conversion and sleep states whenever the CS pin is LOW.
This bit is HIGH during the conversion and goes LOW
when the conversion is complete.
Bit 30 (second output bit) is a dummy bit (DMY) and is
always LOW.
AslongasthevoltageontheIN+ andIN– pinsismaintained
within the –0.3V to (VCC + 0.3V) absolute maximum
operating range, a conversion result is generated for any
differential input voltage VIN from –FS = –0.5 • VREF to
+FS=0.5•VREF.Fordifferentialinputvoltagesgreaterthan
+FS, the conversion result is clamped to the value corre-
sponding to the +FS + 1LSB. For differential input voltages
below –FS, the conversion result is clamped to the value
corresponding to –FS – 1LSB.
Bit 29 (third output bit) is the conversion result sign indi-
cator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this
bit is LOW.
Bit 28 (fourth output bit) is the most significant bit (MSB)
of the result. This bit in conjunction with Bit 29 also
provides the underrange or overrange indication. If both
Bit 29 and Bit 28 are HIGH, the differential input voltage is
above +FS. If both Bit 29 and Bit 28 are LOW, the
differential input voltage is below –FS.
The function of these bits is summarized in Table 1.
Offset Accuracy and Drift
Table 1. LTC2415/LTC2415-1 Status Bits
Bit 31 Bit 30 Bit 29 Bit 28
Unlike the LTC2410/LTC2413 and the entire LTC2400 fam-
ily, the LTC2415/LTC2415-1 do not perform an offset
calibration every cycle. The reason for this is to increase the
data output rate while maintaining line frequency rejection.
Input Range
EOC
DMY
SIG
MSB
V
≥ 0.5 • V
0
0
0
0
0
1
1
0
1
0
IN
REF
0V ≤ V < 0.5 • V
0
1
IN
REF
While the initial accuracy of the LTC2415/LTC2415-1
offset is within 2mV (see Figure 4) several unique proper-
ties of the LTC2415/LTC2415-1 architecture nearly elimi-
natethedriftoftheoffseterrorwithrespecttotemperature
and supply.
–0.5 • V ≤ V < 0V
0
0
REF
IN
V
< –0.5 • V
0
0
IN
REF
Bits 28-5 are the 24-bit conversion result MSB first.
Bit 5 is the least significant bit (LSB).
AsshowninFigure5, theoffsetvariationwithtemperature
is less than 0.6ppm over the complete temperature range
of–50°Cto100°C.Thiscorrespondstoatemperaturedrift
Bits 4-0 are sub LSBs below the 24-bit level. Bits 4-0 may
be included in averaging or discarded without loss of
resolution.
of 0.004ppm/°C.
sn2415 24151fs
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While the variation in offset with supply voltage is propor-
tional to VCC (see Figure 4), several characteristics of this
variation can be used to eliminate the effects. First, the
variation with respect to supply voltage is linear. Second,
the magnitude of the offset error decreases with de-
creased supply voltage. Third, the offset error increases
with increased reference voltage with an equal and oppo-
site magnitude to the supply voltage variation. As a result,
by tying VCC to VREF, the variation with supply can be
nearly eliminated, see Figure 6. The variation with supply
is less than 2ppm over the entire 2.7V to 5.5V supply
range.
Table 2. LTC2415/LTC2415-1 Output Data Format
Differential Input Voltage
V *
Bit 31
EOC
Bit 30
DMY
Bit 29
SIG
Bit 28
MSB
Bit 27
Bit 26
Bit 25
…
Bit 0
IN
V * ≥ 0.5 • V **
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
0
0
0
0
0
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
…
…
…
…
…
…
…
…
…
…
0
1
0
1
0
1
0
1
0
IN
REF
0.5 • V ** – 1LSB
REF
0.25 • V **
REF
0.25 • V ** – 1LSB
REF
0
–1LSB
–0.25 • V **
REF
–0.25 • V ** – 1LSB
REF
–0.5 • V **
REF
V * < –0.5 • V **
0
1
IN
REF
+
–
+
–
*The differential input voltage V = IN – IN .
**The differential reference voltage V = REF – REF .
IN
REF
CS
BIT 31
EOC
BIT 30
“0”
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 5
BIT 0
SDO
SCK
LSB
24
Hi-Z
1
2
3
4
5
26
27
32
SLEEP
DATA OUTPUT
CONVERSION
2415 F03
Figure 3. Output Data Timing
–103.0
–103.5
–104.0
–104.5
–105.0
–105.5
50
0
–103.8
–104.0
–104.2
–104.4
–104.6
V
V
= 5V
= 5V
V
= 0V
IN
CC
REF
+
IN = GND
PART NO.1
PART NO.2
+
–
REF = 5V IN = GND
–
REF = GND F = GND
O
–50
T
=100°C
=25°C
A
–100
–150
–200
–250
T
A
T
= –50°C
A
PART NO.3
V
A
= 2.5V
3.0
REF
T
= 25°C
2.5
3.5
4.0
(V)
4.5
5.0
5.5
2.7
3.5 3.9 4.3
V
4.7
(V)
5.5
3.1
5.1
–50 –25
0
25
50
75
100
V
TEMPERATURE (°C)
AND V
REF
CC
CC
2415 F04
2415 F06
2415 F05
Figure 4. Offset vs VCC
Figure 5. Offset vs Temperature
Figure 6. Offset vs VCC (VREF = VCC)
sn2415 24151fs
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Frequency Rejection Selection LTC2415 (F )
Table 3a summarizes the duration of each state and the
O
achievable output data rate as a function of F .
O
The LTC2415 internal oscillator provides better than 110dB
normalmoderejectionatthelinefrequencyanditsharmon-
Frequency Rejection Selection LTC2415-1 (FO)
ics for 50Hz ±2% or 60Hz ±2%. For 60Hz rejection, F
O
O
The LTC2415-1 internal oscillator provides better than
87dB normal mode rejection over the range of 49Hz to
61.2HzasshowninFigure7b.Forsimultaneous50Hz/60Hz
should be connected to GND while for 50Hz rejection the F
pin should be connected to V .
CC
Theselectionof50Hzor60Hzrejectioncanalsobemadeby
rejection, F should be connected to GND.
O
driving F to an appropriate logic level. A selection change
O
In order to achieve 87dB normal mode rejection of 50Hz
±2% and 60Hz ±2%, two consecutive conversions must be
averaged. By performing a continuous running average of
the two most current results, both simultaneous rejection
is achieved and a 2× increase in throughput is realized
relativetotheLTC2413(seeNormalModeRejection, Ouput
Rate and Running Averages sections of this data sheet).
during the sleep or data output states will not disturb the
converter operation. If the selection is made during the
conversion state, the result of the conversion in progress
may be outside specifications but the following conver-
sions will not be affected.
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2415 can
operate with an external conversion clock. The converter
automatically detects the presence of an external clock
When a fundamental rejection frequency different from
therange49Hzto61.2Hzisrequiredorwhentheconverter
must be synchronized with an outside source, the
LTC2415-1canoperatewithanexternalconversionclock.
The converter automatically detects the presence of an
external clock signal at the FO pin and turns off the internal
oscillator. The frequency fEOSC of the external signal must
beatleast2560Hztobedetected.Theexternalclocksignal
duty cycle is not significant as long as the minimum and
maximumspecificationsforthehighandlowperiods,tHEO
and tLEO, are observed.
signal at the F pin and turns off the internal oscillator. The
O
EOSC
frequency f
of the external signal must be at least
2560Hz (1Hz notch frequency) to be detected. The external
clock signal duty cycle is not significant as long as the
minimumandmaximumspecificationsforthehighandlow
periods t
and t
are observed.
LEO
HEO
While operating with an external conversion clock of a
frequency f , the LTC2415 provides better than 110dB
EOSC
normal mode rejection in a frequency range f
/2560
EOSC
While operating with an external conversion clock of a
frequency fEOSC, the LTC2415-1 provides better than
±4% and its harmonics. The normal mode rejection as a
function of the input frequency deviation from f
is shown in Figure 7a.
/2560
EOSC
110dB normal mode rejection in a frequency range fEOSC
/
2560 ±4%. The normal mode rejection as a function of the
input frequency deviation from fEOSC/2560 is shown in
Figure 7a and Figure 7c shows the normal mode rejection
with running averages included.
Whenever an external clock is not present at the F pin, the
O
converter automatically activates its internal oscillator and
enters the Internal Conversion Clock mode. The LTC2415
operation will not be disturbed if the change of conversion
clock source occurs during the sleep state or during the
data output state while the converter uses an external serial
clock. If the change occurs during the conversion state, the
result of the conversion in progress may be outside speci-
fications but the following conversions will not be affected.
If the change occurs during the data output state and the
converter is in the Internal SCK mode, the serial clock duty
cycle may be affected but the serial data stream will remain
valid.
Whenever an external clock is not present at the FO pin the
converterautomaticallyactivatesitsinternaloscillatorand
enterstheInternalConversionClockmode.TheLTC2415-1
operation will not be disturbed if the change of conversion
clock source occurs during the sleep state or during the
data output state while the converter uses an external
serial clock. If the change occurs during the conversion
state, the result of the conversion in progress may be
outside specifications but the following conversions will
notbeaffected.Ifthechangeoccursduringthedataoutput
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Serial Interface Pins
state and the converter is in the Internal SCK mode, the
serial clock duty cycle may be affected but the serial data
stream will remain valid.
The LTC2415/LTC2415-1 transmit the conversion results
and receive the start of conversion command through a
synchronous 3-wire interface. During the conversion and
sleep states, this interface can be used to assess the
converter status and during the data output state it is used
to read the conversion result.
Table 3b summarizes the duration of each state and the
achievable output data rate as a function of FO.
–80
–90
–60
–70
–80
–85
–90
–80
–95
–100
–100
–120
–130
–140
–100
–105
–110
–115
–120
–125
–130
–135
–140
–90
–100
–110
–120
–130
–140
–12
–8
–4
0
4
8
12
48
50
52
54
56
58
60
62
–12
–8
–4
0
4
8
12
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
DIFFERENTIAL INPUT SIGNAL FREQUENCY
DEVIATION FROM NOTCH FREQUENCY f
/2560(%)
EOSC
2415 F07a
2415 F07b
2415 F07c
Figure 7a. LTC2415/LTC2415-1 Normal Mode
Rejection When Using an External Oscillator
of Frequency fEOSC without Running Averages
Figure 7b. LTC2415-1 Normal Mode
Rejection When Using an Internal
Oscillator with Running Averages
Figure 7c. LTC2415/LTC2415-1
Normal Mode Rejection When Using
an External Oscillator of Frequency
fEOSC with Running Averages
Table 3a. LTC2415 State Duration
State
Operating Mode
Duration
CONVERT
Internal Oscillator
F = LOW, (60Hz Rejection)
66.6ms, Output Data Rate ≤ 15 Readings/s
80ms, Output Data Rate ≤ 12.4 Readings/s
F = External Oscillator with Frequency 10278/f s, Output Data Rate ≤ f /10278 Readings/s
O
F = HIGH, (50Hz Rejection)
O
External Oscillator
O
EOSC
EOSC
f
kHz (f /2560 Rejection)
EOSC
EOSC
SLEEP
As Long As CS = HIGH Until CS = LOW and SCK
DATA OUTPUT Internal Serial Clock F = LOW/HIGH, (Internal Oscillator)
As Long As CS = LOW But Not Longer Than 1.67ms (32 SCK cycles)
O
F = External Oscillator with
O
As Long As CS = LOW But Not Longer Than 256/f
ms (32 SCK cycles)
EOSC
Frequency f
kHz
EOSC
External Serial Clock with Frequency f
kHz
As Long As CS = LOW But Not Longer Than 32/f ms (32 SCK cycles)
SCK
SCK
Table 3b. LTC2415-1 State Duration
State
Operating Mode
Duration
CONVERT
Internal Oscillator
F = LOW
Simultaneous 50Hz/60Hz Rejection
72.8ms, Output Data Rate ≤ 14 Readings/s
O
External Oscillator
F = External Oscillator with Frequency 10278/f
s, Output Data Rate ≤ f
/10278 Readings/s
EOSC
O
EOSC
f
kHz (f /2560 Rejection)
EOSC
EOSC
SLEEP
As Long As CS = HIGH Until CS = LOW and SCK
DATA OUTPUT Internal Serial Clock F = LOW (Internal Oscillator)
As Long As CS = LOW But Not Longer Than 1.83ms (32 SCK cycles)
O
F = External Oscillator with
O
As Long As CS = LOW But Not Longer Than 256/f
ms (32 SCK cycles)
EOSC
Frequency f
kHz
EOSC
External Serial Clock with Frequency f
kHz
As Long As CS = LOW But Not Longer Than 32/f ms (32 SCK cycles)
SCK
SCK
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Serial Clock Input/Output (SCK)
conversionstatusandtoenablethedataoutputtransferas
described in the previous sections.
The serial clock signal present on SCK (Pin 13) is used to
synchronizethedatatransfer.Eachbitofdataisshiftedout
the SDO pin on the falling edge of the serial clock.
In addition, the CS signal can be used to trigger a new
conversion cycle before the entire serial data transfer has
been completed. The LTC2415/LTC2415-1 will abort any
serial data transfer in progress and start a new conversion
cycle anytime a LOW-to-HIGH transition is detected at the
CSpinaftertheconverterhasenteredthedataoutputstate
(i.e., after the first rising edge of SCK occurs with
CS = LOW).
In the Internal SCK mode of operation, the SCK pin is an
output and the LTC2415/LTC2415-1 create their own se-
rial clock by dividing the internal conversion clock by 8. In
the External SCK mode of operation, the SCK pin is used
as input. The internal or external SCK mode is selected on
power-up and then reselected every time a HIGH-to-LOW
transition is detected at the CS pin. If SCK is HIGH or float-
ing at power-up or during this transition, the converter
enters the internal SCK mode. If SCK is LOW at power-up
or during this transition, the converter enters the external
SCK mode.
Finally, CS can be used to control the free-running modes
of operation, see Serial Interface Timing Modes section.
Grounding CS will force the ADC to continuously convert
at the maximum output rate selected by FO. Tying a
capacitor to CS will reduce the output rate and power
dissipation by a factor proportional to the capacitor’s
value, see Figures 15 to 17.
Serial Data Output (SDO)
The serial data output pin, SDO (Pin 12), provides the
result of the last conversion as a serial bit stream (MSB
first) during the data output state. In addition, the SDO pin
is used as an end of conversion indicator during the
conversion and sleep states.
SERIAL INTERFACE TIMING MODES
The LTC2415/LTC2415-1 3-wire interface is SPI and
MICROWIRE compatible. This interface offers several
flexible modes of operation. These include internal/exter-
nal serial clock, 2- or 3-wire I/O, single cycle conversion
and autostart. The following sections describe each of
these serial interface timing modes in detail. In all these
cases, the converter can use the internal oscillator (FO =
LOW or FO = HIGH) or an external oscillator connected to
the FO pin. Refer to Table 4 for a summary.
When CS (Pin 11) is HIGH, the SDO driver is switched to
a high impedance state. This allows sharing the serial
interface with other devices. If CS is LOW during the
convert or sleep state, SDO will output EOC. If CS is LOW
duringtheconversionphase, theEOCbitappearsHIGHon
the SDO pin. Once the conversion is complete, EOC goes
LOW. The device remains in the sleep state until the first
rising edge of SCK occurs while CS = LOW.
External Serial Clock, Single Cycle Operation
(SPI/MICROWIRE Compatible)
Chip Select Input (CS)
This timing mode uses an external serial clock to shift out
the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 8.
The active LOW chip select, CS (Pin 11), is used to test the
Table 4. LTC2415/LTC2415-1 Interface Timing Modes
Conversion
Cycle
Control
Data
Output
Control
Connection
and
Waveforms
SCK
Configuration
Source
External
External
Internal
Internal
Internal
External SCK, Single Cycle Conversion
External SCK, 2-Wire I/O
CS and SCK
SCK
CS and SCK
SCK
Figures 8, 9
Figure 10
Internal SCK, Single Cycle Conversion
Internal SCK, 2-Wire I/O, Continuous Conversion
Internal SCK, Autostart Conversion
CS ↓
CS ↓
Figures 11, 12
Figure 13
Continuous
Internal
Internal
C
EXT
Figure 14
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The serial clock mode is selected on the falling edge of CS.
Toselecttheexternalserialclockmode,theserialclockpin
(SCK) must be LOW during each CS falling edge.
Alternatively, CS may be driven HIGH setting SDO to Hi-Z.
As described above, CS may be pulled LOW at any time in
order to monitor the conversion status.
The serial data output pin (SDO) is Hi-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
While CS is pulled LOW, EOC is output to the SDO pin.
EOC = 1 while a conversion is in progress and EOC = 0 if
the device is in the sleep state. Independent of CS, the
device automatically enters the sleep state once the con-
version is complete. While in the sleep state, if CS is high,
the LTC2415/LTC2415-1 power consumption is reduced
by an order of magnitude
Typically, CS remains LOW during the data output state.
However, the data output state may be aborted by pulling
CS HIGH anytime between the first rising edge and the
32nd falling edge of SCK, see Figure 9. On the rising edge
of CS, the device aborts the data output state and imme-
diately initiates a new conversion. This is useful for sys-
tems not requiring all 32 bits of output data, aborting an
invalid conversion cycle or synchronizing the start of a
conversion.
External Serial Clock, 2-Wire I/O
When the device is in the sleep state (EOC = 0), its
conversion result is held in an internal static shift regis-
ter. The device remains in the sleep state until the first
risingedgeofSCKisseenwhileCSisLOW.Dataisshifted
out the SDO pin on each falling edge of SCK. This enables
external circuitry to latch the output on the rising edge of
SCK. EOC can be latched on the first rising edge of SCK
and the last bit of the conversion result can be latched on
the 32nd rising edge of SCK. On the 32nd falling edge of
SCK, thedevicebeginsanewconversion. SDOgoesHIGH
(EOC = 1) indicating a conversion is in progress.
This timing mode utilizes a 2-wire serial I/O interface. The
conversion result is shifted out of the device by an exter-
nallygeneratedserialclock(SCK)signal,seeFigure10.CS
may be permanently tied to ground, simplifying the user
interface or isolation barrier.
The external serial clock mode is selected at the end of the
power-on reset (POR) cycle. The POR cycle is concluded
approximately 0.5ms after VCC exceeds 2.2V. The level
applied to SCK at this time determines if SCK is internal or
external. SCK must be driven LOW prior to the end of POR
in order to enter the external serial clock timing mode.
At the conclusion of the data cycle, CS may remain LOW
and EOC monitored as an end-of-conversion interrupt.
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION (LTC2415)
2
14
13
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
O
LTC2415/
LTC2415-1
+
3
4
REFERENCE
VOLTAGE
REF
REF
SCK
–
0.1V TO V
CC
3-WIRE
SPI INTERFACE
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
REF
REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
CS
TEST EOC
TEST EOC
TEST EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 26
BIT 5
LSB
BIT 0
SDO
SUB LSB
Hi-Z
Hi-Z
Hi-Z
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2415 F08
Figure 8. External Serial Clock, Single Cycle Operation
sn2415 24151fs
19
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION (LTC2415)
2
14
13
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
LTC2415/
O
LTC2415-1
+
3
REFERENCE
VOLTAGE
0.1V TO V
REF
REF
SCK
4
–
CC
3-WIRE
SPI INTERFACE
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
REF
REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
CS
TEST EOC
TEST EOC
TEST EOC
BIT 0
EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 9
BIT 8
SDO
Hi-Z
Hi-Z
Hi-Z
Hi-Z
SCK
(EXTERNAL)
SLEEP
CONVERSION
DATA OUTPUT
SLEEP
DATA OUTPUT
CONVERSION
2415 F09
Figure 9. External Serial Clock, Reduced Data Output Length
Since CS is tied LOW, the end-of-conversion (EOC) can be
continuously monitored at the SDO pin during the convert
and sleep states. EOC may be used as an interrupt to an
external controller indicating the conversion result is
ready. EOC = 1 while the conversion is in progress and
EOC = 0oncetheconversionentersthesleepstate. Onthe
falling edge of EOC, the conversion result is loaded into an
internal static shift register. The device remains in the
sleep state until the first rising edge of SCK. Data is shifted
out the SDO pin on each falling edge of SCK enabling
external circuitry to latch data on the rising edge of SCK.
EOC can be latched on the first rising edge of SCK. On the
32nd falling edge of SCK, SDO goes HIGH (EOC = 1)
indicating a new conversion has begun.
enter the internal serial clock mode if SCK is driven LOW
on the falling edge of CS. An internal weak pull-up resistor
is active on the SCK pin during the falling edge of CS;
therefore, the internal serial clock timing mode is auto-
matically selected if SCK is not externally driven.
The serial data output pin (SDO) is Hi-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
Once CS is pulled LOW, SCK goes LOW and EOC is output
to the SDO pin. EOC = 1 while a conversion is in progress
and EOC = 0 if the device is in the sleep state.
WhentestingEOC,iftheconversioniscomplete(EOC=0),
thedevicewillexitthesleepstateandenterthedataoutput
state if CS remains LOW. In order to prevent the device
from exiting the sleep state, CS must be pulled HIGH
before the first rising edge of SCK. In the internal SCK
timing mode, SCK goes HIGH and the device begins
outputting data at time tEOCtest after the falling edge of CS
(if EOC = 0) or tEOCtest after EOC goes LOW (if CS is LOW
duringthefallingedgeofEOC).ThevalueoftEOCtest is23µs
(LTC2415), 26µs (LTC2415-1) if the device is using its
internal oscillator (F0 = logic LOW or HIGH). If FO is driven
sn2415 24151fs
Internal Serial Clock, Single Cycle Operation
This timing mode uses an internal serial clock to shift out
the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 11.
In order to select the internal serial clock timing mode, the
serial clock pin (SCK) must be floating (Hi-Z) or pulled
HIGH prior to the falling edge of CS. The device will not
20
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION (LTC2415)
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
2
14
13
V
F
CC
O
LTC2415/
LTC2415-1
+
3
REFERENCE
VOLTAGE
0.1V TO V
REF
REF
SCK
4
–
2-WIRE
INTERFACE
CC
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
REF REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
CS
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 26
BIT 5
LSB
BIT 0
SDO
24
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2415 F10
Figure 10. External Serial Clock, CS = 0 Operation (2-Wire)
2.7V TO 5.5V
V
CC
V
CC
1µF
= 50Hz REJECTION (LTC2415)
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
2
14
13
V
F
CC
LTC2415/
O
10k
LTC2415-1
+
3
4
REFERENCE
VOLTAGE
REF
REF
SCK
–
0.1V TO V
CC
3-WIRE
SPI INTERFACE
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
REF REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
<t
EOCtest
CS
TEST EOC
TEST EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 26
BIT 5
LSB
BIT 0
SDO
24
Hi-Z
Hi-Z
Hi-Z
Hi-Z
SCK
(INTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2415 F11
Figure 11. Internal Serial Clock, Single Cycle Operation
sn2415 24151fs
21
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
by an external oscillator of (LTC2415-1) frequency fEOSC
,
new conversion. This is useful for systems not requiring
all 32 bits of output data, aborting an invalid conversion
cycle, or synchronizing the start of a conversion. If CS is
pulled HIGH while the converter is driving SCK LOW, the
internal pull-up is not available to restore SCK to a logic
HIGH state. This will cause the device to exit the internal
serial clock mode on the next falling edge of CS. This can
be avoided by adding an external 10k pull-up resistor to
theSCKpinorbyneverpullingCSHIGHwhenSCKisLOW.
then tEOCtest is 3.6/fEOSC. If CS is pulled HIGH before time
tEOCtest, the device remains in the sleep state. The conver-
sion result is held in the internal static shift register.
If CS remains LOW longer than tEOCtest, the first rising
edge of SCK will occur and the conversion result is serially
shiftedoutoftheSDOpin. Thedataoutputcyclebeginson
this first rising edge of SCK and concludes after the 32nd
rising edge. Data is shifted out the SDO pin on each falling
edgeofSCK.Theinternallygeneratedserialclockisoutput
to the SCK pin. This signal may be used to shift the
conversion result into external circuitry. EOC can be
latchedonthefirstrisingedgeofSCKandthelastbitofthe
conversionresultonthe32ndrisingedgeofSCK. Afterthe
32nd rising edge, SDO goes HIGH (EOC = 1), SCK stays
HIGH and a new conversion starts.
Whenever SCK is LOW, the LTC2415/LTC2415-1 internal
pull-up at pin SCK is disabled. Normally, SCK is not
externally driven if the device is in the internal SCK timing
mode.However,certainapplicationsmayrequireanexter-
nal driver on SCK. If this driver goes Hi-Z after outputting
a LOW signal, the LTC2415/LTC2415-1 internal pull-up
remains disabled. Hence, SCK remains LOW. On the next
falling edge of CS, the device is switched to the external
SCK timing mode. By adding an external 10k pull-up
resistortoSCK,thispingoesHIGHoncetheexternaldriver
goes Hi-Z. On the next CS falling edge, the device will
remain in the internal SCK timing mode.
Typically, CS remains LOW during the data output state.
However, the data output state may be aborted by pulling
CS HIGH anytime between the first and 32nd rising edge
of SCK, see Figure 12. On the rising edge of CS, the device
aborts the data output state and immediately initiates a
2.7V TO 5.5V
V
CC
V
CC
1µF
= 50Hz REJECTION (LTC2415)
2
14
13
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
O
10k
LTC2415/
LTC2415-1
+
3
REFERENCE
REF
REF
SCK
VOLTAGE
4
–
0.1V TO V
CC
3-WIRE
SPI INTERFACE
5
6
12
11
+
ANALOG INPUT RANGE
–0.5V TO 0.5V
IN
SDO
CS
REF REF
–
IN
1, 7, 8, 9, 10, 15, 16
<t
EOCtest
GND
>t
EOCtest
CS
TEST EOC
TEST EOC
TEST EOC
BIT 0
EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 26
BIT 8
SDO
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
SCK
(INTERNAL)
SLEEP
CONVERSION
DATA OUTPUT
SLEEP
DATA OUTPUT
CONVERSION
2415 F12
Figure 12. Internal Serial Clock, Reduced Data Output Length
sn2415 24151fs
22
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
A similar situation may occur during the sleep state when
CS is pulsed HIGH-LOW-HIGH in order to test the conver-
sionstatus.Ifthedeviceisinthesleepstate(EOC=0),SCK
will go LOW. Once CS goes HIGH (within the time period
defined above as tEOCtest), the internal pull-up is activated.
For a heavy capacitive load on the SCK pin, the internal
pull-up may not be adequate to return SCK to a HIGH level
before CS goes low again. This is not a concern under
normal conditions where CS remains LOW after detecting
EOC = 0. This situation is easily overcome by adding an
external 10k pull-up resistor to the SCK pin.
weak pull-up is active during the POR cycle; therefore, the
internal serial clock timing mode is automatically selected
if SCK is not externally driven LOW (if SCK is loaded such
that the internal pull-up cannot pull the pin HIGH, the
external SCK mode will be selected).
During the conversion, the SCK and the serial data output
pin (SDO) are HIGH (EOC = 1). Once the conversion is
complete, SCK and SDO go LOW (EOC = 0) indicating the
conversion has finished and the device has entered the
low power sleep state. The part remains in the sleep state
a minimum amount of time (1/2 the internal SCK period)
thenimmediatelybeginsoutputtingdata.Thedataoutput
cyclebeginsonthefirstrisingedgeofSCKandendsafter
the 32nd rising edge. Data is shifted out the SDO pin on
each falling edge of SCK. The internally generated serial
clock is output to the SCK pin. This signal may be used
to shift the conversion result into external circuitry. EOC
can be latched on the first rising edge of SCK and the last
bit of the conversion result can be latched on the 32nd
rising edge of SCK. After the 32nd rising edge, SDO goes
HIGH(EOC=1)indicatinganewconversionisinprogress.
SCK remains HIGH during the conversion.
Internal Serial Clock, 2-Wire I/O,
Continuous Conversion
This timing mode uses a 2-wire, all output (SCK and SDO)
interface. The conversion result is shifted out of the device
by an internally generated serial clock (SCK) signal, see
Figure 13. CS may be permanently tied to ground, simpli-
fying the user interface or isolation barrier.
The internal serial clock mode is selected at the end of the
power-on reset (POR) cycle. The POR cycle is concluded
approximately 0.5ms after VCC exceeds 2.2V. An internal
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION (LTC2415)
2
14
13
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
LTC2415/
O
LTC2415-1
+
3
REFERENCE
REF
REF
SCK
VOLTAGE
4
–
2-WIRE
INTERFACE
0.1V TO V
CC
5
6
12
11
+
ANALOG INPUT RANGE
IN
SDO
CS
–0.5V
TO 0.5V
REF
REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
CS
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
MSB
BIT 27
BIT 26
BIT 5
LSB
BIT 0
SDO
24
SCK
(INTERNAL)
CONVERSION
DATA OUTPUT
CONVERSION
2415 F13
SLEEP
Figure 13. Internal Serial Clock, Continuous Operation
sn2415 24151fs
23
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
Internal Serial Clock, Autostart Conversion
conversion is immediately started. This is useful in appli-
cations requiring periodic monitoring and ultralow power.
Figure 17 shows the average supply current as a function
of capacitance on CS.
This timing mode is identical to the internal serial clock,
2-wire I/O described above with one additional feature.
Instead of grounding CS, an external timing capacitor is
tied to CS.
It should be noticed that the external capacitor discharge
current is kept very small in order to decrease the con-
verter power dissipation in the sleep state. In the autostart
mode, the analog voltage on the CS pin cannot be ob-
served without disturbing the converter operation using a
regular oscilloscope probe. When using this configura-
tion, it is important to minimize the external leakage
current at the CS pin by using a low leakage external
capacitor and properly cleaning the PCB surface.
While the conversion is in progress, the CS pin is held
HIGH by an internal weak pull-up. Once the conversion is
complete, the device enters the low power sleep state and
an internal 25nA current source begins discharging the
capacitor tied to CS, see Figure 14. The time the converter
spends in the sleep state is determined by the value of the
external timing capacitor, see Figures 15 and 16. Once the
voltageatCSfallsbelowaninternalthreshold(≈1.4V), the
device automatically begins outputting data. The data
output cycle begins on the first rising edge of SCK and
ends on the 32nd rising edge. Data is shifted out the SDO
pin on each falling edge of SCK. The internally generated
serial clock is output to the SCK pin. This signal may be
used to shift the conversion result into external circuitry.
After the 32nd rising edge, CS is pulled HIGH and a new
The internal serial clock mode is selected every time the
voltage on the CS pin crosses an internal threshold volt-
age. An internal weak pull-up at the SCK pin is active while
CS is discharging; therefore, the internal serial clock
timing mode is automatically selected if SCK is floating. It
is important to ensure there are no external drivers pulling
SCK LOW while CS is discharging.
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION (LTC2415)
2
14
13
= EXTERNAL OSCILLATOR
= 60Hz REJECTION (LTC2415)
= 50Hz/60Hz REJECTION (LTC2415-1)
V
F
CC
O
LTC2415/
LTC2415-1
+
3
REFERENCE
REF
REF
SCK
VOLTAGE
4
–
2-WIRE
INTERFACE
0.1V TO V
CC
5
6
12
11
+
ANALOG INPUT RANGE
IN
SDO
CS
–0.5V
TO 0.5V
REF
REF
–
IN
1, 7, 8, 9, 10, 15, 16
GND
C
EXT
V
CC
CS
GND
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 0
SDO
Hi-Z
Hi-Z
SCK
(INTERNAL)
CONVERSION
DATA OUTPUT
CONVERSION
SLEEP
2415 F14
Figure 14. Internal Serial Clock, Autostart Operation
sn2415 24151fs
24
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
7
66.6msandtheconversiontimeoftheLTC2413is146ms,
while the LTC2415-1 is 73ms. In systems where the SDO
pin is monitored for the end-of-conversion signal (SDO
goes low once the conversion is complete) these two
devices can be interchanged. In cases where SDO is not
monitored, a wait state is inserted between conversions,
thedurationofthiswaitstatemustbegreaterthan66.6ms
for the LTC2415, greater than 133ms for the LTC2410,
greater than 146ms for the LTC2413 and greater than
73ms for the LTC2415-1.
6
5
4
3
2
V
= 5V
CC
1
0
V
= 3V
CC
10000 100000
CAPACITANCE ON CS (pF)
1
10
100
1000
2415 F15
Figure 15. CS Capacitance vs tSAMPLE
PRESERVING THE CONVERTER ACCURACY
The LTC2415/LTC2415-1 are designed to reduce as much
as possible conversion result sensitivity to device
decoupling, PCB layout, antialiasing circuits, line fre-
quency perturbations and so on. Nevertheless, in order to
preserve the extreme accuracy capability of this part,
some simple precautions are desirable.
8
7
6
V
= 5V
CC
5
V
CC
= 3V
4
3
2
1
0
Digital Signal Levels
0
10
100
10000 100000
1000
The LTC2415/LTC2415-1 digital interface is easy to use.
Its digital inputs (FO, CS and SCK in External SCK mode of
operation)acceptstandardTTL/CMOSlogiclevelsandthe
internalhysteresisreceiverscantolerateedgeratesasslow
as 100µs. However, some considerations are required to
takeadvantageoftheexceptionalaccuracyandlowsupply
current of this converter.
CAPACITANCE ON CS (pF)
2415 F16
Figure 16. CS Capacitance vs Output Rate
300
250
V
V
= 5V
= 3V
CC
The digital output signals (SDO and SCK in Internal SCK
mode of operation) are less of a concern because they are
not generally active during conversion.
200
150
CC
100
50
0
While a digital input signal is in the range 0.5V to
(VCC – 0.5V), the CMOS input receiver draws additional
current from the power supply. It should be noted that,
when any one of the digital input signals (FO, CS and SCK
inExternalSCKmodeofoperation)iswithinthisrange,the
LTC2415/LTC2415-1 power supply current may increase
even if the signal in question is at a valid logic level. For
micropower operation, it is recommended to drive all
digital input signals to full CMOS levels [VIL < 0.4V and
VOH > (VCC – 0.4V)].
1
10
100
1000
10000 100000
CAPACITANCE ON CS (pF)
2415 F17
Figure 17. CS Capacitance vs Supply Current
Timing Compatibility with the LTC2410/LTC2413
All timing modes described above are identical with re-
specttotheLTC2410/LTC2413andLTC2415/LTC2415-1,
with one exception. The conversion time of the LTC2410
is 133ms while the conversion time of the LTC2415 is
During the conversion period, the undershoot and/or
overshootofafastdigitalsignalconnectedtotheLTC2415/
sn2415 24151fs
25
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
LTC2415-1pinsmayseverelydisturbtheanalogtodigital
conversion process. Undershoot and overshoot can oc-
cur because of the impedance mismatch at the converter
pin when the transition time of an external control signal
is less than twice the propagation delay from the driver to
LTC2415/LTC2415-1. For reference, on a regular FR-4
board, signal propagation velocity is approximately
183ps/inch for internal traces and 170ps/inch for surface
traces. Thus, a driver generating a control signal with a
minimum transition time of 1ns must be connected to the
converterpinthroughatraceshorterthan2.5inches.This
problem becomes particularly difficult when shared con-
trollinesareusedandmultiplereflectionsmayoccur. The
solution is to carefully terminate all transmission lines
close to their characteristic impedance.
nals. When the FO signal is parallel terminated near the
converter, substantial AC current is flowing in the loop
formedbytheFO connectiontrace, theterminationandthe
ground return path. Thus, perturbation signals may be
inductively coupled into the converter input and/or refer-
ence. In this situation, the user must reduce to a minimum
the loop area for the FO signal as well as the loop area for
the differential input and reference connections.
Driving the Input and Reference
The input and reference pins of the LTC2415/LTC2415-1
convertersaredirectlyconnectedtoanetworkofsampling
capacitors. Depending upon the relation between the
differential input voltage and the differential reference
voltage, these capacitors are switching between these
four pins transferring small amounts of charge in the
process. A simplified equivalent circuit is shown in
Figure 18.
Parallel termination near the LTC2415/LTC2415-1 pins
will eliminate this problem but will increase the driver
powerdissipation.Aseriesresistorbetween27Ωand56Ω
placed near the driver or near the LTC2415/LTC2415-1
pins will also eliminate this problem without additional
powerdissipation. Theactualresistorvaluedependsupon
the trace impedance and connection topology.
For a simple approximation, the source impedance RS
driving an analog input pin (IN+, IN–, REF+ or REF–) can be
considered to form, together with RSW and CEQ (see
Figure 18), a first order passive network with a time
constant τ = (RS + RSW) • CEQ. The converter is able to
sample the input signal with better than 1ppm accuracy if
the sampling period is at least 14 times greater than the
input circuit time constant τ. The sampling process on the
four input analog pins is quasi-independent so each time
constant should be considered by itself and, under worst-
case circumstances, the errors may add.
An alternate solution is to reduce the edge rate of the
control signals. It should be noted that using very slow
edges will increase the converter power supply current
during the transition time. The multiple ground pins used
in this package configuration, as well as the differential
input and reference architecture, reduce substantially the
converter’s sensitivity to ground currents.
Whenusingtheinternaloscillator(FO =LOWorHIGH), the
LTC2415’sfront-endswitched-capacitornetworkisclocked
at 76800Hz corresponding to a 13µs sampling period and
the LTC2415-1’s front end is clocked at 69900Hz corre-
sponding to 14.2µs. Thus, for settling errors of less than
1ppm, the driving source impedance should be chosen
such that τ ≤ 13µs/14 = 920ns (LTC2415) and τ <14.2µs/
14 = 1.01µs (LTC2415-1).. When an external oscillator of
frequency fEOSC is used, the sampling period is 2/fEOSC
Particular attention must be given to the connection of the
FO signal when the LTC2415/LTC2415-1 are used with an
external conversion clock. This clock is active during the
conversion time and the normal mode rejection provided
by the internal digital filter is not very high at this fre-
quency. A normal mode signal of this frequency at the
converter reference terminals may result into DC gain and
INL errors. A normal mode signal of this frequency at the
converterinputterminalsmayresultintoaDCoffseterror.
Such perturbations may occur due to asymmetric capaci-
tivecouplingbetweentheFO signaltraceandtheconverter
input and/or reference connection traces. An immediate
solution is to maintain maximum possible separation
between the FO signal trace and the input/reference sig-
and, for a settling error of less than 1ppm, τ ≤ 0.14/fEOSC
.
Input Current
If complete settling occurs on the input, conversion re-
sults will be unaffected by the dynamic input current. An
sn2415 24151fs
26
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
V
CC
V
IN + VINCM − VREFCM
I IN+
=
=
I
+
+
(
)
)
REF
AVG
AVG
0.5• REQ
−VIN + VINCM − VREFCM
0.5• REQ
R
R
(TYP)
SW
I
I
LEAK
20k
I IN−
(
V
REF
V2
LEAK
1.5• VREF − VINCM + VREFCM
IN
I REF+
=
−
(
)
V
AVG
0.5• REQ
V
REF • REQ
CC
I
IN
+
V2
(TYP)
20k
−1.5• VREF − VINCM + VREFCM
0.5• REQ
SW
IN
I REF−
=
+
I
I
LEAK
(
)
AVG
V
REF • REQ
V
+
IN
where:
C
EQ
LEAK
VREF = REF+ −REF−
18pF
(TYP)
V
CC
REF+ + REF−
I
–
–
VREFCM
=
IN
R
R
(TYP)
2
SW
I
I
LEAK
20k
SWITCHING FREQUENCY
V
IN = IN+ −IN−
V
IN
f
= 76800Hz INTERNAL
SW
IN+ − IN−
OSCILLATOR (LTC2415)
LEAK
V
INCM
=
(F = LOW OR HIGH)
2
O
V
CC
f
= 69900Hz INTERNAL
SW
REQ = 3.61MΩ INTERNAL OSCILLATOR 60Hz Notch F = LOW LTC2415
I
–
–
(
)
O
REF
OSCILLATOR (LTC2415-1)
(TYP)
20k
SW
I
I
LEAK
REQ = 4.32MΩ INTERNAL OSCILLATOR 50Hz Notch F = HIGH LTC2415
(F = LOW)
(
)
O
O
f
= 0.5 • f
EXTERNAL OSCILLATOR
EOSC
SW
REQ = 0.555• 1012 / fEOSC EXTERNAL OSCILLATOR
V
REF
(
)
LEAK
REQ = 3.97MΩ INTERNAL OSCILLATOR 50Hz /60Hz Notch F = LOW LTC2415-1
(
)
O
2415 F18
Figure 18. LTC2415/LTC2415-1 Equivalent Analog Input Circuit
50
incomplete settling of the input signal sampling process
may result in gain and offset errors, but it will not degrade
theINLperformanceoftheconverter. Figure18showsthe
mathematical expressions for the average bias currents
flowing through the IN+ and IN– pins as a result of the
sampling charge transfers when integrated over a sub-
stantial time period (longer than 64 internal clock cycles).
C
= 0.01µF
IN
C
= 0.001µF
IN
C
40
30
20
10
0
= 100pF
IN
C
= 0pF
IN
V
= 5V
CC
+
–
REF = 5V
REF = GND
+
–
IN = 5V
IN = 2.5V
The effect of this input dynamic current can be analyzed
using the test circuit of Figure 19. The CPAR capacitor
includes the LTC2415/LTC2415-1 pin capacitance (5pF
typical) plus the capacitance of the test fixture used to
obtain the results shown in Figures 20 and 21. A careful
implementation can bring the total input capacitance (CIN
+ CPAR) closer to 5pF thus achieving better performance
than the one predicted by Figures 20 and 21. For simplic-
ity, two distinct situations can be considered.
F
= GND
O
A
T
= 25°C
1
10
100
1k
(Ω)
10k
100k
R
SOURCE
2415 F20
Figure 20. +FS Error vs RSOURCE at IN+ or IN– (Small CIN)
0
V
= 5V
CC
+
–
REF = 5V
REF = GND
–10
–20
–30
–40
–50
+
–
IN = GND
R
IN = 2.5V
SOURCE
+
F
= GND
IN
O
A
T
= 25°C
C
PAR
20pF
V
+ 0.5V
C
INCM
IN
IN
LTC2415/
LTC2415-1
C
= 0.01µF
IN
C
= 0.001µF
IN
C
R
SOURCE
–
IN
= 100pF
IN
2415 F19
C
= 0pF
IN
C
PAR
20pF
V
– 0.5V
C
INCM
IN
IN
1
10
100
1k
10k
100k
R
(Ω)
SOURCE
2415 F21
Figure 21. –FS Error vs RSOURCE at IN+ or IN– (Small CIN)
Figure 19. An RC Network at IN+ and IN–
sn2415 24151fs
27
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
For relatively small values of input capacitance (CIN
<
In addition to this gain error, an offset error term may also
appear. The offset error is proportional to the mismatch
between the source impedance driving the two input pins
IN+ and IN– and with the difference between the input and
reference common mode voltages. While the input drive
circuit nonzero source impedance combined with the
converter average input current will not degrade the INL
performance,indirectdistortionmayresultfromthemodu-
lation of the offset error by the common mode component
of the input signal. Thus, when using large CIN capacitor
values, itisadvisabletocarefullymatchthesourceimped-
ance seen by the IN+ and IN– pins. When FO = LOW
(internal oscillator and 60Hz notch), every 1Ω mismatch
in source impedance transforms a full-scale common
mode input signal into a differential mode input signal of
0.28ppm. When FO = HIGH (internal oscillator and 50Hz
notch), every 1Ω mismatch in source impedance trans-
forms a full-scale common mode input signal into a
differential mode input signal of 0.23ppm. When FO is
0.01µF), the voltage on the sampling capacitor settles
almost completely and relatively large values for the
source impedance result in only small errors. Such values
for CIN will deteriorate the converter offset and gain
performancewithoutsignificantbenefitsofsignalfiltering
and the user is advised to avoid them. Nevertheless, when
small values of CIN are unavoidably present as parasitics
of input multiplexers, wires, connectors or sensors, the
LTC2415/LTC2415-1canmaintaintheirexceptionalaccu-
racy while operating with relative large values of source
resistance as shown in Figures 20 and 21. These mea-
sured results may be slightly different from the first order
approximation suggested earlier because they include the
effect of the actual second order input network together
with the nonlinear settling process of the input amplifiers.
For small CIN values, the settling on IN+ and IN– occurs
almost independently and there is little benefit in trying to
match the source impedance for the two pins.
driven by an external oscillator with a frequency fEOSC
,
Larger values of input capacitors (CIN > 0.01µF) may be
required in certain configurations for antialiasing or gen-
eral input signal filtering. Such capacitors will average the
input sampling charge and the external source resistance
will see a quasi constant input differential impedance.
When FO = LOW (internal oscillator and 60Hz notch), the
typical differential input resistance is 1.8MΩ (LTC2415),
1.97MΩ (LTC2415-1) which will generate a gain error of
approximately 0.28ppm for each ohm of source resis-
tancedrivingIN+ orIN–. FortheLTC2415, whenFO =HIGH
(internaloscillatorand50Hznotch), thetypicaldifferential
input resistance is 2.16MΩ which will generate a gain
error of approximately 0.23ppm for each ohm of source
resistance driving IN+ or IN–. When FO is driven by an
externaloscillatorwithafrequencyfEOSC (externalconver-
sion clock operation), the typical differential input resis-
tance is 0.28 • 1012/fEOSCΩ and each ohm of
source resistance driving IN+ or IN– will result in
1.78 • 10–6 • fEOSCppm gain error. The effect of the source
resistance on the two input pins is additive with respect to
thisgainerror.Thetypical+FSand–FSerrorsasafunction
of the sum of the source resistance seen by IN+ and IN– for
large values of CIN are shown in Figures 22 and 23.
every 1Ω mismatch in source impedance transforms a
full-scale common mode input signal into a differential
mode input signal of 1.78 • 10–6 • fEOSCppm. Figure 24
shows the typical offset error due to input common mode
voltage for various values of source resistance imbalance
between the IN+ and IN– pins when large CIN values are
used.
If possible, it is desirable to operate with the input signal
common mode voltage very close to the reference signal
common mode voltage as is the case in the ratiometric
measurement of a symmetric bridge. This configuration
eliminates the offset error caused by mismatched source
impedances.
Themagnitudeofthedynamicinputcurrentdependsupon
thesizeoftheverystableinternalsamplingcapacitorsand
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typical better than 0.5%. Such
a specification can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
sn2415 24151fs
28
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
used for the external source impedance seen by IN+ and
IN–, the expected drift of the dynamic current, offset and
gain errors will be insignificant (about 1% of their respec-
tive values over the entire temperature and voltage range).
Even for the most stringent applications, a one-time
calibration operation may be sufficient.
In addition to the input sampling charge, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA max), results
inasmalloffsetshift. A100Ωsourceresistancewillcreate
a 0.1µV typical and 1µV maximum offset voltage.
300
0
V
= 5V
CC
C
= 1µF, 10µF
IN
+
C
= 0.01µF
= 0.1µF
IN
REF = 5V
–
REF = GND
240
180
120
60
–60
–120
–180
–240
–300
+
–
IN = 3.75V
IN = 1.25V
F
= GND
= 25°C
O
A
T
C
IN
= 0.1µF
C
IN
V
= 5V
CC
+
REF = 5V
–
REF = GND
+
–
IN = 1.25V
C
IN
= 0.01µF
IN = 3.75V
F
= GND
= 25°C
O
A
C
IN
= 1µF, 10µF
T
0
0
100 200 300 400 500 600 700 800 9001000
(Ω)
0
100 200 300 400 500 600 700 800 9001000
(Ω)
R
SOURCE
R
SOURCE
2415 F22
2415 F23
Figure 22. +FS Error vs RSOURCE at IN+ or IN– (Large CIN)
Figure 23. –FS Error vs RSOURCE at IN+ or IN– (Large CIN)
120
V
= 5V
CC
+
100
REF = 5V
A
–
REF = GND
IN = IN = V
80
+
–
INCM
60
B
40
C
20
D
0
E
–20
F
–40
–60
F
= GND
O
A
G
T
= 25°C
–80
–100
–120
R
C
– = 500Ω
SOURCEIN
= 10µF
IN
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
V
INCM
(V)
A: ∆R = +400Ω
E: ∆R = –100Ω
IN
IN
B: ∆R = +200Ω
F: ∆R = –200Ω
IN
IN
C: ∆R = +100Ω
G: ∆R = –400Ω
IN
IN
D: ∆R = 0Ω
2415 F24
IN
Figure 24. Offset Error vs Common Mode Voltage
(VINCM = IN+ = IN–) and Input Source Resistance Imbalance
(∆RIN = RSOURCEIN+ – RSOURCEIN–) for Large CIN Values (CIN ≥ 1µF)
sn2415 24151fs
29
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
Reference Current
filtering and the user is advised to avoid them.
In a similar fashion, the LTC2415/LTC2415-1 sample the
differential reference pins REF+ and REF– transferring
small amount of charge to and from the external driving
circuits thus producing a dynamic reference current. This
current does not change the converter offset, but it may
degrade the gain and INL performance. The effect of this
current can be analyzed in the same two distinct situa-
tions.
Larger values of reference capacitors (CREF > 0.01µF) may
be required as reference filters in certain configurations.
Suchcapacitorswillaveragethereferencesamplingcharge
and the external source resistance will see a quasi con-
stant reference differential impedance. For the LTC2415,
when FO = LOW (internal oscillator and 60Hz notch), the
typical differential reference resistance is 1.3MΩ which
will generate a gain error of approximately 0.38ppm for
each ohm of source resistance driving REF+ or REF–.
When FO = HIGH (internal oscillator and 50Hz notch), the
typical differential reference resistance is 1.56MΩ which
will generate a gain error of approximately 0.32ppm for
each ohm of source resistance driving REF+ or REF–. For
the LTC2415-1, the typical differential reference resis-
Forrelativelysmallvaluesoftheexternalreferencecapaci-
tors(CREF <0.01µF),thevoltageonthesamplingcapacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for CREF will deteriorate the converter offset and
gainperformancewithoutsignificantbenefitsofreference
0
50
C
= 0.01µF
REF
V
= 5V
CC
+
REF = 5V
C
= 0.001µF
REF
–
REF = GND
–10
–20
–30
–40
–50
40
30
20
10
0
+
–
IN = 5V
C
= 100pF
REF
IN = 2.5V
C
= 0pF
REF
F
= GND
= 25°C
O
A
T
V
= 5V
CC
+
–
REF = 5V
C
= 0.01µF
REF = GND
REF
+
–
IN = GND
C
= 0.001µF
REF
IN = 2.5V
F
= GND
O
C
= 100pF
REF
T
= 25°C
A
C
= 0pF
REF
1
10
100
R
1k
(Ω)
10k
100k
1
10
100
R
1k
10k
100k
(Ω)
SOURCE
SOURCE
2415 F25
2415 F26
Figure 25. +FS Error vs RSOURCE at REF+ or REF– (Small CIN)
Figure 26. –FS Error vs RSOURCE at REF+ or REF– (Small CIN)
0
450
C
REF
= 0.01µF
V
= 5V
CC
C
REF
= 1µF, 10µF
+
REF = 5V
–
–90
–180
–270
–360
–450
REF = GND
360
270
180
90
+
–
IN = 1.25V
IN = 3.75V
F
= GND
= 25°C
O
A
C
= 0.1µF
REF
T
C
= 0.1µF
REF
V
= 5V
CC
+
REF = 5V
–
REF = GND
+
–
IN = 3.75V
C
= 0.01µF
REF
IN = 1.25V
C
= 1µF, 10µF
F
= GND
= 25°C
REF
O
A
T
0
0
100 200 300 400 500 600 700 800 9001000
(Ω)
0
100 200 300 400 500 600 700 800 9001000
(Ω)
R
R
SOURCE
SOURCE
2415 F27
2415 F28
Figure 27. +FS Error vs RSOURCE at REF+ and REF– (Large CREF
)
Figure 28. –FS Error vs RSOURCE at REF+ and REF– (Large CREF
)
sn2415 24151fs
30
LTC2415/LTC2415-1
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APPLICATIO S I FOR ATIO
tance is 1.43MΩ. When FO is driven by an external
oscillator with a frequency fEOSC (external conversion
clock operation), the typical differential reference resis-
tance is 0.20 • 1012/fEOSCΩ and each ohm of source
resistance driving REF+ or REF– will result in
2.47 • 10–6 • fEOSCppm gain error. The effect of the source
resistance on the two reference pins is additive with
respect to this gain error. The typical +FS and –FS errors
for various combinations of source resistance seen by the
REF+ and REF– pins and external capacitance CREF
connected to these pins are shown in Figures 25, 26, 27
and 28.
large CREF values are used. The effect of the source
resistance on the two reference pins is additive with
respect to this INL error. In general, matching of source
impedance for the REF+ and REF– pins does not help the
gain or the INL error. The user is thus advised to minimize
the combined source impedance driving the REF+ and
REF– pins rather than to try to match it.
The magnitude of the dynamic reference current depends
upon the size of the very stable internal sampling capaci-
tors and upon the accuracy of the converter sampling
clock. The accuracy of the internal clock over the entire
temperature and power supply range is typical better than
0.5%. Such a specification can also be easily achieved by
an external clock. When relatively stable resistors
(50ppm/°C) are used for the external source impedance
seen by REF+ and REF–, the expected drift of the dynamic
current gain error will be insignificant (about 1% of its
valueovertheentiretemperatureandvoltagerange). Even
for the most stringent applications a one-time calibration
operation may be sufficient.
In addition to this gain error, the converter INL perfor-
mance is degraded by the reference source impedance.
WhenFO =LOW(internaloscillatorand60Hznotch),every
100ΩofsourceresistancedrivingREF+ orREF– translates
intoabout1.34ppmadditionalINLerror. FortheLTC2415,
whenFO =HIGH(internaloscillatorand50Hznotch),every
100ΩofsourceresistancedrivingREF+ orREF– translates
into about 1.1ppm additional INL error; and for the
LTC2415-1 operating with simultaneous 50Hz/60Hz re-
jection, every 100Ω of source resistance leads to an
additional 1.22ppm of additional INL error. When FO is
Inadditiontothereferencesamplingcharge,thereference
pinsESDprotectiondiodeshaveatemperaturedependent
leakage current. This leakage current, nominally 1nA
(±10nA max), results in a small gain error. A 100Ω source
resistance will create a 0.05µV typical and 0.5µV maxi-
mum full-scale error.
driven by an external oscillator with a frequency fEOSC
,
every 100Ω of source resistance driving REF+ or REF–
translatesintoabout8.73 •10–6 •fEOSCppmadditionalINL
error. Figure 26 shows the typical INL error due to the
source resistance driving the REF+ or REF– pins when
15
V
= 5V
CC
R
= 1000Ω
REF+ = 5V
12
9
SOURCE
REF– = GND
V
+
–
= 0.5 • (IN + IN ) = 2.5V
INCM
R
= 500Ω
SOURCE
F
= GND
O
6
C
= 10µF
REF
3
T
= 25°C
A
0
–3
–6
–9
–12
–15
R
= 100Ω
SOURCE
–0.5–0.4–0.3–0.2–0.1
0
/V
0.1 0.2 0.3 0.4 0.5
V
INDIF REFDIF
2415 F29
Figure 29. INL vs Differential Input Voltage (VIN = IN+ – IN–) and Reference
Source Resistance (RSOURCE at REF+ and REF– for Large CREF Values (CREF ≥ 1µF)
sn2415 24151fs
31
LTC2415/LTC2415-1
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Normal Mode Rejection, Output Rate and Running
Averages
running average can be performed. By averaging two
consecutiveADCreadings,aSinc1 notchiscombinedwith
the Sinc4 digital filter yielding the frequency response
shown in Figures 33 and 34. In order to preserve the 2×
output rate, adjacent results are averaged with the follow-
ing algorithm:
The LTC2415/LTC2415-1 both contain an identical Sinc4
digital filter (see Figures 30 and 31) which offers excellent
line frequency noise rejection. For the LTC2415, a notch
frequency of either 50Hz or 60Hz (see Figure 32) is user
selectablebytyingpinFO highorLow, respectively. Onthe
other hand, the LTC2415-1 offers simultaneous rejection
of 50Hz and 60Hz by tying FO low. This sets the notch
frequency to approximately 55Hz (see Figure 32).
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
Result 3 = average (sample 2, sample 3)
…
At a notch frequency of 55Hz, the LTC2415-1 rejects 50Hz
±2% and 60Hz ±2% better than 72dB. In order to achieve
better than 87dB rejection of both 50Hz and 60Hz ±2%, a
Result N = average (sample n-1, sample n)
0
0
–20
–40
–60
–70
V
V
V
= 5V
CC
= 5V
REF
–20
= 2.5V
IN
= 0
F
O
–80
–40
–60
–90
–60
–80
–100
–110
–120
–130
–140
–80
–100
–120
–100
–120
–140
1
50
100
150
IN
200
250
0
f /2
S
f
–12
–8
–4
0
4
8
12
S
FREQUENCY AT V (Hz)
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
INPUT FREQUENCY
2415 F30
2415 F31
2415 F32
Figure 30. Rejection vs Frequency at VIN
Figure 32. Rejection vs Frequency at VIN
Figure 31. Rejection vs Frequency at VIN
–80
–90
0
–20
–100
–100
–120
–130
–140
–40
–60
–80
–100
–120
48
50
52
54
56
58
60
62
0
20
40
60
80
100
120
140
160
180
200
220
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
2415 F34
2415 F33
Figure 33. Normal Mode Rejection
when Using an Internal Oscillator
Figure 34. Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 100% of Full Scale
sn2415 24151fs
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LTC2415/LTC2415-1
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Sample Driver for LTC2415/LTC2415-1 SPI Interface
The code begins by declaring variables and allocating four
memory locations to store the 32-bit conversion result.
ThisisfollowedbyinitializingPORTD’sSPIconfiguration.
The program then enters the main sequence. It activates
the LTC2415/LTC2415-1 serial interface by setting the SS
outputlow,sendingalogiclowtoCS.Itnextwaitsinaloop
for a logic low on the data line, signifying end-of-conver-
sion. After the loop is satisfied, four SPI transfers are
completed, retrieving the conversion. The main sequence
ends by setting SS high. This places the LTC2415/
LTC2415-1 serial interface in a high impedance state and
initiates another conversion.
Figure 35 shows the use of an LTC2415/LTC2415-1 with
a differential multiplexer. This is an inexpensive multi-
plexer that will contribute some error due to leakage if
used directly with the output from the bridge, or if
resistors are inserted as a protection mechanism from
overvoltage.Althoughthebridgeoutputmaybewithinthe
input range of the A/D and multiplexer in normal opera-
tion, some thought should be given to fault conditions
that could result in full excitation voltage at the inputs to
the multiplexer or ADC. The use of amplification prior to
the multiplexer will largely eliminate errors associated
with channel leakage developing error voltages in the
source impedance.
The performance of the LTC2415/LTC2415-1 can be
verified using the demonstration board DC291A, see
Figure 40 for the schematic. This circuit uses the
computer’s serial port to generate power and the SPI
digital signals necessary for starting a conversion and
reading the result. It includes a Labview application
software program (see Figure 39) which graphically cap-
tures the conversion results. It can be used to determine
noise performance, stability and with an external source,
linearity. As exemplified in the schematic, the LTC2415/
LTC2415-1 are extremely easy to use. This demonstra-
tion board and associated software is available by con-
tacting Linear Technology.
The LTC2415/LTC2415-1 have a very simple serial inter-
face that makes interfacing to microprocessors and
microcontrollers very easy.
The listing in Figure 38 is a simple assembler routine for
the68HC11microcontroller.ItusesPORTD,configuring
it for SPI data transfer between the controller and the
LTC2415/LTC2415-1. Figure 36 shows the simple 3-wire
SPI connection.
5V
5V
+
16
2
47µF
12
14
15
11
V
CC
3
4
+
–
REF
REF
LTC2415/
LTC2415-1
74HC4052
1
5
5
6
13
3
+
IN
–
IN
2
4
TO OTHER
DEVICES
GND
1, 7, 8, 9,
10, 15, 16
6
8
9
10
A0
A1
2415 F35
Figure 35. Use a Differential Multiplexer to Expand Channel Capability
68HC11
13
12
11
SCK
SDO
CS
SCK (PD4)
MISO (PD2)
SS (PD5)
LTC2415/
LTC2415-1
2415 F36
Figure 36. Connecting the LTC2415/LTC2415-1 to a 68HC11 MCU Using the SPI Serial Interface
sn2415 24151fs
33
LTC2415/LTC2415-1
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Correlated Double Sampling with the
LTC2415/LTC2415-1
tors above 10MHz. The conversion spikes that remain at
the output of other bipolar amplifiers pass through the
feedback network and often overdrive the input of the
amplifier, producing envelope detection. RFI may also be
present on the signal lines from the bridge; C3 and C4
provide RFI suppression at the signal input, as well as
suppressing transient voltages during bridge commuta-
tion.
Figure 37 shows the LTC2415/LTC2415-1 in a correlated
double sampling circuit that achieves a noise floor of
under 100nV. In this scheme, the polarity of the bridge is
alternatedeveryothersampleandtheresultistheaverage
of a pair of samples of opposite sign. This technique has
the benefit of canceling any fixed DC error components in
the bridge, amplifiers and the converter, as these will
alternate in polarity relative to the signal. Offset voltages
and currents, thermocouple voltages at junctions of dis-
similarmetalsandthelowerfrequencycomponentsof1/f
noise are virtually eliminated.
The wideband noise density of the LT1219 is 33nV√Hz,
seemingly much noisier than the lowest noise amplifiers.
However, in the region just below the 1/f corner that is not
well suppressed by the correlated double sampling, the
average noise density is similar to the noise density of
many low noise amplifiers. If the amplifier is rolled off
below about 1500Hz, the total noise bandwidth is deter-
mined by the converter’s Sinc4 filter at about 12Hz. The
use of correlated double sampling involves averaging
even numbers of samples; hence, in this situation, two
samples would be averaged to give an input-referred
The LTC2415/LTC2415-1 have the virtue of being able to
digitize an input voltage that is outside the range defined
by the reference, thereby providing a simple means to
implement a ratiometric example of correlated double
sampling.
Thiscircuitusesabipolaramplifier(LT1219—U1andU2)
that has neither the lowest noise nor the highest gain. It
does, however, have an output stage that can effectively
suppress the conversion spikes from the LTC2415/
LTC2415-1. The LT1219 is a C-LoadTM stable amplifier
that, bydesign, needsatleast0.1µFoutputcapacitanceto
remain stable. The 0.1µF ceramic capacitors at the out-
puts(C1andC2)shouldbeplacedandroutedtominimize
lead inductance or their effectiveness in preventing enve-
lope detection in the input stage will be reduced. Alterna-
tively, several smaller capacitors could be placed so that
leadinductanceisfurtherreduced. Thisisaconsideration
because the frequency content of the conversion spikes
extends to 50MHz or more. The output impedance of
most op amps increases dramatically with frequency but
the effective output impedance of the LT1219 remains
low, determined by the ESR and inductance of the capaci-
noise level of about 100nVRMS
.
Level shift transistors Q4 and Q5 are included to allow
excitation voltages up to the maximum recommended for
the bridge. In the case shown, if a 10V supply is used, the
excitation voltage to the bridge is 8.5V and the outputs of
the bridge are above the supply rail of the ADC. U1 and U2
are also used to produce a level shift to bring the outputs
within the input range of the converter. This instrumenta-
tion amplifier topology does not require well-matched
resistors in order to produce good CMRR. However, the
use of R2 requires that R3 and R6 match well, as the
commonmodegainisapproximately–12dB. Ifthebridge
is composed of four equal 350Ω resistors, the differential
component associated with mismatch of R3 and R6 is
nearly constant with either polarity of excitation and, as
with offset, its contribution is canceled.
C-Load is a trademark of Linear Technology Corporation.
sn2415 24151fs
34
LTC2415/LTC2415-1
U
W U U
APPLICATIO S I FOR ATIO
10V
ELIMINATE FOR 5V
1.5k
1.5k
DIFFERENCE
AMP
OPERATION (CONNECT 2.7k
RESISTORS TO 100Ω
RESISTORS)
10V
0.1µf
Q2
Q3
100Ω
7
3
2
+
–
100Ω
5k
U1
LT1219
6
R2
27k
22Ω
5
C1
0.1µF
22Ω
4
SHDN
5V
Q4
1k
Q5
5V
5V
R4
499Ω
R3 10k
1000pF
1000pF
2.7k
2.7k
5
6
3
4
+
–
C3 2.2nF
C4 2.2nF
IN
IN
350Ω
×4
R5
499Ω
R6 10k
10V
LTC2415/
LTC2415-1
+
POL
REF
REF
0.1µf
1k
74HC04
–
+
7
2
3
–
5k
U2
6
LT1219
Q1
GND
5
C2
22Ω
4
0.1µF
SHDN
R1
61.9Ω
0.1%
33Ω
100Ω
SILICONIX Si9802DY (800) 554-5565
MMBD2907
MMBD3904
Q1:
Q2, Q3:
Q4, Q5:
30pF
30pF
22Ω
2415 F37
Figure 37. Correlated Double Sampling Resolves 100nV
sn2415 24151fs
35
LTC2415/LTC2415-1
U
TYPICAL APPLICATIO S
************************************************************
* This example program transfers the LTC2415/LTC2415-1 32-bit output *
* conversion result into four consecutive 8-bit memory locations.
*
************************************************************
*68HC11 register definition
PORTD EQU
$1008
Port D data register
" – , – , SS* ,CSK ;MOSI,MISO,TxD ,RxD"
Port D data direction register
SPI control register
"SPIE,SPE ,DWOM,MSTR;SPOL,CPHA,SPR1,SPR0"
SPI status register
*
DDRD
SPSR
*
EQU
EQU
$1009
$1028
SPSR
*
SPDR
*
EQU
EQU
$1029
$102A
"SPIF,WCOL, – ,MODF; – , – , – , – "
SPI data register; Read-Buffer; Write-Shifter
* RAM variables to hold the LTC2415/LTC2415-1’s 32 conversion result
*
DIN1
DIN2
DIN3
DIN4
*
EQU
EQU
EQU
EQU
$00
$01
$02
$03
This memory location holds the LTC2415/LTC2415-1’s bits 31 - 24
This memory location holds the LTC2415/LTC2415-1’s bits 23 - 16
This memory location holds the LTC2415/LTC2415-1’s bits 15 - 08
This memory location holds the LTC2415/LTC2415-1’s bits 07 - 00
**********************
* Start GETDATA Routine *
**********************
*
ORG
LDS
$C000
Program start location
INIT1
*
#$CFFF Top of C page RAM, beginning location of stack
#$2F
LDAA
–,–,1,0;1,1,1,1
–, –, SS*-Hi, SCK-Lo, MOSI-Hi, MISO-Hi, X, X
STAA
LDAA
STAA
PORTD Keeps SS* a logic high when DDRD, bit 5 is set
#$38
–,–,1,1;1,0,0,0
SS*, SCK, MOSI are configured as Outputs
MISO, TxD, RxD are configured as Inputs
DDRD
*
*DDRD’s bit 5 is a 1 so that port D’s SS* pin is a general output
LDAA
STAA
#$50
SPCR
The SPI is configured as Master, CPHA = 0, CPOL = 0
and the clock rate is E/2
(This assumes an E-Clock frequency of 4MHz. For higher E-
Clock frequencies, change the above value of $50 to a value
that ensures the SCK frequency is 2MHz or less.)
*
*
*
*
GETDATA PSHX
PSHY
PSHA
LDX
#$0
The X register is used as a pointer to the memory locations
that hold the conversion data
*
LDY
#$1000
BCLR
PORTD, Y %00100000
This sets the SS* output bit to a logic
low, selecting the LTC2415/LTC2415-1
*
*
sn2415 24151fs
36
LTC2415/LTC2415-1
U
TYPICAL APPLICATIO S
********************************************
* The next short loop waits for the
* LTC2415/LTC2415-1’s conversion to finish before
* starting the SPI data transfer
*
*
*
********************************************
*
CONVEND LDAA
PORTD
#%00000100
Retrieve the contents of port D
Look at bit 2
ANDA
*
*
Bit 2 = Hi; the LTC2415/LTC2415-1’s conversion is not
complete
*
Bit 2 = Lo; the LTC2415/LTC2415-1’s conversion is complete
Branch to the loop’s beginning while bit 2 remains
high
BNE
CONVEND
*
*
********************
* The SPI data transfer *
********************
*
TRFLP1 LDAA
#$0
SPDR
Load accumulator A with a null byte for SPI transfer
This writes the byte in the SPI data register and starts
the transfer
STAA
*
WAIT1
LDAA
BPL
SPSR
This loop waits for the SPI to complete a serial
transfer/exchange by reading the SPI Status Register
The SPIF (SPI transfer complete flag) bit is the SPSR’s MSB
and is set to one at the end of an SPI transfer. The branch
will occur while SPIF is a zero.
WAIT1
*
*
LDAA
SPDR
0,X
Load accumulator A with the current byte of LTC2415/LTC2415-1 data
that was just received
Transfer the LTC2415/LTC2415-1’s data to memory
STAA
INX
Increment the pointer
CPX
BNE
#DIN4+1 Has the last byte been transferred/exchanged?
TRFLP1 If the last byte has not been reached, then proceed to the
next byte for transfer/exchange
PORTD,Y %00100000 This sets the SS* output bit to a logic high,
de-selecting the LTC2415/LTC2415-1
Restore the A register
*
*
BSET
PULA
PULY
PULX
RTS
Restore the Y register
Restore the X register
Figure 38. This is an Example of 68HC11 Code That Captures the LTC2415/LTC2415-1
Conversion Results Over the SPI Serial Interface Shown in Figure 40
sn2415 24151fs
37
LTC2415/LTC2415-1
U
TYPICAL APPLICATIO S
Figure 39. Display Graphic
U
W
PCB LAYOUT A D FIL
LTC2415CGN
Differential Input 24-Bit ADC
with 2× Output Rate
Demo Circuit DC382
www.linear-tech.com
LTC Confidential For Customer Use Only
Silkscreen Top
Top Layer
sn2415 24151fs
38
LTC2415/LTC2415-1
U
W
PCB LAYOUT A D FIL
Bottom Layer
U
PACKAGE DESCRIPTIO
GN Package
16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
0.189 – 0.196*
(4.801 – 4.978)
0.009
(0.229)
0.015 ± 0.004
(0.38 ± 0.10)
16 15 14 13 12 11 10 9
REF
× 45° 0.053 – 0.068
0.004 – 0.0098
(0.102 – 0.249)
(1.351 – 1.727)
0.007 – 0.0098
(0.178 – 0.249)
0° – 8° TYP
0.229 – 0.244
(5.817 – 6.198)
0.150 – 0.157**
(3.810 – 3.988)
0.016 – 0.050
(0.406 – 1.270)
0.0250
(0.635)
BSC
0.008 – 0.012
(0.203 – 0.305)
* DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
GN16 (SSOP) 1098
1
2
3
4
5
6
7
8
sn2415 24151fs
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
39
LTC2415/LTC2415-1
U
TYPICAL APPLICATIO
D1
BAV74LT1
2
U1
U2
V
CC
JP1
V
JP2
1
1
J1
EXT
CC
LT1460ACN8-2.5
LT1236ACN8-5
R1
JUMPER
JUMPER
V
10Ω
1
3
1
2
6
2
6
2
3
1
V
V
IN
V
V
IN
OUT
GND
OUT
GND
J2
GND
2
+
+
+
+
C1
C2
C3
C4
4
4
10µF
35V
22µF
10µF
100µF
25V
35V
16V
P1
DB9
R2
3Ω
1
6
2
7
3
8
4
9
5
JP3
JUMPER
U3E
U3F
74HC14
74HC14
1
3
R3
51k
10
11
12
13
2
JP4
V
CC
JUMPER
1
3
1
1
J3
CC
V
+
C5
2
BANANA JACK
C6
10µF
35V
1
U3B
74HC14
U3A
74HC14
J4
0.1µF
J5
GND
V
EXT
R4
51k
2
11
CS
4
5
3
6
2
9
1
8
BANANA JACK
V
CC
1
3
14
13
12
16
15
10
J6
+
–
+
REF
F
O
REF
4
5
6
REF
SCK
SDO
GND
GND
GND
U3C
74HC14
U3D
74HC14
BANANA JACK
R5
R6
3k
V
V
+
IN
IN
1
J7
49.9Ω
–
–
U4
REF
LTC2415/
LTC2415-1
1
BANANA JACK
R7
22k
1
J8
IN
3
2
GND GND GND GND
V
+
Q1
MMBT3904LT1
1
7
8
9
BANANA JACK
R8
51k
1
2
1
J9
IN
JP5
JUMPER
V
–
V
CC
BANANA JACK
NOTES:
1
J10
GND
BYPASS CAP
FOR U3
C7
0.1µF
INSTALL JUMBER JP1 AT PIN 1 AND PIN 2
INSTALL JUMBER JP2 AT PIN 1 AND PIN 2
INSTALL JUMBER JP3 AT PIN 1 AND PIN 2
2415 F40
Figure 40. 24-Bit A/D Demo Board Schematic
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1019
Precision Bandgap Reference, 2.5V, 5V
3ppm/°C Drift, 0.05% Max Initial Accuracy
80µA Supply Current, 0.5°C Initial Accuracy
LT1025
Micropower Thermocouple Cold Junction Compensator
LTC1043
Dual Precision Instrumentation Switched Capacitor
Building Block
Precise Charge, Balanced Switching, Low Power
LTC1050
LT1236A-5
LT1460
Precision Chopper Stabilized Op Amp
Precision Bandgap Reference, 5V
Micropower Series Reference
No External Components 5µV Offset, 1.6µV Noise
P-P
0.05% Max Initial Accuracy, 5ppm/°C Drift
0.075% Max Initial Accuracy, 10ppm/°C Max Drift,
0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
0.6ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2400
24-Bit, No Latency ∆Σ ADC in SO-8
LTC2401/LTC2402 1-/2-Channel, 24-Bit, No Latency ∆Σ ADCs in MSOP
LTC2414/LTC2418 4-/8-Channel, 24-Bit, No Latency ∆Σ ADCs with Differential Inputs 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2410
LTC2411
LTC2413
LTC2420
24-Bit, No Latency ∆Σ ADC with Differential Inputs
24-Bit, No Latency ∆Σ ADC with Differential Inputs in MSOP
24-Bit, No Latency ∆Σ ADC with Differential Inputs
20-Bit, No Latency ∆Σ ADC in SO-8
800nV
Noise, Pin Compatible with LTC2415
Noise, 4ppm INL
RMS
1.45µV
RMS
Simultaneous 50Hz/60Hz Rejection, 800nV
Noise
RMS
1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400
sn2415 24151fs
LT/TP 0202 2K • PRINTED IN USA
40 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2001
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