LTC2400CS8#PBF [Linear]
LTC2400 - 24-Bit µPower No Latency Delta-Sigma ADC in SO-8; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C;
| 型号： | LTC2400CS8#PBF |
| 厂家： | 
Linear |
| 描述： | LTC2400 - 24-Bit µPower No Latency Delta-Sigma ADC in SO-8; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C 转换器 模数转换器 光电二极管 |
| 文件： | 总40页 (文件大小：457K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC2400
24-Bit µPower
No Latency ∆ΣTM ADC in SO-8
U
FEATURES
DESCRIPTIO
■
24-Bit ADC in SO-8 Package
The LTC®2400 is a 2.7V to 5.5V micropower 24-bit
converter with an integrated oscillator, 4ppm INL and
0.3ppm RMS noise. It uses delta-sigma technology and
provides single cycle settling time for multiplexed appli-
cations. Through a single pin the LTC2400 can be config-
uredforbetterthan110dBrejectionat50Hzor60Hz ±2%,
or it can be driven by an external oscillator for a user
defined rejection frequency in the range 1Hz to 120Hz.
The internal oscillator requires no external frequency
setting components.
■
4ppm INL, No Missing Codes
■
4ppm Full-Scale Error
■
Single Conversion Settling Time
for Multiplexed Applications
■
0.5ppm Offset
0.3ppm Noise
■
■
Internal Oscillator—No External Components
Required
■
110dB Min, 50Hz/60Hz Notch Filter
■
Reference Input Voltage: 0.1V to VCC
The converter accepts any external reference voltage from
0.1V to VCC. With its extended input conversion range of
–12.5% VREF to 112.5% VREF, the LTC2400 smoothly
resolves the offset and overrange problems of preceding
sensors or signal conditioning circuits.
■
Live Zero—Extended Input Range Accommodates
12.5% Overrange and Underrange
Single Supply 2.7V to 5.5V Operation
■
■
Low Supply Current (200µA) and Auto Shutdown
U
The LTC2400 communicates 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.
APPLICATIO S
■
Weight Scales
■
Direct Temperature Measurement
■
Gas Analyzers
Strain-Gage Transducers
Instrumentation
Data Acquisition
Industrial Process Control
6-Digit DVMs
■
■
■
■
■
U
TYPICAL APPLICATIO
Total Unadjusted Error vs Output Code
10
V
V
= 5V
CC
2.7V TO 5.5V
8
6
= 5V
REF
V
CC
1µF
T
= 25°C
A
O
= INTERNAL OSC/50Hz REJECTION
F
= LOW
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/60Hz REJECTION
V
CC
F
O
4
2
LTC2400
REFERENCE
VOLTAGE
0
V
V
SCK
REF
–2
–4
–6
–8
–10
0.1V TO V
CC
3-WIRE
SPI INTERFACE
ANALOG
SDO
CS
INPUT RANGE
IN
–0.12V
TO 1.12V
REF
REF
GND
2400 TA01
0
8,338,608
OUTPUT CODE (DECIMAL)
16,777,215
2400 TA02
1
LTC2400
W W U W
U
W
U
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Notes 1, 2)
ORDER PART NUMBER
TOP VIEW
Supply Voltage (VCC) to GND.......................–0.3V to 7V
Analog Input Voltage to GND ....... –0.3V to (VCC + 0.3V)
Reference Input 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
LTC2400C ............................................... 0°C to 70°C
LTC2400I............................................ –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
V
1
2
3
4
8
7
6
5
F
O
CC
LTC2400CS8
LTC2400IS8
V
REF
SCK
SDO
CS
V
IN
GND
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
2400
2400I
TJMAX = 125°C, θJA = 130°C/W
Consult factory for Military grade parts.
U
CONVERTER CHARACTERISTICS The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
0.1V ≤ V ≤ V , (Note 5)
MIN
TYP
MAX
UNITS
Resolution (No Missing Codes)
Integral Nonlinearity
●
24
Bits
REF
CC
V
REF
V
REF
= 2.5V (Note 6)
= 5V (Note 6)
●
●
2
4
10
15
ppm of V
ppm of V
REF
REF
Offset Error
2.5V ≤ V ≤ V
●
0.5
0.01
4
2
ppm of V
REF
CC
CC
CC
CC
REF
Offset Error Drift
Full-Scale Error
2.5V ≤ V ≤ V
ppm of V /°C
REF
REF
2.5V ≤ V ≤ V
●
10
ppm of V
REF
REF
Full-Scale Error Drift
Total Unadjusted Error
2.5V ≤ V ≤ V
0.02
ppm of V /°C
REF
REF
V
REF
V
REF
= 2.5V
= 5V
5
10
ppm of V
ppm of V
REF
REF
Output Noise
V
= 0V (Note 13)
1.5
130
130
100
110
110
µV
RMS
IN
Normal Mode Rejection 60Hz ±2%
Normal Mode Rejection 50Hz ±2%
Power Supply Rejection, DC
Power Supply Rejection, 60Hz ±2%
Power Supply Rejection, 50Hz ±2%
(Note 7)
(Note 8)
●
●
110
110
dB
dB
dB
dB
dB
V
REF
V
REF
V
REF
= 2.5V, V = 0V
IN
= 2.5V, V = 0V, (Notes 7, 15)
IN
= 2.5V, V = 0V, (Notes 8, 15)
IN
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
–0.125 • V
0.1
TYP
MAX
UNITS
V
V
V
C
C
Input Voltage Range
(Note 14)
●
●
1.125 • V
REF
IN
REF
Reference Voltage Range
Input Sampling Capacitance
Reference Sampling Capacitance
Input Leakage Current
Reference Leakage Current
V
CC
V
REF
10
15
1
pF
S(IN)
pF
S(REF)
IN(LEAK)
REF(LEAK)
I
I
CS = V
●
●
–10
–10
10
10
nA
nA
CC
V
REF
= 2.5V, CS = V
1
CC
2
LTC2400
U
U
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
V
V
V
High Level Input Voltage
●
●
●
●
●
●
2.5
2.0
V
V
IH
IL
IH
IL
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
– 0.5V
– 0.5V
OH
OL
OH
OL
CC
CC
Low Level Output Voltage
SDO
I = 1.6mA
O
0.4V
V
High Level Output Voltage
SCK
I = –800µA (Note 10)
O
V
Low Level Output Voltage
SCK
I = 1.6mA (Note 10)
O
0.4V
10
V
I
High-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
●
2.7
5.5
V
CC
I
CC
Conversion Mode
Sleep Mode
CS = 0V (Note 12)
●
●
200
20
300
30
µA
µA
CS = V (Note 12)
CC
3
LTC2400
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
0.5
TYP
MAX
307.2
390
UNITS
kHz
µs
f
t
t
t
External Oscillator Frequency Range
External Oscillator High Period
External Oscillator Low Period
Conversion Time
●
●
●
EOSC
HEO
0.5
390
µs
LEO
F
= 0V
O
●
●
●
130.66
156.80
133.33
160
EOSC
136
163.20
(in kHz)
ms
ms
ms
CONV
F = V
O
CC
External Oscillator (Note 11)
20480/f
f
Internal SCK Frequency
Internal Oscillator (Note 10)
External Oscillator (Notes 10, 11)
19.2
kHz
kHz
ISCK
f
/8
EOSC
D
Internal SCK Duty Cycle
(Note 10)
(Note 9)
(Note 9)
(Note 9)
●
●
●
●
45
55
%
kHz
ns
ISCK
f
t
t
t
External SCK Frequency Range
External SCK Low Period
2000
ESCK
250
250
1.64
LESCK
External SCK High Period
ns
HESCK
DOUT_ISCK
Internal SCK 32-Bit Data Output Time
Internal Oscillator (Notes 10, 12)
External Oscillator (Notes 10, 11)
●
●
1.67
1.70
ms
ms
256/f
(in kHz)
EOSC
t
t
External SCK 32-Bit Data Output Time
CS ↓ to SDO Low Z
CS ↑ to SDO High Z
CS ↓ to SCK ↓
(Note 9)
●
●
●
●
●
●
●
●
●
32/f
(in kHz)
ms
ns
ns
ns
ns
ns
ns
ns
ns
DOUT_ESCK
1
ESCK
0
0
150
150
150
t2
t3
t4
(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 ↓
200
KQMAX
KQMIN
(Note 5)
15
50
t
t
5
6
50
Note 1: Absolute Maximum Ratings are those values beyond which the
life of the device may be impaired.
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 11: The external oscillator is connected to the FO pin. The external
oscillator frequency, fEOSC, is expressed in kHz.
Note 2: All voltage values are with respect to GND.
Note 3: VCC = 2.7 to 5.5V unless otherwise specified.
Note 4: Internal Conversion Clock source with the FO pin tied
to GND or to VCC or to external conversion clock source with
fEOSC = 153600Hz unless otherwise specified.
Note 12: The converter uses the internal oscillator.
FO = 0V or FO = VCC
.
Note 13: The output noise includes the contribution of the internal
calibration operations.
Note 14: For reference voltage values VREF > 2.5V the extended input
of –0.125 • VREF to 1.125 • VREF is limited by the absolute maximum
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.
Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz ±2%
(external oscillator).
Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz ±2%
(external oscillator).
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.
rating of the Analog Input Voltage pin (Pin 3). For 2.5V < VREF
≤
0.267V + 0.89 • VCC the input voltage range is –0.3V to 1.125 • VREF
For 0.267V + 0.89 • VCC < VREF ≤ VCC the input voltage range is –0.3V
to VCC + 0.3V.
Note 15: The DC voltage at VCC = 4.1V, and the AC voltage applied to
VCC is 2.8VP-P
.
4
LTC2400
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Total Unadjusted Error
(3V Supply)
Negative Input Extended Total
Unadjusted Error (3V Supply)
INL (3V Supply)
10
5
10
5
10
5
V
V
= 3V
= 3V
V
V
= 3V
= 3V
V
V
= 3V
= 3V
CC
REF
CC
REF
CC
REF
T
= 90°C
A
T
= –55°C, –45°C, 25°C, 90°C
A
T
= 25°C
A
0
0
0
T
T
= –45°C
= –55°C
A
A
T
A
= –55°C, –45°C, 25°C, 90°C
–5
–5
–5
–10
–10
–10
0
0.5
1.0
1.5
2.0
2.5
3.0
0
0.5
1.0
1.5
2.0
2.5
3.0
0
–0.05 –0.10 –0.15 –0.20 –0.25 –0.30
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
2400 G01
2400 G02
2400 G03
Total Unadjusted Error
(5V Supply)
Positive Input Extended Total
Unadjusted Error (3V Supply)
INL (5V Supply)
10
8
10
5
10
5
V
V
= 5V
= 5V
CC
REF
V
V
= 3V
REF
CC
= 3V
6
4
T
A
= –55°C
2
T
= –45°C
A
0
0
0
T
= –55°C, –45°C, 25°C, 90°C
A
–2
–4
–6
–8
–10
T
A
= 90°C
T = 25°C
A
–5
–5
–10
–10
0
1
2
3
4
5
0
1
2
3
4
5
3.0
3.1
3.2
3.3
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
2400 G05
2400 G06
2400 G04
Positive Input Extended Total
Unadjusted Error (5V Supply)
Negative Input Extended Total
Unadjusted Error (5V Supply)
Offset Error vs Reference Voltage
6
5
10
5
10
5
V
V
= 5V
= 5V
V
T
= 5V
V
V
= 5V
= 5V
CC
REF
CC
A
CC
REF
= 25°C
T
= 90°C
A
4
3
T
= –55°C
A
T
= 25°C
A
0
0
T
A
= –45°C
2
T
= –45°C
= –55°C
A
T
A
= 90°C
T = 25°C
A
1
–5
–5
T
A
0
–1
–10
–10
0
–0.05 –0.10 –0.15 –0.20 –0.25 –0.30
INPUT VOLTAGE (V)
5.0
5.1
5.2
5.3
0
1
2
3
4
5
INPUT VOLTAGE (V)
REFERENCE VOLTAGE
2400 G07
2400 G08
2400 G09
5
LTC2400
TYPICAL PERFOR A CE CHARACTERISTICS
U W
RMS Noise vs Reference Voltage
Offset Error vs VCC
RMS Noise vs VCC
5.0
2.5
20
15
10
5
5.0
2.5
0
V
T
= 2.5V
V
T
= 5V
V
A
= 2.5V
REF
REF
A
CC
A
= 25°C
= 25°C
T
= 25°C
0
–2.5
–5.0
0
0
1
2
3
4
5
2.7
3.2
3.7
4.2
4.7
5.2
2.7
3.2
3.7
4.2
4.7
5.2
V
CC
V
CC
REFERENCE VOLTAGE (V)
2400 G10
2400 G11
2400 G12
RMS Noise vs Code Out
Noise Histogram
Offset Error vs Temperature
1500
1000
500
0
5.0
2.5
1.00
0.75
0.50
0.25
0
V
V
V
= 5V
= 5V
V
V
V
= 5V
= 5V
V
V
V
T
= 5V
= 5V
CC
REF
IN
CC
REF
IN
CC
REF
IN
A
= 0V
= 0V
= –0.3V TO 5.3V
= 25°C
0
–2.5
–5.0
–1.0
–0.5
0
0.5
1.0
1.5
–55 –30 –5 20 45 70 95 120
0
7FFFFF
FFFFFF
TEMPERATURE (°C)
OUTPUT CODE (ppm)
CODE OUT (HEX)
2400 G14
2400 G13
2400 G18
Full-Scale Error
vs Reference Voltage
Full-Scale Error vs Temperature
Full-Scale Error vs VCC
5.0
2.5
10.0
7.5
5.0
2.5
0
6
V
V
V
= 5V
REF
= 5V
V
= 5V
REF
CC
CC
IN
= 5V
V
= V
5
4
3
2
1
0
IN
0
–2.5
V
V
A
= 2.5V
REF
IN
= 2.5V
T
= 25°C
–5.0
–55 –30 –5 20 45 70 95 120
0
1
2
3
4
5
2.7
3.2
3.7
4.2
4.7
5.2
TEMPERATURE (°C)
V
CC
REFERENCE VOLTAGE (V)
2400 G15
2400 G16
2400 G17
6
LTC2400
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Sleep Current vs Temperature
Conversion Current vs Temperature
PSRR vs Frequency at VCC
0
230
30
25
20
15
V
CC
V
IN
= 4.1V
= 0V
220
210
V
= 5.5V
= 4.1V
= 2.7V
CC
CC
–20
T
= 25°C
A
O
F
= 0
V
CC
= 2.7V, 5.5V
–40
–60
V
200
190
180
170
160
V
CC
–80
–100
–120
10
5
15,360Hz
153,600Hz
1M
0
150
1
100
10k
45
TEMPERATURE (°C)
95 120
–30
–5
45
70
95 120
–55 –30
–5
20
70
–55
20
FREQUENCY AT V (Hz)
TEMPERATURE (°C)
CC
2400 G23
2400 G20
2400 G19
PSRR vs Frequency at VCC
PSRR vs Frequency at VCC
Rejection vs Frequency at VIN
–10
–30
0
0
–20
V
V
T
= 4.1V
= 0V
V
V
V
F
= 5V
V
V
T
= 4.1V
= 0V
CC
IN
CC
CC
IN
= 5V
REF
–20
= 25°C
= 2.5V
= 25°C
A
0
IN
A
O
F
= 0
= 0
F
= 0
O
–50
–70
–40
–60
–40
–60
–90
–110
–130
–80
–100
–120
–80
–100
–120
0
50
100
150
200
250
1
50
100
150
200
250
15200
15300 15350 15400 15450 15500
15250
FREQUENCY AT V (Hz)
FREQUENCY AT V (Hz)
FREQUENCY AT V (Hz)
CC
CC
IN
2400 G21
2400 G24
1635 G22
Rejection vs Frequency at VIN
Rejection vs Frequency at VIN
Rejection vs Frequency at VIN
0
–60
–70
0
–20
–40
V
V
V
= 5V
CC
= 5V
REF
–20
= 2.5V
IN
F
= 0
O
–80
–40
–60
–90
–60
–80
–100
–110
–120
–130
–140
–80
–100
–120
–100
–120
–140
SAMPLE RATE = 15.36kHz ±2%
15100
15200
15300
15400
15500
–12
–8
–4
0
4
8
12
0
f /2
S
f
S
FREQUENCY AT V (Hz)
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
INPUT FREQUENCY
IN
2400 G26
2400 G25
2400 F26
7
LTC2400
U W
TYPICAL PERFOR A CE CHARACTERISTICS
INL vs Output Rate
Resolution vs Output Rate
24
22
20
18
16
14
12
10
8
24
22
20
18
16
14
12
10
8
V
V
= 5V
REF
= 25°C
V
V
= 5V
REF
= 25°C
CC
CC
= 5V
= 5V
T
T
A
0
A
O
F
= EXTERNAL
F
= EXTERNAL
LOG(V /RMS NOISE)
REF
*RESOLUTION =
LOG (2)
0
5
10 15 20 25 30 35 40 45 50 55 60
0
5
10 15 20 25 30 35 40 45 50 55 60
OUTPUT RATE (Hz)
OUTPUT RATE (Hz)
2400 G27
2400 G28
U
U
U
PIN FUNCTIONS
SDO (Pin 6): Three-State Digital Output. During the data
output period, this pin is used for 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
periodsthispincanbeusedasaconversionstatusoutput.
TheconversionstatuscanbeobservedbypullingCSLOW.
VCC (Pin 1): Positive Supply Voltage. Bypass to GND
(Pin 4) with a 10µF tantalum capacitor in parallel with
0.1µF ceramic capacitor as close to the part as possible.
VREF (Pin2):ReferenceInput.Thereferencevoltagerange
is 0.1V to VCC.
VIN (Pin 3): Analog Input. The input voltage range is
–0.125 • VREF to 1.125 • VREF. For VREF > 2.5V, the input
voltage range may be limited by the pin absolute maxi-
mum rating of –0.3V to VCC + 0.3V.
SCK (Pin 7): Bidirectional Digital Clock Pin. In Internal
Serial Clock Operation mode, SCK is used as digital output
fortheinternalserialinterfaceclockduringthedataoutput
period. In External Serial Clock Operation mode, SCK is
used as digital input for the external serial interface. A
weak internal pull-up is automatically activated in Internal
Serial Clock Operation mode. The Serial Clock mode is
determinedbythelevelappliedtoSCKatpowerupandthe
falling edge of CS.
GND (Pin 4): Ground. Shared pin for analog ground,
digitalground,referencegroundandsignalground.Should
be connected directly to a ground plane through a mini-
mum length trace or it should be the single-point-ground
in a single point grounding system.
CS (Pin 5): 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 on CS wakes up the ADC. A
LOW-to-HIGH transition on this pin disables the SDO
digitaloutput.ALOW-to-HIGHtransitiononCSduringthe
Data Output transfer aborts the data transfer and starts a
new conversion.
FO (Pin 8): Frequency Control Pin. Digital input that
controls the ADC’s notch frequencies and conversion
time. When the FO pin is connected to VCC (FO = VCC), the
converter uses its internal oscillator and the digital filter
first null is located at 50Hz. When the FO pin is connected
to GND (FO = OV), the converter uses its internal oscillator
and the digital filter first null is located at 60Hz. When FO
isdrivenbyanexternalclocksignalwithafrequencyfEOSC,
the converter uses this signal as its clock and the digital
filter first null is located at a frequency fEOSC/2560.
8
LTC2400
U
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FU CTIO AL BLOCK DIAGRA
INTERNAL
OSCILLATOR
V
CC
GND
AUTOCALIBRATION
AND CONTROL
F
O
(INT/EXT)
V
IN
∫
∫
∫
SDO
SERIAL
INTERFACE
∑
ADC
SCK
CS
V
REF
DECIMATING FIR
DAC
2400 FD
TEST CIRCUITS
V
CC
3.4k
SDO
SDO
3.4k
C
LOAD
= 20pF
C
= 20pF
LOAD
HI-Z TO V
OH
OH
V
OL
V
OH
TO V
HI-Z TO V
TO HI-Z
2400 TA03
OL
V
V
TO V
OL
OH
OL
TO HI-Z
2400 TA04
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APPLICATIONS INFORMATION
Converter Operation Cycle
CONVERT
The LTC2400 is a low power, delta-sigma analog-to-
digitalconverterwithaneasytouse3-wireserialinterface.
Its operation is simple and made up of three states. The
converter operating cycle begins with the conversion,
followed by a low power sleep state and concluded with
the data output (see Figure 1). The 3-wire interface con-
sists of serial data output (SDO), a serial clock (SCK) and
a chip select (CS).
SLEEP
1
CS AND
SCK
0
DATA OUTPUT
Initially, the LTC2400 performs a conversion. Once the
conversion is complete, the device enters the sleep state.
Whileinthissleepstate,powerconsumptionisreducedby
2400 F01
Figure 1. LTC2400 State Transition Diagram
9
LTC2400
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an order of magnitude. The part remains in the sleep state
as long as CS is logic HIGH. The conversion result is held
indefinitely in a static shift register while the converter is
in the sleep state.
conversion and the output data. Therefore, multiplexing
an analog input voltage is easy.
The LTC2400 performs offset and full-scale calibrations
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
extreme stability of offset and full-scale readings with re-
specttotime,supplyvoltagechangeandtemperaturedrift.
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
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 cycle and the
cycle repeats.
Power-Up Sequence
The LTC2400 automatically enters an internal reset state
when the power supply voltage VCC drops below approxi-
mately 2.2V. This feature guarantees the integrity of the
conversion result and of the serial interface mode selec-
tion which is performed at the initial power-up. (See the
2-wire I/O sections in the Serial Interface Timing Modes
section.)
Through timing control of the CS and SCK pins, the
LTC2400 offers several flexible modes of operation
(internal or external SCK and free-running conversion
modes). These various modes do not require program-
ming configuration registers; moreover, they do not dis-
turbthecyclicoperationdescribedabove. Thesemodesof
operation are described in detail in the Serial Interface
Timing Modes section.
When the VCC voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with duration of approximately 0.5ms. The POR
signal clears all internal registers. Following the POR
signal, the LTC2400 starts a normal conversion cycle and
follows the normal succession of states described above.
The first conversion result following POR is accurate
within the specifications of the device.
Conversion Clock
A major advantage delta-sigma converters offer over
conventional type converters is an on-chip digital filter
(commonly known as Sinc or Comb filter). For high
resolution, low frequency applications, this filter is typi-
cally designed to reject line frequencies of 50 or 60Hz plus
their harmonics. In order to reject these frequencies in
excess of 110dB, a highly accurate conversion clock is
required. The LTC2400 incorporates an on-chip highly
accurate oscillator. This eliminates the need for external
frequency setting components such as crystals or oscilla-
tors. Clocked by the on-chip oscillator, the LTC2400
rejects line frequencies (50 or 60Hz ±2%) a minimum of
110dB.
Reference Voltage Range
The LTC2400 can accept a reference voltage from 0V to
VCC. The converter output noise is determined by the
thermal noise of the front-end circuits, and as such, its
value in microvolts is nearly constant with reference
voltage. A decrease in reference voltage will not signifi-
cantly improve the converter’s effective resolution. On the
other hand, a reduced reference voltage will improve the
overall converter INL performance. The recommended
range for the LTC2400 voltage reference is 100mV to VCC.
Input Voltage Range
The converter is able to accommodate system level offset
and gain errors as well as system level overrange situa-
tions due to its extended input range, see Figure 2. The
LTC2400 converts input signals within the extended input
Ease of Use
The LTC2400 data output has no latency, filter settling or
redundant data associated with the conversion cycle.
There is a one-to-one correspondence between the
range of –0.125 • VREF to 1.125 • VREF
.
10
LTC2400
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V
CC
+ 0.3V
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.
9/8V
REF
V
REF
ABSOLUTE
MAXIMUM
INPUT
NORMAL
INPUT
RANGE
EXTENDED
INPUT
RANGE
1/2V
REF
Bit 30 (second output bit) is a dummy bit (DMY) and is
always LOW.
RANGE
0
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
bitisLOW.Thesignbitchangesstateduringthezerocode.
–1/8V
REF
–0.3V
2400 F02
Bit 28 (forth output bit) is the extended input range (EXR)
indicator. If the input is within the normal input range
0 ≤ VIN ≤ VREF, this bit is LOW. If the input is outside the
normal input range, VIN > VREF or VIN < 0, this bit is HIGH.
Figure 2. LTC2400 Input Range
For large values of VREF this range is limited by the
absolutemaximumvoltagerangeof–0.3Vto(VCC +0.3V).
Beyond this range the input ESD protection devices begin
to turn on and the errors due to the input leakage current
increase rapidly.
The function of these bits is summarized in Table 1.
Table 1. LTC2400 Status Bits
Bit 31
EOC
Bit 30
DMY
Bit 29
SIG
Bit 28
EXR
Input Range
> V
Input signals applied to VIN may extend below ground by
–300mV and above VCC by 300mV. In order to limit any
fault current, a resistor of up to 5k may be added in series
with the VIN pin without affecting the performance of the
device. In the physical layout, it is important to maintain
the parasitic capacitance of the connection between this
series resistance and the VIN pin as low as possible;
therefore, the resistor should be located as close as
practical to the VIN pin. The effect of the series resistance
on the converter accuracy can be evaluated from the
curves presented in the Analog Input/Reference Current
section. In addition a series resistor will introduce a
temperature dependent offset error due to the input leak-
age 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.
V
0
0
0
0
0
0
0
0
1
1
1
0
0
1
IN
REF
0 < V ≤ V
IN
REF
+
–
V
V
= 0 /0
1/0
0
IN
IN
< 0
Bit 27 (fifth output bit) is the most significant bit (MSB).
Bits 27-4 are the 24-bit conversion result MSB first.
Bit 4 is the least significant bit (LSB).
Bits 3-0 are sub LSBs below the 24-bit level. Bits 3-0 may
be included in averaging or discarded without loss of
resolution.
DataisshiftedoutoftheSDOpinundercontroloftheserial
clock (SCK), see Figure 3. Whenever CS is HIGH, SDO
remains high impedance and any SCK clock pulses are
ignored by the internal data out shift register.
Output Data Format
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
The LTC2400 serial output data stream is 32 bits long. The
first 4 bits represent status information indicating the
sign,inputrangeandconversionstate.Thenext24bitsare
the conversion result, MSB first. The remaining 4 bits are
sub LSBs beyond the 24-bit level that may be included in
averaging or discarded without loss of resolution.
11
LTC2400
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to the value corresponding to 1.125 • VREF. For input
voltages below –0.125 • VREF, the conversion result is
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
edge of the 32nd SCK pulse, SDO goes HIGH indicating a
new conversion cycle has been initiated. This bit serves as
EOC (Bit 31) for the next conversion cycle. Table 2 sum-
marizes the output data format.
clamped to the value corresponding to –0.125 • VREF
.
Frequency Rejection Selection (FO Pin Connection)
TheLTC2400internaloscillatorprovidesbetterthan110dB
normal mode rejection at the line frequency and all its
harmonics for 50Hz ±2% or 60Hz ±2%. For 60Hz rejec-
tion, FO (Pin 8) should be connected to GND (Pin 4) while
for 50Hz rejection the FO pin should be connected to VCC
(Pin 1).
As long as the voltage on the VIN pin is maintained within
the –0.3V to (VCC + 0.3V) absolute maximum operating
range, a conversion result is generated for any input value
from –0.125 • VREF to 1.125 • VREF. For input voltages
greaterthan1.125•VREF,theconversionresultisclamped
CS
BIT 31
EOC
BIT 30
“0”
BIT 29
SIG
BIT 28
EXT
BIT 27
MSB
BIT 4
BIT 0
SDO
SCK
LSB
24
Hi-Z
1
2
3
4
5
27
28
32
SLEEP
DATA OUTPUT
CONVERSION
2400 F03
Figure 3. Output Data Timing
Table 2. LTC2400 Output Data Format
Bit 31
EOC
Bit 30
DMY
Bit 29
SIG
Bit 28
EXR
Bit 27
MSB
Bit 26
Bit 25
Bit 24
Bit 23
…
Bit 4
LSB
Bit 3-0
SUB LSBs*
Input Voltage
> 9/8 • V
V
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
0
0
0
1
1
1
1
0
0
0
0
1
1
1
0
0
0
1
1
0
0
1
1
0
0
1
1
1
0
0
0
1
0
1
0
1
0
1
0
1
1
1
1
1
0
1
0
1
0
1
0
1
0
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
0
...
...
...
...
...
...
...
...
...
...
...
...
...
...
1
X
X
X
X
X
X
X
X
X
X
X
X
X
X
IN
REF
9/8 • V
1
1
REF
V
V
+ 1LSB
1
0
REF
REF
1
1
3/4V + 1LSB
1
0
REF
3/4V
1
1
REF
1/2V + 1LSB
1
0
REF
1/2V
1
1
REF
1/4V + 1LSB
1
0
REF
1/4V
1
1
REF
+
–
0 /0
1/0**
0
–1LSB
0
0
0
1
–1/8 • V
0
REF
V
< –1/8 • V
0
IN
REF
*The sub LSBs are valid conversion results beyond the 24-bit level that may be included in averaging or discarded without loss of resolution.
**The sign bit changes state during the 0 code.
12
LTC2400
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–60
–70
The selection of 50Hz or 60Hz rejection can also be made
by driving FO to an appropriate logic level. A selection
change 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
conversions will not be affected.
–80
–90
–100
–110
–120
–130
–140
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2400 can
operate with an external conversion clock. 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 be at least
2560Hz (1Hz notch frequency) to be detected. The exter-
nal clock signal duty cycle is not significant as long as the
minimum and maximum specifications for the high and
low periods tHEO and tLEO are observed.
–12
–8
–4
0
4
8
12
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
2400 G25
Figure 4. LTC2400 Normal Mode Rejection When
Using an External Oscillator of Frequency fEOSC
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
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.
While operating with an external conversion clock of a
frequency fEOSC, the LTC2400 provides better than 110dB
normal mode rejection in a frequency range fEOSC/2560
±4% and its harmonics. The normal mode rejection as a
function of the input frequency deviation from fEOSC/2560
is shown in Figure 4.
WheneveranexternalclockisnotpresentattheFO pin, the
converterautomaticallyactivatesitsinternaloscillatorand
enters the Internal Conversion Clock mode. The LTC2400
Table 3 summarizes the duration of each state as a
function of FO.
Table 3. LTC2400 State Duration
State
Operating Mode
Duration
CONVERT
Internal Oscillator
F = LOW
(60Hz Rejection)
133ms
O
F = HIGH
O
160ms
(50Hz Rejection)
External Oscillator
F = External Oscillator
20480/f
s
EOSC
O
with Frequency f
kHz
EOSC
(f
EOSC
/2560 Rejection)
SLEEP
As Long As CS = HIGH Until CS = 0 and SCK
DATA OUTPUT
Internal Serial Clock
External Serial Clock with
F = LOW/HIGH
(Internal Oscillator)
As Long As CS = LOW But Not Longer Than 1.67ms
(32 SCK cycles)
O
F = External Oscillator with
As Long As CS = LOW But Not Longer Than 256/f
ms
EOSC
O
Frequency f
kHz
(32 SCK cycles)
EOSC
As Long As CS = LOW But Not Longer Than 32/f ms
SCK
Frequency f
kHz
(32 SCK cycles)
SCK
13
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SERIAL INTERFACE
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 = 0.
The LTC2400 transmits the conversion results and re-
ceives the start of conversion command through a syn-
chronous 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.
Chip Select Input (CS)
The active LOW chip select, CS (Pin 5), is used to test the
conversionstatusandtoenablethedataoutputtransferas
described in the previous sections.
Serial Clock Input/Output (SCK)
In addition, the CS signal can be used to trigger a new
conversion cycle before the entire serial data transfer has
been completed. The LTC2400 will abort any serial data
transfer in progress and start a new conversion cycle
anytime a LOW-to-HIGH transition is detected at the CS
pin after the converter has entered the data output state
(i.e., after the first rising edge of SCK occurs with CS = 0).
The serial clock signal present on SCK (Pin 7) is used to
synchronizethedatatransfer.Eachbitofdataisshiftedout
the SDO pin on the falling edge of the serial clock.
In the Internal SCK mode of operation, the SCK pin is an
output and the LTC2400 creates its own serial clock by
dividing the internal conversion clock by 8. In the External
SCK mode of operation, the SCK pin is used as input. The
internalorexternalSCKmodeisselectedonpower-upand
then reselected every time a HIGH-to-LOW transition is
detected at the CS pin. If SCK is HIGH or floating at power-
up or during this transition, the converter enters the inter-
nal 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 12 to 14.
Serial Data Output (SDO)
SERIAL INTERFACE TIMING MODES
The serial data output pin, SDO (Pin 6), drives the serial
data during the data output state. In addition, the SDO pin
is used as an end of conversion indicator during the
conversion and sleep states.
The LTC2400’s 3-wire interface is SPI and MICROWIRE
compatible. This interface offers several flexible modes of
operation. These include internal/external serial clock,
2-or3-wireI/O,singlecycleconversionandautostart.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 5) 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
Table 4. LTC2400 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 5, 6
Figure 7
Internal SCK, Single Cycle Conversion
Internal SCK, 2-Wire I/O, Continuous Conversion
Internal SCK, Autostart Conversion
CS ↓
CS ↓
Figures 8, 9
Figure 10
Figure 11
Continuous
Internal
Internal
C
EXT
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External Serial Clock, Single Cycle Operation
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.
(SPI/MICROWIRE Compatible)
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 5.
The serial clock mode is selected on the falling edge of CS.
Toselecttheexternalserialclockmode,theserialclockpin
(SCK) must be LOW during each CS falling edge.
At the conclusion of the data cycle, CS may remain LOW
and EOC monitored as an end-of-conversion interrupt.
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.
WhileCSispulledLOW, EOCisoutputtotheSDOpin. 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 low power sleep state once the
conversion is complete.
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 6. 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.
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
2.7V TO 5.5V
1µF
V
CC
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
CC
F
O
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
GND
CS
TEST EOC
TEST EOC
TEST EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
BIT 26
BIT 4
LSB
BIT 0
SDO
SUB LSB
Hi-Z
Hi-Z
Hi-Z
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F05
Figure 5. External Serial Clock, Single Cycle Operation
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APPLICATIONS INFORMATION
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2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
F
O
CC
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
GND
CS
TEST EOC
TEST EOC
TEST EOC
BIT 0
EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
BIT 9
BIT 8
SDO
Hi-Z
Hi-Z
Hi-Z
Hi-Z
SCK
(EXTERNAL)
SLEEP
CONVERSION
DATA OUTPUT
SLEEP
DATA OUTPUT
CONVERSION
2400 F06
Figure 6. External Serial Clock, Reduced Data Output Length
External Serial Clock, 2-Wire I/O
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.
This timing mode utilizes a 2-wire serial I/O interface. The
conversion result is shifted out of the device by an exter-
nally generated serial clock (SCK) signal, see Figure 7. CS
maybepermanentlytiedtoground(Pin4), simplifyingthe
user interface or isolation barrier.
Internal Serial Clock, Single Cycle Operation
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.
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 8.
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
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.
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=1whiletheconversionisinprogressandEOC
= 0 once the conversion enters the low power sleep state.
On the 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
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.
16
LTC2400
U
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APPLICATIONS INFORMATION
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
CC
F
O
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
GND
CS
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
BIT 26
BIT 4
BIT 0
SDO
LSB
24
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F07
Figure 7. External Serial Clock, CS = 0 Operation
2.7V TO 5.5V
V
CC
V
CC= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
1µF
V
CC
F
O
10k
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
GND
<t
EOCtest
CS
TEST EOC
TEST EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
BIT 26
BIT 4
BIT 0
SDO
LSB
24
Hi-Z
Hi-Z
Hi-Z
Hi-Z
SCK
(INTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F08
Figure 8. Internal Serial Clock, Single Cycle Operation
17
LTC2400
U
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APPLICATIONS INFORMATION
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.
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.
WhentestingEOC,iftheconversioniscomplete(EOC=0),
thedevicewillexitthesleepstateandenterthedataoutput
state if CS remains LOW. In order to prevent the device
from exiting the low power 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
if the device is using its internal oscillator (F0 = logic LOW
or HIGH). If FO is driven by an external oscillator of
frequency fEOSC, then tEOCtest is 3.6/fEOSC. If CS is pulled
HIGH before time tEOCtest, the device remains in the sleep
state. The conversion result is held in the internal static
shift register.
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 9. On the rising edge of CS, the device
aborts the data output state and immediately initiates a
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
If CS remains LOW longer than tEOCtest, the first rising
edge of SCK will occur and the conversion result is serially
2.7V TO 5.5V
V
CC
V
CC= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
1µF
V
CC
F
O
10k
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
GND
>t
<t
EOCtest
EOCtest
CS
TEST EOC
TEST EOC
TEST EOC
BIT 0
EOC
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
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
2400 F09
Figure 9. Internal Serial Clock, Reduced Data Output Length
18
LTC2400
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APPLICATIONS INFORMATION
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.
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.
Whenever SCK is LOW, the LTC2400’s 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,
certainapplicationsmayrequireanexternaldriveronSCK.
If this driver goes HI-Z after outputting a LOW signal, the
LTC2400’s 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 resistor to SCK, this pin goes HIGH
once the external driver goes HI-Z. On the next CS falling
edge, the device will remain in the internal SCK timing
mode.
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 10. CS may be permanently tied to ground (Pin 4),
simplifying 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
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).
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
2.7V TO 5.5V
V
CC= 50Hz REJECTION
1µF
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
F
O
CC
LTC2400
V
REF
0.1V TO V
V
V
SCK
REF
CC
V
IN
SDO
CS
IN
–0.12V
REF
TO 1.12V
REF
GND
CS
BIT 31
EOC
BIT 30
BIT 29
SIG
BIT 28
EXR
BIT 27
MSB
BIT 26
BIT 4
BIT 0
SDO
LSB
24
SCK
(INTERNAL)
CONVERSION
DATA OUTPUT
CONVERSION
2400 F10
SLEEP
Figure 10. Internal Serial Clock, Continuous Operation
19
LTC2400
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APPLICATIONS INFORMATION
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, Autostart Conversion
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.
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 11. The time the converter
spends in the sleep state is determined by the value of the
external timing capacitor, see Figures 12 and 13. 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
2.7V TO 5.5V
V
CC
1µF
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
F
O
CC
LTC2400
V
REF
CC
V
V
SCK
REF
0.1V TO V
V
IN
SDO
CS
IN
–0.12V
TO 1.12V
REF
REF
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
2400 F11
Figure 11. Internal Serial Clock, Autostart Operation
20
LTC2400
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APPLICATIONS INFORMATION
7
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
conversion is immediately started. This is useful in appli-
cations requiring periodic monitoring and ultralow power.
Figure 14 shows the average supply current as a function
of capacitance on CS.
6
5
4
3
2
V
= 5V
CC
1
0
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
modetheanalogvoltageontheCSpincannotbeobserved
without disturbing the converter operation using a regular
oscilloscope probe. When using this configuration, 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.
V
CC
= 3V
10
100
100000
1
1000
10000
CAPACITANCE ON CS (pF)
2400 F12
Figure 12. CS Capacitance vs tSAMPLE
8
7
6
5
V
= 5V
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.
CC
V
= 3V
CC
4
3
2
1
0
10
100
10000
100000
0
1000
CAPACITANCE ON CS (pF)
DIGITAL SIGNAL LEVELS
2400 F13
The LTC2400’s digital interface is easy to use. Its digital
inputs(FO,CSandSCKinExternalSCKmodeofoperation)
accept standard TTL/CMOS logic levels and the internal
hysteresis receivers can tolerate edge rates as slow as
100µs.However,someconsiderationsarerequiredtotake
advantageofexceptionalaccuracyandlowsupplycurrent.
Figure 13. CS Capacitance vs Output Rate
300
250
V
V
= 5V
= 3V
CC
CC
200
150
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 the conversion state.
100
50
0
In order to preserve the LTC2400’s accuracy, it is very
important to minimize the ground path impedance which
mayappearinserieswiththeinputand/orreferencesignal
and to reduce the current which may flow through this
path.TheGNDpinshouldbeconnectedtoalowresistance
ground plane through a minimum length trace. The use of
multiple via holes is recommended to further reduce the
1
10
100
1000
10000 100000
CAPACITANCE ON CS (pF)
2400 F14
Figure 14. CS Capacitance vs Supply Current
21
LTC2400
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APPLICATIONS INFORMATION
connection resistance. The LTC2400’s power supply cur-
rent flowing through the 0.01Ω resistance of the common
ground pin will develop a 2.5µV offset signal. For a
reference voltage VREF = 2.5V, this represents a 1ppm
offset error.
Parallel termination near the LTC2400 pin will eliminate
thisproblembutwillincreasethedriverpowerdissipation.
A series resistor between 27Ω and 56Ω placed near the
driver or near the LTC2400 pin will also eliminate this
problem without additional power dissipation. The actual
resistor value depends upon the trace impedance and
connection topology.
Inanalternativeconfiguration,theGNDpinoftheconverter
canbethesingle-point-groundinasinglepointgrounding
system. The input signal ground, the reference signal
ground, the digital drivers ground (usually the digital
ground)andthepowersupplyground(theanalogground)
should be connected in a star configuration with the com-
mon point located as close to the GND pin as possible.
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched ca-
pacitor network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
andthereference(VREF). Theresultissmallcurrentspikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.
The power supply current during the conversion state
should be kept to a minimum. This is achieved by restrict-
ing the number of digital signal transitions occurring
during this period.
V
CC
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
LTC2400 power supply current may increase even if the
signal in question is at a valid logic level. For micropower
operationandinordertominimizethepotentialerrorsdue
to additional ground pin current, it is recommended to
drive all digital input signals to full CMOS levels
[VIL < 0.4V and VOH > (VCC – 0.4V)].
R
SW
5k
I
I
REF(LEAK)
REF(LEAK)
V
REF
V
CC
I
IN
IN
AVERAGE INPUT CURRENT:
= 0.25(V – 0.5 • V )fC
REF EQ
R
SW
5k
I
I
I
IN(LEAK)
IN
IN
V
C
EQ
10pF (TYP)
IN(LEAK)
R
SW
5k
2400 F15
GND
SWITCHING FREQUENCY
f = 153.6kHz FOR INTERNAL OSCILLATOR (f = LOGIC LOW OR HIGH)
f = f
O
FOR EXTERNAL OSCILLATORS
EOSC
Severe ground pin current disturbances can also occur
due to the undershoot of fast digital input signals. Under-
shootandovershootcanoccurbecauseoftheimpedance
mismatch at the converter pin when the transition time of
an external control signal is less than twice the propaga-
tion delay from the driver to LTC2400. For reference, on
a regular FR-4 board, signal propagation velocity is ap-
proximately183ps/inchforinternaltracesand170ps/inch
for surface traces. Thus, a driver generating a control
signal with a minimum transition time of 1ns must be
connected to the converter pin through a trace shorter
than 2.5 inches. This problem becomes particularly diffi-
cult when shared control lines are used and multiple
reflections may occur. The solution is to carefully termi-
nate all transmission lines close to their characteristic
impedance.
Figure 15. LTC2400 Equivalent Analog Input Circuit
The key to understanding the effects of this dynamic input
current is based on a simple first order RC time constant
model. Using the internal oscillator, the LTC2400’s inter-
nal switched capacitor network is clocked at 153,600Hz
corresponding to a 6.5µs sampling period. Fourteen time
constantsarerequiredeachtimeacapacitorisswitchedin
order to achieve 1ppm settling accuracy.
Therefore, the equivalent time constant at VIN and VREF
should be less than 6.5µs/14 = 460ns in order to achieve
1ppm accuracy.
22
LTC2400
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APPLICATIONS INFORMATION
Input Current (VIN)
If the total capacitance at VIN (see Figure 17) is small
(<0.01µF), relativelylargeexternalsourceresistances(up
to 20k for 20pF parasitic capacitance) can be tolerated
withoutanyoffset/full-scaleerror.Figures18and19show
a family of offset and full-scale error curves for various
small valued input capacitors (CIN < 0.01µF) as a function
of input source resistance.
If complete settling occurs on the input, conversion re-
sultswillbeuneffectedbythedynamicinputcurrent. Ifthe
settling is incomplete, it does not degrade the linearity
performance of the device. It simply results in an offset/
full-scale shift, see Figure 16. To simplify the analysis of
input dynamic current, two separate cases are assumed:
large capacitance at VIN (CIN > 0.01µF) and small capaci-
tance at VIN (CIN < 0.01µF).
For large input capacitor values (CIN > 0.01µF), the input
spikesareaveragedbythecapacitorintoaDCcurrent. The
gain shift becomes a linear function of input source
resistance independent of input capacitance, see Figures
20 and 21. The equivalent input impedance is 1.66MΩ.
This results in ±1.5µA of input dynamic current at the
extreme values of VIN (VIN = 0V and VIN = VREF, when
TUE
0
V
V
V
= 5V
CC
= 5V
REF
= 5V
IN
–10
–20
–30
–40
–50
T
= 25°C
A
C
IN
= 1000pF
= 0pF
IN
0
V
/2
V
REF
REF
C
IN
= 100pF
C
V
2400 F16
IN
C
= 0.01µF
IN
Figure 16. Offset/Full-Scale Shift
R
SOURCE
V
IN
1
10
100
1k
10k
100k
INTPUT
SIGNAL
C
PAR
20pF
LTC2400
C
R
(Ω)
IN
SOURCE
SOURCE
2400 F19
2400 F17
Figure 19. Full-Scale Error vs RSOURCE (Small C)
Figure 17. An RC Network at VIN
300
50
40
30
20
10
C
= 1µF
= 10µF
V
V
V
= 5V
= 5V
IN
= 25°C
IN
CC
REF
V
V
V
= 5V
CC
C
IN
= 5V
REF
250
= 0V
= 0V
IN
T
A
T
= 25°C
A
200
150
C
IN
= 0.1µF
C
= 0pF
= 100pF
IN
C
IN
C
IN
= 1000pF
100
50
0
C
IN
= 0.01µF
C
= 0.01µF
IN
0
1
0
100 200 300 400 500 600 700 800 900 1000
(Ω)
10
100
1k
10k
100k
R
SOURCE
R
(Ω)
SOURCE
2400 F20
2400 F18
Figure 20. Offset vs RSOURCE (Large C)
Figure 18. Offset vs RSOURCE (Small C)
23
LTC2400
U
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APPLICATIONS INFORMATION
600
500
0
V
V
V
= 5V
CC
V
V
V
= 5V
CC
= 5V
REF
= 5V
REF
C
= 0.01µF
IN
= 5V
IN
= 5V
–50
IN
T
= 25°C
A
T
= 25°C
A
400
300
–100
–150
C
= 0.1µF
C
= 10µF
IN
VREF
C
IN
= 1µF
= 10µF
IN
C
= 1µF
VREF
C
200
100
0
–200
–250
–300
C
= 0.1µF
VREF
C
= 0.01µF
VREF
0
200
400
600
800
(Ω)
1000
0
200
400
R
600
(Ω)
800
1000
RESISTANCE AT V
REF
SOURCE
2400 F22
2400 F21
Figure 21. Full-Scale Error vs RSOURCE (Large C)
Figure 22. Full-Scale Error vs RVREF (Large C)
VREF = 5V). This corresponds to a 0.3ppm shift in offset
and full-scale readings for every 1Ω of input source
resistance.
50
V
V
V
= 5V
= 5V
CC
REF
IN
40
= 5V
T
= 25°C
A
In addition to the input current spikes, the input ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA
max), resultsinafixedoffsetshiftof10µVfora10ksource
resistance.
30
20
C
VREF
= 100pF
C
= 1000pF
VREF
C
= 0.01µF
VREF
10
0
C
VREF
= 0pF
–10
–20
Reference Current (VREF
)
Similar to the analog input, the reference input has a
dynamic input current. This current has negligible effect
on the offset. However, the reference current at VIN = VREF
is similar to the input current at full-scale. For large values
of reference capacitance (CVREF > 0.01µF), the full-scale
error shift is 0.3ppm/Ω of external reference resistance
independent of the capacitance at VREF, see Figure 22. If
the capacitance tied to VREF is small (CVREF < 0.01µF), an
input resistance of up to 20k (20pF parasitic capacitance
at VREF) may be tolerated, see Figure 23.
10
100
10k
100k
1
1k
RESISTANCE AT V (Ω)
REF
2400 F23
Figure 23. Full-Scale Error vs RVREF (Small C)
50
V
V
A
= 5V
CC
REF
= 5V
40
T
= 25°C
C
VREF
= 0pF
VREF
C
= 100pF
30
20
C
VREF
= 1000pF
Unlike the analog input, the integral nonlinearity of the
device can be degraded with excessive external RC time
constants tied to the reference input. If the capacitance at
node VREF is small (CVREF < 0.01µF), the reference input
can tolerate large external resistances without reduction
in INL, see Figure 24. If the external capacitance is large
(CVREF > 0.01µF), the linearity will be degraded by
0.15ppm/Ω independent of capacitance at VREF, see
Figure 25.
C
= 0.01µF
VREF
10
0
–10
1
10
100
1k
10k
(Ω)
100k
RESISTANCE AT V
REF
2400 F24
Figure 24. INL Error vs RVREF (Small C)
24
LTC2400
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APPLICATIONS INFORMATION
160
0
–20
–40
V
V
A
= 5V
= 5V
CC
REF
= 25°C
140
120
100
80
T
C
= 0.1µF
VREF
VREF
VREF
C
= 1µF
C
= 10µF
–60
–80
60
C
VREF
= 0.01µF
40
–100
–120
–140
20
0
–20
0
800
1000
0
f /2
S
f
200
400
600
S
RESISTANCE AT V
(Ω)
INPUT FREQUENCY
REF
2400 F25
2400 F26
Figure 26. Sinc4 Filter Rejection
Figure 25. INL Error vs RVREF (Large C)
In addition to the dynamic reference current, the VREF ESD
protection diodes have a temperature dependent leakage
current.Thisleakagecurrent,nominally1nA(±10nAmax),
results in a fixed full-scale shift of 10µV for a 10k source
resistance.
digitalfilterisnarrow(≈0.2%)comparedtothebandwidth
of the frequencies rejected.
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2400. If passive RC components are placed in
front of the LTC2400 the input dynamic current should be
considered (see Input Current section). In cases where
large effective RC time constants are used, an external
buffer amplifier may be required to minimize the effects of
input dynamic current.
ANTIALIASING
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2400 significantly
simplifies antialiasing filter requirements.
The modulator contained within the LTC2400 can handle
large-signal level perturbations without saturating. Signal
levels up to 40% of VREF do not saturate the analog modu-
lator. ThesesignalsarelimitedbytheinputESDprotection
to 300mV below ground and 300mV above VCC.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(fS), see Figure 26. The modulator sampling frequency is
256 • FO, where FO is the notch frequency (typically 50Hz
or 60Hz). The bandwidth of signals not rejected by the
25
LTC2400
U
TYPICAL APPLICATIONS
SYNCHRONIZATION OF MULTIPLE LTC2400s
Increasing the Output Rate Using Multiple LTC2400s
Since the LTC2400’s absolute accuracy (total unadjusted
error) is 10ppm, applications utilizing multiple matched
ADCs are possible.
A second application uses multiple LTC2400s to increase
the effective output rate by 4×, see Figure 28. In this case,
four LTC2400s are interleaved under the control of sepa-
rate CS signals. This increases the effective output rate
from 7.5Hz to 30Hz (up to a maximum of 60Hz). Addition-
ally, the one-shot output spectrum is unfolded allowing
further digital signal processing of the conversion results.
SCK and SDO may be common to all four LTC2400s. The
four CS rising edges equally divide one LTC2400 conver-
sion cycle (7.5Hz for 60Hz notch frequency). In order to
synchronize the start of conversion to CS, 31 or less SCK
clock pulses must be applied to each ADC.
Simultaneous Sampling with Two LTC2400s
OnesuchapplicationissynchronizingmultipleLTC2400s,
see Figure 27. The start of conversion is synchronized to
the rising edge of CS. In order to synchronize multiple
LTC2400s, CS is a common input to all the ADCs.
To prevent the converters from autostarting a new con-
version at the end of data output read, 31 or fewer SCK
clocksignalsareappliedtotheLTC2400insteadof32(the
32nd falling edge would start a conversion). The exact
timing and frequency for the SCK signal is not critical
since it is only shifting out the data. In this case, two
LTC2400’s simultaneously start and end their conversion
cycles under the external control of CS.
Both the synchronous and 4×output rate applications use
the external serial clock and single cycle operation with
reduced data output length (see Serial Interface Timing
Modes section and Figure 6). An external oscillator clock
is applied commonly to the FO pin of each LTC2400 in
order to synchronize the sampling times. Both circuits
may be extended to include more LTC2400s.
SCK2
SCK1
EXTERNAL OSCILLATOR
(153,600HZ)
LTC2400
#1
LTC2400
#2
V
V
V
F
V
V
V
F
O
CC
REF
IN
O
CC
REF
IN
µCONTROLLER
SCK
SDO
CS
SCK
SDO
CS
GND
GND
CS
SDO1
SDO2
V
REF
(0.1V TO V
)
CC
CS
SCK1
SCK2
SDO1
SDO2
31 OR LESS CLOCK CYCLES
31 OR LESS CLOCK CYCLES
2400 F27
Figure 27. Synchronous Conversion—Extendable
26
LTC2400
U
TYPICAL APPLICATIONS
V
REF
(0.1V TO V
)
CC
EXTERNAL OSCILLATOR
(153,600HZ)
LTC2400
#1
LTC2400
#2
LTC2400
#3
LTC2400
#4
V
V
V
F
V
V
V
F
V
V
V
F
V
V
V
F
O
CC
REF
IN
O
CC
REF
IN
O
CC
REF
IN
O
CC
REF
IN
SCK
SDO
CS
SCK
SDO
CS
SCK
SDO
CS
SCK
SDO
CS
GND
GND
GND
GND
µCONTROLLER
SCK
SDO
CS1
CS2
CS3
CS4
CS1
CS2
CS3
CS4
31 OR LESS
CLOCK PULSES
SCK
SDO
2400 F28
Figure 28. 4× Output Rate LTC2400 System
Differential to Single-Ended
Analog Conditioning
Simple Differential Front-End
for the LTC2400
The circuits in Figures 29 and 30 use the LTC1043 dual
precision, switched capacitor building block. Each circuit
uses one-half of an LTC1043 to perform a differential to
single-ended conversion over an input common mode
range that includes the power supplies. The LTC1043
samples a differential input voltage, holds it on CS and
transfers it to a ground-referenced capacitor CH. The
voltage on CH is applied to the LTC2400’s input and
converted to a digital value.
The circuit in Figure 29 is ideal for wide dynamic range
differential signals in applications where absolute accu-
racy is secondary to high resolution, have large signal
swings, source impedances under 500Ω and use a 5V or
±5V supply.
The circuit achieves a nonlinearity of ±35ppm (a linearity
accuracy of 14.5 bits), noise of 1.5µVRMS and 21-bit
resolution. The circuit exhibits a typical 2.75mV zero
offset. However, this is not an offset that simply shifts the
outputcodebyaconstantvalue.Itisagainerrorthatalters
the transfer function’s slope. The gain error revolves
aroundmidscale(VREF/2).Thisgainerrorcanbecorrected
in software by measuring the error at 0V input and using
the result to create a correction factor.
The LTC1043 achieves its best differential to single-ended
conversion when its internal switching frequency oper-
atesatanominal300Hz, assetbythe0.01µFcapacitorC1,
and when 1µF capacitors are used for CS and CH. CS and
CH should be a film-type capacitor such as mylar or
polypropylene.
27
LTC2400
U
TYPICAL APPLICATIONS
V
5V
0.1µF
REFIN
5V
4
0.1µF
1
V
CC
5
6
7
2
CHIP SELECT
SERIAL
CS
V
REF
3
7
8
V
LTC2400 SDO
IN
SERIAL
SCK
11
12
LARGE
MAGNITUDE
DIFFERENTIAL
INPUT
GND
F
O
C
S
C
H
1µF
4
8
1µF
EXT
13
16
14
C1
0.01µF
1/2
LTC1043
17
0.1µF
–5V
2400 F29
Figure 29. Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals
Multiple Inputs
LTC2400 High Accuracy Differential to Single-Ended
Converter for ±5V Supplies
The simple circuit shown in Figure 31 takes advantage of
the LTC2400’s single conversion settling. The LTC1391
serially programmed multiplexer allows accurate conver-
sionsoneachofitseightchannelswithoutintroducingany
offset, gain or linearity errors with its input signal between
0V and VREF, as long as the total capacitance connected to
the LTC2400’s input is less than 1000pF. A small 2ppm
(typ) error occurs when an active input channel’s signal
voltage reaches –300mV (typ). If the excursion below
ground is above –200mV (typ), the error is less than the
LTC2400’s0.3ppmRMS noise. Onthetopside, theselected
input signal’s magnitude can go above the 5V supply with
no linearity degradation or increased noise. Figure 31’s
circuit can tolerate overdrive on the unselected channel
withoutconversiondegradationaslongastheoverdriveis
less than 250mV above the supply voltage or 250mV
below ground. The linearity performance is similar to that
shownintheTypicalPerformanceCharacteristicssection.
The circuit in Figure 30 is ideal for low level differential
signals in applications that have a ±5V supply and need
highaccuracywithoutcalibration.Thecircuitcombinesan
LTC1043 and LTC1050 as a differential to single-ended
amplifier that has an input common mode range that
includes the power supplies. Resistors R1 and R2 set the
LTC1050’s gain at 101.
The circuit schematic shows an optional resistor RS. This
resistor can be placed in series with the LTC2400’s input
to limit current if the input goes below –300mV. The
resistor does not degrade the converter’s performance as
long as any capacitance, stray or otherwise, connected
between the LTC2400’s input and ground is less than
100pF. Higher capacitance will increase offset and full-
scale errors (see Input Current section).
The circuit achieves a nonlinearity of ±1ppm, input re-
ferred noise of 0.05µVRMS (averaging 64 samples), 19.6
bitsresolutionforafull-scaleinputof40mV,andanoverall
accuracy of 20 bits when using an LTC1236-5 precision
5V reference.
Errors caused by channel-to-channel crosstalk are less
than the LTC2400’s typical input noise. This remains the
case for a frequency range of 1Hz to 153.6kHz (the
LTC2400’s internal clock frequency or 10fS). When the
frequencyreaches1.536MHz(4VP-P),theRMSnoisetypi-
callydoublesandthelinearityisdegradedby30ppm(typ).
28
LTC2400
U
TYPICAL APPLICATIONS
5V
V
5V
0.1µF
REFIN
0.1µF
5V
0.1µF
0.1µF
BRIDGE-
TYPICAL
INPUT
1
4
V
= 40mV
FS
V
CC
5
6
7
3
2
2
7
R *
S
7
8
+
CHIP SELECT
SERIAL
CS
V
REF
5.1k
3
6
LTC1050
V
IN
LTC2400 SDO
11
12
350Ω 350Ω
C
S
SERIAL
SCK
–
C
4
DIFFERENTIAL
INPUT
H
GND
F
O
1µF
1µF
EXT
4
8
R1
9.09k
350Ω 350Ω
–5V
13
16
14
R2
90.9k
AGND OR
–V
C1
1/2
LTC1043
0.01µF
EXT
*OPTIONAL: LIMITS INPUT CURRENT IF THE
INPUT VOLTAGE GOES BELOW –300mV
17
0.1µF
–5V
2400 F30
Figure 30. Differential to Single-Ended Converter for Low Level Inputs, Such as Bridges, Maintains the LTC2400’s High Accuracy
5V
V
5V
0.1µF
REFIN
1
0.1µF
LTC1391
V
CC
5
6
7
2
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
+
CS
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
S0
S1
S2
V
CS
V
REF
3
SDO
SCK
D
V
LTC2400 SDO
IN
–
V
SCK
GND
F
O
S3 DATA 2
S4 DATA 1
4
8
S5
S6
S7
CS
CLK
GND
2400 F31
Figure 31. Multiplex 8-Signal Sources with the LTC1391 and Maintain the LTC2400’s Conversion Accuracy
29
LTC2400
U
TYPICAL APPLICATIONS
Sample Driver for LTC2400 SPI Interface
retrieve the 32-bit result. A fourth port line is used to
power the LTC2400, a vivid example of the converter’s
micropower operation. The program’s main sequence
activates the LTC2400’s serial interface, uses a loop to
retrieve the 32 conversion bits, and then places the
converter’s interface in a high impedance state and start-
ing the next conversion. All bits are retained in variables
ADlo and ADhi. The code can be found on their web site,
www.parallaxinc.com.
TheLTC2400hasaverysimpleserialinterfacethatmakes
interfacingtomicroprocessorsandmicrocontrollersvery
easy. Shown in Figures 32 and 34 are listings of sample
source codes that can be used to initiate conversions and
retrieve data from the LTC2400.
ThelistinginFigure32wascreatedbyParallax, Inc. (916-
624-8333), for the BASIC Stamp. This code uses indi-
vidual port lines to control the LTC2400’s conversion and
'LTC2400
Sample Driver
'03/17/99
This program is an example showing how to access the
LTC2400 using the Basic Stamp2 from Parallax. Since
the BS2 is based on a 16-bit architecture, only the
upper 16 bits of the 24-bit result are displayed,
although all 24 bits are retrieved.
'
'
'
'
ADlo
ADhi
Ctr
var
var
var
var
word
word
byte
bit
'A/D result - lower 16 bits
'A/D result - upper 8 bits
'loop counter
Temp
'temporary bit used for shift
SDO
SCK
CS
con
con
con
con
0
1
2
3
'Serial data connected to P0
'Serial clock connected to P1
'Chip Select connected to P2
'Stamp supplies power connected to P3
'(Uses only 0.3mA!)
Pwr
Init
dira = $E
outa = $0
'Set up data direction
'Pwr, CS, and SCK are outputs
'SDO is an input
'Initialize outputs
'Pwr, CS, and SCK are low
'Wait 100mS for I/O to settle
'Power up the LTC2400
'Wait 1mS for power-on sequence
'Disable the device until we
'wish to read it.
pause 100
high Pwr
pause 1
high CS
Start
pause 125
low CS
'Eight times second
'Enable the LTC2400
for Ctr = 0 to 31
high SCK
'Cycle clock 32 times
gosub ShiftL
30
LTC2400
U
TYPICAL APPLICATIONS
ADlo.bit0 = in0
low SCK
next
'and sample data line
high CS
'Disable the LTC2400
ADhi = (ADhi<<4)+((ADlo&$F000)>>12)
debug ?ADhi
goto Start
'Discard the lower eight bits
'and display (debug command).
ShiftL
Temp = ADlo.bit15
ADlo = ADlo<<1
ADhi = ADhi<<1
ADhi.bit0 = Temp
return
'This routine simply
'performs a 1 bit
'left shift on two
'16 bit variables
Figure 32. This BASIC Stamp Code is an Example of How Easy it is to Retrieve Data from the LTC2400
The listing in Figure 34 is a simple assembler routine for
the68HC11microcontroller.ItusesPORTD,configuring
it for SPI data transfer between the controller and the
LTC2400. Figure 33 shows the simple 3-wire SPI
connection.
the LTC2400’s serial interface by setting the SS output
low, sending a logic low to CS. It next waits in a loop for
a logic low on the data line, signifying end-of-conversion.
After the loop is satisfied, four SPI transfers are com-
pleted,retrievingtheconversion.Themainsequenceends
by setting SS high. This places the LTC2400’s serial
interface in a high impedance state and initiates another
conversion.
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
68HC11
SCK (PD4)
MISO (PD2)
SS (PD5)
7
6
5
SCK
LTC2400 SDO
CS
2400 F33
Figure 33. Connecting the LTC2400 to a 68HC11 MCU Using the SPI Serial Interface
*****************************************************
* This example program transfers the LTC2400's 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 LTC2400's 32 conversion result
31
LTC2400
U
TYPICAL APPLICATIONS
*
DIN1
DIN2
DIN3
DIN4
*
EQU
EQU
EQU
EQU
$00
$01
$02
$03
This memory location holds the LTC2400's bits 31 - 24
This memory location holds the LTC2400's bits 23 - 16
This memory location holds the LTC2400's bits 15 - 08
This memory location holds the LTC2400'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 LTC2400
*
TRFLP1 LDAA
STAA
*
#$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
WAIT1
LDAA
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.
BPL
WAIT1
*
*
LDAA
SPDR
0,X
Load accumulator A with the current byte of LTC2400 data
that was just received
Transfer the LTC2400's data to memory
Increment the pointer
STAA
INX
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
*
*
BSET
PORTD,Y %00100000 This sets the SS* output bit to a logic high,
de-selecting the LTC2400
Restore the A register
Restore the Y register
Restore the X register
PULA
PULY
PULX
RTS
Figure 34. This is an Example of 68HC11 Code That Captures the LTC2400’s
Conversion Results Over the SPI Serial Interface Shown in Figure 33
32
LTC2400
U
TYPICAL APPLICATIONS
This circuit produces a DC offset at the cold junction
referencepoint,of1mVto15mV,whichmustbenulledout
in software. This DC offset, resulting from the forward
voltage of the diode, is variable from device to device and
must be calibrated for each unit.
Thermocouple Applications
Figure 35 shows a thermocouple interface circuit that
demonstrates the practicality of direct connection to the
LTC2400 using even the lowest output thermocouples (in
this case, a type S thermocouple, with a full-scale output
of 18mV).
Since the temperature coefficient of the 1N4148 diode is
not guaranteed, a trim should be provided to accommo-
date a coefficient from 1.7mV/°C to 2.3mV/°C. Alterna-
tively, a transistor can be used as a sensor with Omega
Engineering thermocouple circuit board connectors that
are available with TO-92 transistor retainer clips, placing
the transistor in physical contact with the cold junction.
This topology is the least costly solution for thermocouple
sensing. As shown, it is capable of resolving approxi-
mately0.25°Cwithoutaveraging.SincetheLTC2400does
not exhibit any easily discernible quantization effects,
averaging can significantly extend the resolution for slow
changing processes.
The 1M resistor RTC shown is intended as an open-circuit
detection scheme, producing full scale at the input of the
LTC2400. Note that this resistor contributes to the offset
and must have low TC, as should the resistors R2 and R3.
Since R1 provides forward bias for the diode, its tempera-
ture coefficient is not as critical.
In this circuit, a 1N4148 diode provides cold junction
compensationbyproducing, atthepositiveterminalofthe
thermocouple, an approximation of the average Seebeck
coefficientforatypeSthermocoupleoverthetemperature
range expected at the cold junction (0°C to 40°C). If the
operating range is less, the coefficient can be adjusted to
produce a better match for the range anticipated. This
basic circuit can be used with other thermocouples by
changing the divide ratio to suit the Seebeck coefficient of
the type chosen (see table).
The circuit in Figure 35 uses only 12% of the LTC2400’s
input range and is able to accommodate the full-scale
output of all thermocouple types. The commonly used
5V
0.1µF
1
R
R1
43.2k
COLD JUNCTION
ISOTHERMAL
TC
V
CC
1M
5
6
7
2
CS
V
REF
3
Cu
–
V
LTC2400 SDO
IN
THERMOCOUPLE
SCK
Cu
+
GND
4
F
O
8
1N4148
–2mV/°C
R2*
10k
5V
60Hz
50Hz
–SB
R3*
100Ω
*25ppm, 1% TOLERANCE
SINGLE POINT GROUND
THERMOCOUPLE
SEEBECK
COEFFICIENT*
TYPE
R2
J
K
S
50.2µV/°C
39.2µV/°C
6.15µV/°C
3.83k
4.99k
32.4k
2400 F35
*20°C ≤ T ≤ 50°C
A
Figure 35. Diode Cold Junction Compensation
33
LTC2400
U
TYPICAL APPLICATIONS
thermocouple with the highest output is type E, at about
70mV. This circuit does not provide curvature correction
for the Seebeck effect at the cold junction. If the applica-
tion requires very high accuracy, the temperature of the
cold junction should be determined via a separate input
to the A/D, using an RTD for example. The cold junction
compensation can be performed by implementing the
thermocouple’s NBS polynominal curvature correction
in software. (The input to the LTC2400 can be multi-
plexed using the LTC1391 with little degradation.) If a
separate temperature sensor is used to monitor the cold
junction, the connection from the thermocouple to the
LTC2400 can be direct. The junctions formed at the point
where the thermocouple leads meet different metal (e.g.,
copper traces) must be equal in temperature, and the
cold junction sensor must be mounted at that point. Any
temperature differential between the leads, or any differ-
ential between the leads and the temperature sensor will
introduce an error into the reading.
Figure36showsaninexpensivecircuitwithremovalofthe
DCoffset. TheoutputoftheLT®1077isattenuatedinorder
to produce the required coefficient, as well as reduce the
noise and offset error contribution. If used with a ther-
mistor, this circuit can be modified to produce curvature
correction.Theremovaloftheoffsetassociatedwithdiode
forward voltage, or the 273°K overhead on some mono-
lithic temperature sensors, simplifies the use of substan-
tial gain after the thermocouple. Chopper amplifiers such
as the LTC1050 can extend the noise floor of the LTC2400
by as much as a factor of 10 to 20. The use of a gain of 20
in front of the LTC2400 can extend the resolution of a
thermocouple application to 0.02°C or better.
If absolute accuracy is not important, the use of a low
noise bipolar amplifier, such as the LT1028, can extend
the resolution an additional order of magnitude.
NotethatachievinghighaccuracyinthecircuitinFigure36
requires a calibration sequence for circuit offset and gain
correction.
5V
0.1µF
1
+
V
R2
R
LM334
SO-8
174k*
–
5V
R1
226Ω*
V
V
CC
5
3
2
7
R5
1k
+
CS
V
REF
1mV/°C
6.1µV/°C
–
+
3
6
6
7
LT1077
V
LTC2400 SDO
IN
2
SCK
–
4
GND
F
O
R3
1k*
SELECT R3 FOR
R6
6.19Ω
4
8
THERMOCOUPLE TYPE
R4
10k*
10k
50Hz
S: 6.19Ω
K: 39.2Ω
J: 49.9Ω
E: 61.9Ω
5V
60Hz
2400 F35
*RECOMMENDED 0.1%, ±5ppm IRC AFD SERIES CHIP RESISTORS
Figure 36. Inexpensive Amplifier Improves Cold Junction Compensation
34
LTC2400
U
TYPICAL APPLICATIONS
Simple Platinum RTD Interface
A simpler, and potentially less expensive solution is the
use of the LT1025 as shown in Figure 37.
If high temperature resolution is required over a more
limited range, Figure 38 can resolve approximately
0.01°C without additional amplification. The resistance of
a platinum RTD changes by approximately 0.31Ω/°C at
TA = 25°C. The 100Ω to 300Ω source impedance of this
circuit does not compromise the stability, accuracy or
noise level of the LTC2400.
The LT1025 incorporates the functions of temperature
sensor, a precision divider chain required to produce the
appropriate correction for five different types of thermo-
couples,aswellascurvaturecorrection.TheLT1025must
be located at the cold junction. The use of a thermal mass
around the cold junction, as well as protection from air
currents, is advisable.
5V
2
0.1µF
1
V
V
CC
IN
5
2
CS
LTC2400 SDO
SCK
V
REF
LT1025
–
+
3
6
6
7
V
S
–
IN
GND
4
R
GND
F
O
5
TYPE
S
4
8
10k
50Hz
5V
60Hz
2400 F36
Figure 37. The LT1025 Complete Cold Junction Solution
5V
5V 0.1µF
1
R1*
V
CC
12.1k
5
6
7
2
CS
V
REF
3
F
S
V
LTC2400 SDO
IN
SCK
GND
4
F
O
Pt RTD
100Ω
8
10k
50Hz
5V
60Hz
*VISHAY S102 OR EQUIVALENT
2400 F37
Figure 38. Simplest Platinum RTD Interface
35
LTC2400
U
TYPICAL APPLICATIONS
The 12.1k resistor should be a precision resistor such as
a Vishay S102 series, or must be temperature stabilized.
Theexcitationcurrentislowenoughformostsensorsthat
theself-heatingeffectisnearthenoiseflooroftheLTC2400.
or the resistors must exhibit very low temperature coeffi-
cients. Precision resistor networks are always a good
alternative and are available from Vishay or Caddock.
Half-Bridge Strain Gauge
The use of a bipolar amplifier configuration shown in
Figure 39 offers a potential resolution of 0.001°C
The circuit in Figure 40 is a ratiometric half-bridge circuit
with direct connection to the LTC2400. The use of two
thin-film strain gauges in a half-bridge configuration can
produce 2mV/V output and approximately 12-bit resolu-
tion. The 175Ω source impedance seen by the LTC2400
does not compromise operation.
In order to achieve these results, the following effects
must be considered. Variation in the self-heating of the
RTD element due to air currents is the most difficult
challenge. If the RTD is mounted in a sealed glass enclo-
sure and painted black, the LTC2400 can detect the arrival
of a person in the room. This is also true of infrared
thermocouplesensors(thermopiles)thatcanalsobeused
directly with the LTC2400. A variation of this circuit with
two RTDs can detect small differential temperatures in
order to determine heat inflow or outflow from a process.
In order for this circuit to be practical, the ambient tem-
perature of the amplifier and resistors must be controlled
The optional resistor shown can be up to 5k and will
provide surge and transient protection for the LTC2400
if the strain gauges are located some distance from the
LTC2400, or if the strain bearing member is not well
grounded and may be subject to ESD discharge. Thin-
film strain elements form coupling capacitance to the
strain bearing member to which they are bonded. If noise
V
REF
R1*
9.09k
5V 0.1µF
R2*
9.09k
S
5V
LT1028
–5V
F
3
2
+
–
1
350Ω
1k
6
STRAIN
R1
5k
OPTIONAL
TO
LTC2400
V
CC
5
6
7
2
ELEMENT
CS
V
REF
3
V
LTC2400 SDO
IN
0.1µF
SCK
300Ω
Pt RTD
100Ω
GND
4
F
O
350Ω
STRAIN
ELEMENT
R3*
9.09k
8
10k
50Hz
2400 F38
5V
R4**
100Ω
60Hz
MUST BE 5ppm/°C OR BETTER,
AN ARRAY IS RECOMMENDED
MUST BE VERY STABLE <5ppm/°C
*
2400 F39
**
Figure 39. Extremely High Resolution RTD Interface
Figure 40. Half-Bridge Connection for Strain Gauges
36
LTC2400
U
TYPICAL APPLICATIONS
pick-up from the strain bearing member is largely 60Hz,
the LTC2400 will reject it. If serious high frequency noise
is present on the strain bearing member, it may be
necessary to add buffering in order to allow the use of
noise suppression.
dividers, but they are too low to be produced directly by a
quartz oscillator. Quartz stability is generally not required,
as the notches are wide enough that an oscillator with
0.1% to 1% stability is adequate.
In instances where digital generation of these frequencies
isnotpracticalduetopower,spaceorcostlimitations,and
notches in the range of 4Hz to 120Hz are required, the
circuit in Figure 41 can be used.
Stable Relaxation Oscillator
for External Clock
Applications that require that the notch produced by the
LTC2400’s sinc4 filter be placed at some frequency other
than 50Hz or 60Hz require an external clock. The fre-
quency required is 2560× the required notch frequency.
Simple relaxation oscillators built from logic gates with
hysteresis such as the 74HC14 are not stable with tem-
perature, supply voltage changes, or from device to
device.
The frequency can be varied over this range by changing
capacitor C1 over the range of 4000pf to 30pF. For the
resistor values shown, the output frequency in kHz is
approximately 9.5e-6 divided by C1 (C1 in pF). The circuit
produces a controlled amount of hysteresis dependent
onlyonresistormatchingandselfbiasesitselfaroundthe
input threshold. All gates must be in the same package,
and no loads should be driven from the outputs driving
feedback paths. If there are spare gates, they can be used
in parallel with gates B and D for improved drive of
feedback paths.
If for example, a remote weigh scale application requires
rejectionofaresonanceat11Hz,thefrequencymustbeset
to 28.16kHz. In many instances, these frequencies could
be produced digitally with a phase lock loop or with digital
R1
100k
HC04
–6
9.5 • 10
A
B
C
D
f
(kHz) =
OUT
C1(pF)
C2
15pF
C3
15pF
C1
R2
47k
R3
47k
2400 F40
C1 MAY VARY FROM 30pF TO 4000pF
STABLE OPERATION AT HIGHER FREQUENCIES
REQUIRES VALUES OF RESISTOR TO BE REDUCED
Figure 41. Stable Relaxation Low Power Oscillator for Notch Tuning
37
LTC2400
U
TYPICAL APPLICATIONS
TheperformanceoftheLTC2400canbeverifiedusingthe
demonstration board DC228, see Figure 42 for the sche-
matic. 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 43)
which graphically captures the conversion results. It can
be used to determine noise performance, stability, and
with an external source, linearity. As exemplified in the
schematic, the LTC2400 is extremely easy to use. This
demonstrationboardandassociatedsoftwareisavailable
by contacting Linear Technology.
D1
BAV74LT1
U2
J6
+
5V
LT1236ACS8-5
EXT V
R3
8V TO 15V
100Ω
6
2
J2
REFOUT
OUT
GND
IN
V
+
C5
C6
22µF
100µF
4
16V
EXTERNAL JP2 ONBOARD
JP1
2
C4
0.1µF
REF
REF
1
2
3
50Hz 1
3 60Hz
J1
REFIN
V
C1
10µF
U1
LTC2400
U3-2
U3-1
J5
DB9
1
2
3
4
8
7
6
5
74HC14
74HC14
R1
V
V
V
F
O
CC
REF
IN
51k
SCLKDTR
4
3
2
1
1
6
2
7
3
8
4
9
5
SCK
SDO
CS
C2
10µF
GND
U3-4
74HC14
U3-3
74HC14
J3
INPUT
DOUTCTS
CSRTS
9
8
5
6
C3
10µF
J4
GROUND
U3-6
74HC14
U3-5
74HC14
R2
51k
NOTES: UNLESS OTHERWISE SPECIFIED
INSTALL SHUNTS ON PIN 2 AND 3 OF JP1 AND JP2
12
13 10
11
2400 F42
Figure 42. 24-Bit A/D Demo Board Schematic
Figure 43. Display Graphic
38
LTC2400
U W
U
PACKAGE I FOR ATIO
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
7
5
8
6
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.053 – 0.069
3
4
2
0.010 – 0.020
(0.254 – 0.508)
× 45°
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
0.050
(1.270)
TYP
0.014 – 0.019
(0.355 – 0.483)
*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
SO8 0996
U
W
PCB LAYOUT A D FIL
Component Side Silkscreen
Solder Side Silkscreen
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
LTC2400
U
W
PCB LAYOUT A D FIL
Component Side Solder Mask
Component Side Paste Mask
Component Side
Solder Side
Solder Side Solder Mask
Solder Side Paste Mask
RELATED PARTS
PART NUMBER
LT1019
DESCRIPTION
COMMENTS
Precision Bandgap Reference, 2.5V, 5V
3ppm/°C Drift, 0.05% Max
LT1025
Micropower Therocouple Cold Junction Compensator
Dual Precision Instrumentation Switched Capacito Building Blockr
Precision Chopper Stabilized Op Amp
LTC1043
LTC1050
LT1236A-5
LT1460
Precise Charge, Balanced Switching, Low Power
No External Components 5µV Offset, 1.6µV Noise
P-P
Precision Bandgap Reference, 5V
0.05% Max, 5ppm/°C Drift
Micropower Series Reference
0.075% Max, 10ppm/°C Max Drift, 2.5V, 5V and 10V Versions
3µV Noise, 10-Pin MSOP Package, Ground Sensing
Same Performance as LTC2400
LTC2401/LTC2402 1-/2-Channel 24-Bits ADCs
LTC2404/LTC2408 4-/8-Channel 24-Bit ADCs
LTC2420
20-Bit Micropower ADC
6µV Noise, Pin-Compatible with LTC2400
2400fa LT/TP 0300 2K REV A • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1998
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
40
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
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