ADS7818EB/250 [TI]
具有 5V 输入电压范围的 12 位高速低功耗采样模数转换器 | DGK | 8 | -40 to 85;
| 型号： | ADS7818EB/250 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 具有 5V 输入电压范围的 12 位高速低功耗采样模数转换器 | DGK | 8 | -40 to 85 光电二极管 转换器 模数转换器 |
| 文件： | 总20页 (文件大小：619K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
®
ADS7818
ADS7818
For most current data sheet and other product
information, visit www.burr-brown.com
12-Bit High Speed Low Power Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
DESCRIPTION
● 500kHz THROUGHPUT RATE
● 2.5V INTERNAL REFERENCE
● LOW POWER: 11mW
The ADS7818 is a 12-bit sampling analog-to-digital
converter (A/D) complete with sample/hold, internal
2.5V reference, and synchronous serial interface. Typi-
cal power dissipation is 11mW at a 500kHz through-
put rate. The device can be placed into a power down
mode which reduces dissipation to just 2.5mW. The
input range is zero to two times the reference voltage,
and the internal reference can be overdriven by an
external voltage.
● SINGLE SUPPLY +5V OPERATION
● DIFFERENTIAL INPUT
● SERIAL INTERFACE
● GUARANTEED NO MISSING CODES
● MINI-DIP-8 AND MSOP-8
● UNIPOLAR INPUT RANGE
Low power, small size, and high-speed make the
ADS7818 ideal for battery operated systems such as
wireless communication devices, portable multi-chan-
nel data loggers, and spectrum analyzers. The serial
interface also provides low-cost isolation for remote
data acquisition. The ADS7818 is available in a plas-
tic mini-DIP-8 or an MSOP-8 package and is guaran-
teed over the –40°C to +85°C temperature range.
APPLICATIONS
● BATTERY OPERATED SYSTEMS
● DIGITAL SIGNAL PROCESSING
● HIGH SPEED DATA ACQUISITION
● WIRELESS COMMUNICATION SYSTEMS
CLK
SAR
CONV
+In
CDAC
Serial
Interface
DATA
–In
Comparator
S/H Amp
Internal
Buffer
+2.5V Ref
VREF
10kΩ ±30%
International Airport Industrial Park
Twx: 910-952-1111
•
Mailing Address: PO Box 11400, Tucson, AZ 85734
•
Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706
•
Tel: (520) 746-1111
•
Internet: http://www.burr-brown.com/
•
Cable: BBRCORP
•
Telex: 066-6491
•
FAX: (520) 889-1510
•
Immediate Product Info: (800) 548-6132
©1998 Burr-Brown Corporation
PDS-1408B
Printed in U.S.A. May, 2000
SBAS078
SPECIFICATIONS
At TA = –40°C to +85°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, internal reference, unless otherwise specified.
ADS7818P, E
TYP
ADS7818PB, EB
PARAMETER
CONDITIONS
MIN
MAX
MIN
TYP
MAX
UNITS
ANALOG INPUT
Full-Scale Input Span(1)
Absolute Input Range
+In – (–In)
+In
0
–0.2
–0.2
5
✻
✻
✻
✻
✻
✻
V
V
V
VCC +0.2
+0.2
–In
Capacitance
Leakage Current
15
1
✻
✻
pF
µA
SYSTEM PERFORMANCE
Resolution
12
✻
Bits
Bits
No Missing Codes
Integral Linearity Error
Differential Linearity Error
Offset Error
12
✻
±1
±0.8
±2
±2
±0.5
±0.5
±1
±1
±1
✻
±15
±35
LSB(2)
LSB
LSB
LSB
LSB
dB
±5
±30
±50
Gain Error(3)
25°C
±12
±7
–40°C to +85°C
DC, 0.2Vp-p
1MHz, 0.2Vp-p
Common-Mode Rejection
70
50
150
1.2
✻
✻
✻
✻
dB
µVrms
LSB
Noise
Power Supply Rejection
Worst Case ∆, +VCC = 5V ±5%
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Aperture Delay
1.625
0.350
✻
✻
µs
µs
kHz
ns
ps
ns
500
–72
✻
5
30
350
✻
✻
✻
Aperture Jitter
Step Response
DYNAMIC CHARACTERISTICS
Signal-to-Noise Ratio
Total Harmonic Distortion(4)
Signal-to-(Noise+Distortion)
Spurious Free Dynamic Range
Usable Bandwidth
VIN = 5Vp-p at 100kHz
VIN = 5Vp-p at 100kHz
VIN = 5Vp-p at 100kHz
VIN = 5Vp-p at 100kHz
SINAD > 68dB
72
–78
70
78
350
✻
–82
72
82
✻
dB
dB
dB
dB
kHz
–75
68
72
70
75
REFERENCE OUTPUT
Voltage
Source Current(5)
Drift
IOUT = 0
Static Load
IOUT = 0
2.475
2.0
2.50
2.525
50
2.48
✻
2.52
✻
V
µA
ppm/°C
mV
20
0.6
✻
✻
Line Regulation
4.75V ≤ VCC ≤ 5.25V
REFERENCE INPUT
Range
Resistance(6)
2.55
✻
✻
✻
✻
V
kΩ
to Internal Reference Voltage
10
✻
✻
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels:
VIH
VIL
VOH
CMOS
|IIH| ≤ +5µA
|IIL| ≤ +5µA
IOH = –500µA
IOL = 500µA
3.0
–0.3
3.5
VCC +0.3
0.8
✻
✻
✻
✻
✻
✻
✻
✻
V
V
V
V
VOL
0.4
Data Format
Straight Binary
✻
POWER SUPPLY REQUIREMENT
+VCC
Quiescent Current
Specified Performance
fSAMPLE = 500kHz
Power Down
fSAMPLE = 500kHz
Power Down
4.75
5.25
20
✻
✻
✻
V
2.2
0.5
11
✻
✻
✻
✻
mA
mA
mW
mW
Power Dissipation
2.5
TEMPERATURE RANGE
Specified Performance
–40
+85
✻
✻
°C
✻ Specifications same as ADS7818P,E.
NOTES: (1) Ideal input span, does not include gain or offset error. (2) LSB means Least Significant Bit, with VREF equal to +2.5V, one LSB is 1.22mV. (3) Measured
relative to an ideal, full-scale input (+In – (–In)) of 4.999V. Thus, gain error includes the error of the internal voltage reference. (4) Calculated on the first nine
harmonics of the input frequency. (5) If the internal reference is required to source current to an external load, the reference voltage will change due to the internal
10kΩ resistor. (6) Can vary ±30%.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN
assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject
to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not
authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
®
ADS7818
2
ABSOLUTE MAXIMUM RATINGS(1)
ELECTROSTATIC
DISCHARGE SENSITIVITY
Electrostatic discharge can cause damage ranging from per-
formance degradation to complete device failure. Burr-
Brown Corporation recommends that all integrated circuits
be handled and stored using appropriate ESD protection
methods.
+VCC to GND............................................................................ –0.3V to 6V
Analog Inputs to GND .............................................. –0.3V to (VCC + 0.3V)
Digital Inputs to GND ............................................... –0.3V to (VCC + 0.3V)
Power Dissipation .......................................................................... 325mW
Maximum Junction Temperature ................................................... +150°C
Operating Temperature Range ......................................... –40°C to +85°C
Storage Temperature Range .......................................... –65°C to +150°C
Lead Temperature (soldering, 10s) ............................................... +300°C
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings” may
cause permanent damage to the device. Exposure to absolute maximum condi-
tions for extended periods may affect device reliability.
ESD damage can range from subtle performance degrada-
tion to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet
published specifications.
PIN CONFIGURATION
Top View
VREF
+In
1
2
3
4
8
7
6
5
+VCC
CLK
VREF
+In
1
2
3
4
8
7
6
5
+VCC
CLK
ADS7818
ADS7818
–In
DATA
CONV
–In
DATA
CONV
GND
GND
Plastic Mini-DIP-8
MSOP-8
PIN ASSIGNMENTS
PIN
NAME
DESCRIPTION
1
2
3
4
5
VREF
+In
Reference Output. Decouple to ground with a 0.1µF ceramic capacitor and a 2.2µF tantalum capacitor.
Non-Inverting Input.
–In
Inverting Input. Connect to ground or to remote ground sense point.
Ground.
GND
CONV
Convert Input. Controls the sample/hold mode, start of conversion, start of serial data transfer, type of serial transfer, and power
down mode. See the Digital Interface section for more information.
6
DATA
Serial Data Output. The 12-bit conversion result is serially transmitted most significant bit first with each bit valid on the rising edge
of CLK. By properly controlling the CONV input, it is possibly to have the data transmitted least significant bit first. See the Digital
Interface section for more information.
7
8
CLK
Clock Input. Synchronizes the serial data transfer and determines conversion speed.
+VCC
Power Supply. Decouple to ground with a 0.1µF ceramic capacitor and a 10µF tantalum capacitor.
PACKAGE/ORDERING INFORMATION
MAXIMUM
INTEGRAL
LINEARITY
ERROR
MAXIMUM
DIFFERENTIAL
LINEARITY
ERROR
PACKAGE
DRAWING
NUMBER(1)
SPECIFICATION
TEMPERATURE
RANGE
PACKAGE
MARKING(2)
ORDERING
NUMBER(3)
TRANSPORT
MEDIA
PRODUCT
(LSB)
(LSB)
PACKAGE
ADS7818E
"
ADS7818EB
"
ADS7818P
ADS7818PB
±2
"
±1
"
±2
±1
N/S(4)
"
MSOP-8
337
"
337
"
006
"
–40°C to +85°C
A18
"
A18
ADS7818E/250 Tape and Reel
ADS7818E/2K5 Tape and Reel
ADS7818EB/250 Tape and Reel
ADS7818EB/2K5 Tape and Reel
"
"
±1
MSOP-8
–40°C to +85°C
"
"
"
"
N/S(4)
±1
Plastic DIP-8
–40°C to +85°C
ADS7818P
ADS7818PB
ADS7818P
Rails
Rails
"
"
ADS7818PB
NOTE: (1) For detail drawing and dimension table, please see end of data sheet or Package Drawing File on Web. (2) Performance Grade information is marked
on the reel. (3) Models with a slash(/) are available only in Tape and reel in quantities indicated (e.g. /250 indicates 250 units per reel, /2K5 indicates 2500 devices
per reel). Ordering 2500 pieces of ”ADS7818E/2K5“ will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to the
www.burr-brown.com web site under Applications and Tape and Reel Orientation and Dimensions. (4) N/S = Not Specified, typical only. However, 12-Bits no missing
codes is guaranteed over temperature.
®
3
ADS7818
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
CHANGE IN FULL-SCALE ERROR
vs TEMPERATURE
CHANGE IN OFFSET vs TEMPERATURE
2
0
2.0
1.6
–2
–4
–6
–8
–10
1.2
0.8
0.4
0.0
–0.4
–40
–20
0
20
40
60
80
100
100
600
–40
–20
0
20
40
60
80
100
100
600
Temperature (°C)
Temperature (°C)
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
SUPPLY CURRENT vs TEMPERATURE
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
470
460
450
440
430
420
410
400
390
fSAMPLE = 500kHz
fSAMPLE = 125kHz
–40
–20
0
20
40
60
80
–40
–20
0
20
40
60
80
Temperature (°C)
Temperature (°C)
CHANGE IN INTEGRAL LINEARITY and
DIFFERENTIAL LINEARITY vs SAMPLE RATE
SUPPLY CURRENT vs SAMPLE RATE
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
0.4
0.3
0.2
0.1
Change in Integral
Linearity (LSB)
0.0
–0.1
–0.2
–0.3
Change in Differential
Linearity (LSB)
100
200
300
400
500
100
200
300
400
500
Sample Rate (kHz)
Sample Rate (kHz)
®
ADS7818
4
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
CHANGE IN FULL-SCALE ERROR
vs EXTERNAL REFERENCE VOLTAGE
CHANGE IN OFFSET
vs EXTERNAL REFERENCE VOLTAGE
0.5
0.4
0.2
0.0
0.3
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
0.2
0.1
0.0
–0.1
–0.2
–0.3
–0.4
2.0
2.1
2.2
2.3
2.4
2.5
1.9
2.1
2.3
2.5
2.7
2.9
3.1
External Reference Voltage (V)
External Reference Voltage (V)
POWER SUPPLY REJECTION
PEAK-TO-PEAK NOISE
vs POWER SUPPLY RIPPLE FREQUENCY
vs EXTERNAL REFERENCE VOLTAGE
30
25
20
15
10
5
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0
1
10
100
1k
10k
100k
1M
2.0
2.1
2.2
2.3
2.4
2.5
Power Supply Ripple Frequency (Hz)
External Reference Voltage (LSB)
FREQUENCY SPECTRUM
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 10.9kHz, –0.2dB)
(4096 Point FFT; fIN = 102kHz, –0.2dB)
0
–20
0
–20
–40
–40
–60
–60
–80
–80
–100
–120
–100
–120
0
62.5
125
187.5
250
0
62.5
125
187.5
250
Frequency (kHz)
Frequency (kHz)
®
5
ADS7818
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
SIGNAL-TO-NOISE RATIO and
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 247kHz, –0.2dB)
SIGNAL-TO-(NOISE+DISTORTION)
vs INPUT FREQUENCY
0
–20
76
74
72
70
68
66
64
SNR
–40
SINAD
–60
–80
–100
–120
0
62.5
125
187.5
250
1k
10k
100k
1M
Input Frequency (Hz)
Frequency (kHz)
SPURIOUS FREE DYNAMIC RANGE AND
TOTAL HARMONIC DISTORTION
vs INPUT FREQUENCY
CHANGE IN SIGNAL-TO-NOISE RATIO
and SIGNAL-TO-(NOISE+DISTORTION)
vs TEMPERATURE
90
85
80
75
70
65
–90
–85
–80
–75
–70
–65
0.3
0.2
SFDR
SNR
THD❉
0.1
0.0
SINAD
–0.1
–0.2
–0.3
❉ First nine harmonics
of the input frequency
fIN = 100kHz, –0.2dB
–40
–20
0
20
40
60
80
100
1k
10k
100k
1M
Temperature (°C)
Input Frequency (Hz)
CHANGE IN SPURIOUS FREE DYNAMIC RANGE
AND TOTAL HARMONIC DISTORTION
vs TEMPERATURE
1.0
0.5
–1.0
–0.5
0.0
fIN = 100kHz, –0.2dB
0.0
THD❉
–0.5
–1.0
–1.5
–2.0
0.5
SFDR
1.0
1.5
❉ First nine harmonics
of the input frequency
2.0
–40
–20
0
20
40
60
80
100
Temperature (°C)
®
ADS7818
6
are common to both inputs. Thus, the –IN input is best used
to sense a remote ground point near the source of the +IN
signal. If the source driving the +IN signal is nearby, the
–IN should be connected directly to ground.
THEORY OF OPERATION
The ADS7818 is a high speed successive approximation
register (SAR) analog-to-digital converter (A/D) with an
internal 2.5V bandgap reference. The architecture is based
on capacitive redistribution which inherently includes a
sample/hold function. The converter is fabricated on a 0.6µ
CMOS process. See Figure 1 for the basic operating circuit
for the ADS7818.
The input current into the analog input depends on input
voltage and sample rate. Essentially, the current into the
device must charge the internal hold capacitor during the
sample period. After this capacitance has been fully charged,
there is no further input current. The source of the analog
input voltage must be able to charge the input capacitance to
a 12-bit settling level within the sample period—which can
be as little as 350ns in some operating modes. While the
converter is in the hold mode or after the sampling capacitor
has been fully charged, the input impedance of the analog
input is greater than 1GΩ.
The ADS7818 requires an external clock to run the conver-
sion process. This clock can vary between 200kHz (12.5Hz
throughput) and 8MHz (500kHz throughput). The duty cycle
of the clock is unimportant as long as the minimum HIGH
and LOW times are at least 50ns and the clock period is at
least 125ns. The minimum clock frequency is set by the
leakage on the capacitors internal to the ADS7818.
Care must be taken regarding the input voltage on the +In
and –IN pins. To maintain the linearity of the converter, the
+In input should remain within the range of GND – 200mV
to +VCC + 200mV. The –IN input should not drop below
GND – 200mV or exceed GND + 200mV. Outside of these
ranges, the converter’s linearity may not meet specifications.
The analog input is provided to two input pins: +IN and –IN.
When a conversion is initiated, the differential input on these
pins is sampled on the internal capacitor array. While a
conversion is in progress, both inputs are disconnected from
any internal function.
The range of the analog input is set by the voltage on the
VREF pin. With the internal 2.5V reference, the input range
is 0 to 5V. An external reference voltage can be placed on
VREF, overdriving the internal voltage. The range for the
external voltage is 2.0V to 2.55V, giving an input voltage
range of 4.0V to 5.1V.
REFERENCE
The reference voltage on the VREF pin directly sets the full-
scale range of the analog input. The ADS7818 can operate
with a reference in the range of 2.0V to 2.55V, for a full-
scale range of 4.0V to 5.1V.
The digital result of the conversion is provided in a serial
manner, synchronous to the CLK input. The result is pro-
vided most significant bit first and represents the result of
the conversion currently in progress—there is no pipeline
delay. By properly controlling the CONV and CLK inputs,
it is possible to obtain the digital result least significant bit
first.
The voltage at the VREF pin is internally buffered and this
buffer drives the capacitor DAC portion of the converter.
This is important because the buffer greatly reduces the
dynamic load placed on the reference source. However, the
voltage at VREF will still contain some noise and glitches
from the SAR conversion process. These can be reduced by
carefully bypassing the VREF pin to ground as outlined in the
sections that follow.
ANALOG INPUT
The +IN and –IN input pins allow for a differential input
signal to be captured on the internal hold capacitor when the
converter enters the hold mode. The voltage range on the
–IN input is limited to –0.2V to 0.2V. Because of this, the
differential input can be used to reject only small signals that
INTERNAL REFERENCE
The ADS7818 contains an on-board 2.5V reference, result-
ing in a 0V to 5V input range on the analog input. The
specification table gives the various specifications for the
+5V
+
+
ADS7818
2.2µF
0.1µF
0.1µF
10µF
1
2
3
4
VREF
+VCC
CLK
8
7
6
5
0 to 5V
Analog Input
Serial Clock
Serial Data
Convert Start
+In
from
Microcontroller
or DSP
–In
DATA
CONV
GND
FIGURE 1. Basic Operation of the ADS7818.
®
7
ADS7818
internal reference. This reference can be used to supply a
small amount of source current to an external load, but the
load should be static. Due to the internal 10kΩ resistor, a
dynamic load will cause variations in the reference voltage,
and will dramatically affect the conversion result. Note that
even a static load will reduce the internal reference voltage
seen at the buffer input. The amount of reduction depends on
the load and the actual value of the internal “10kΩ” resistor.
The value of this resistor can vary by ±30%.
tCKP
tCKH
tCKL
CLK
tCKDS
tCKDH
DATA
FIGURE 2. Serial Data and Clock Timing.
The VREF pin should be bypassed with a 0.1µF capacitor
placed as close as possible to the ADS7818 package. In
addition, a 2.2 µF tantalum capacitor should be used in
parallel with the ceramic capacitor. Placement of this ca-
pacitor is not as critical.
SYMBOL
DESCRIPTION
MIN
TYP
MAX
UNITS
tACQ
tCONV
tCKP
Acquisition Time
Conversion Time
Clock Period
Clock LOW
350
1.5
125
50
ns
µs
ns
ns
ns
ns
5000
tCKL
EXTERNAL REFERENCE
tCKH
Clock HIGH
50
The internal reference is connected to the VREF pin and to the
internal buffer via a 10kΩ series resistor. Thus, the reference
voltage can easily be overdriven by an external reference
voltage. The voltage range for the external voltage is 2.0V
to 2.55V, corresponding to an analog input range of 4.0V to
5.1V.
tCKDH
Clock Falling to Current Data
Bit No Longer Valid
5
15
30
tCKDS
tCVL
Clock Falling to Next Data Valid
CONV LOW
50
ns
ns
ns
ns
ns
ns
ns
40
40
10
10
tCVH
CONV HIGH
tCKCH
tCKCS
tCKDE
tCKDD
CONV Hold after Clock Falls(1)
CONV Setup to Clock Falling(1)
Clock Falling to DATA Enabled
While the external reference will not source significant
current into the VREF pin, it does have to drive the series
10kΩ resistor that is terminated into the 2.5V internal
reference (the exact value of the resistor will vary up to
±30% from part to part). In addition, the VREF pin should
still be bypassed to ground with at least a 0.1 µF ceramic
capacitor (placed as close to the ADS7818 as possible). The
reference will have to be stable with this capacitive load.
Depending on the particular reference and A/D conversion
speed, additional bypass capacitance may be required, such
as the 2.2µF tantalum capacitor shown in Figure 1.
20
70
50
Clock Falling to DATA
High Impedance
100
tCKSP
tCKPD
tCVHD
Clock Falling to Sample Mode
5
50
5
ns
ns
ns
Clock Falling to Power-down Mode
CONV Falling to Hold Mode
(Aperture Delay)
tCVSP
tCVPU
tCVDD
CONV Rising to Sample Mode
CONV Rising to Full Power-up
5
ns
ns
ns
50
70
CONV Changing State to DATA
High Impedance
100
5
tCVPD
CONV Changing State to
Power-down Mode
50
ns
Reasons for choosing an external reference over the internal
reference vary, but there are two main reasons. One is to
achieve a given input range. For example, a 2.048V refer-
ence provides for a 0V to 4.095V input range—or 1mV per
LSB. The other is to provide greater stability over tempera-
ture. The internal reference is typically 20ppm/°C which
translates into a full-scale drift of roughly 1 output code for
every 12°C (this does not take into account other sources of
full-scale drift). If greater stability over temperature is needed,
then an external reference with lower temperature drift will
be required.
tDRP
CONV Falling to Start of CLK
(for hold droop < 0.1 LSB)
µs
Note: (1) This timing is not required under some situations. See text for more information.
TABLE I. Timing Specifications (TA = –40°C to +85°C,
CLOAD = 30pF).
The asynchronous nature of CONV to CLK raises some
interesting possibilities, but also some design consider-
ations. Figure 3 shows that CONV has timing restraints in
relation to CLK (tCKCH and tCKCS). However, if these times
are violated (which could happen if CONV is completely
asynchronous to CLK), the converter will perform a conver-
sion correctly, but the exact timing of the conversion is
indeterminate. Since the setup and hold time between CONV
and CLK has been violated in this example, the start of
conversion could vary by one clock cycle. (Note that the
start of conversion can be detected by using a pull-up
resistor on DATA. When DATA drops out of high-imped-
ance and goes LOW, the conversion has started and that
clock cycle is this first of the conversion.)
DIGITAL INTERFACE
Figure 2 shows the serial data timing and Figure 3 shows the
basic conversion timing for the ADS7818. The specific
timing numbers are listed in Table I. There are several
important items in Figure 3 which give the converter addi-
tional capabilities over typical 8-pin converters. First, the
transition from sample mode to hold mode is synchronous to
the falling edge of CONV and is not dependent on CLK.
Second, the CLK input is not required to be continuous
during the sample mode. After the conversion is complete,
the CLK may be kept LOW or HIGH.
In addition if CONV is completely asynchronous to CLK
and CLK is continuous, then there is possibility that CLK
will transition just prior to CONV going LOW. If this occurs
®
ADS7818
8
faster than the 10ns indicated by tCKCH, then there is a
chance that some digital feedthrough may be coupled onto
the hold capacitor. This could cause a small offset error for
that particular conversion.
Figure 4 shows the typical method for placing the A/D into
the power-down mode. If CONV is kept LOW during the
conversion and is LOW at the start of the 13 clock cycle,
then the device enters the power-down mode. It remains in
this mode until the rising edge of CONV. Note that CONV
must be HIGH for at least tACQ in order to sample the signal
properly as well as to power-up the internal nodes.
Thus, there are two basic ways to operate the ADS7818.
CONV can be synchronous to CLK and CLK can be con-
tinuous. This would be the typical situation when interfacing
the converter to a digital signal processor. The second
method involves having CONV asynchronous to CLK and
gating the operation of CLK (a non-continuous clock). This
method would be more typical of an SPI-like interface on a
microcontroller. This method would also allow CONV to be
generated by a trigger circuit and to initiate (after some
delay) the start of CLK. These two methods are covered
under DSP Interfacing and SPI Interfacing.
There are two different methods for clocking the ADS7818.
The first involves scaling the CLK input in relation to the
conversion rate. For example, an 8MHz input clock and the
timing shown in Figure 3 results in a 500kHz conversion
rate. Likewise, a 1.6MHz clock would result in a 100kHz
conversion rate. The second method involves keeping the
clock input as close to the maximum clock rate as possible
and starting conversions as needed. This timing is similar to
that shown in Figure 4. As an example, a 50kHz conversion
rate would require 160 clock periods per conversion instead
of the 16 clock periods used at 500kHz.
POWER-DOWN TIMING
The conversion timing shown in Figure 3 does not result in
the ADS7818 going into the power-down mode. If the
conversion rate of the device is high (approaching 500kHz),
then there is very little power that can be saved by using the
power-down mode. However, since the power-down mode
incurs no conversion penalty (the very first conversion is
valid), at lower sample rates, significant power can be saved
by allowing the device to go into power-down mode be-
tween conversions.
The main distinction between the two is the amount of time
that the ADS7818 remains in power down. In the first mode,
the converter only remains in power down for a small
number of clock periods (depending on how many clock
periods there are per each conversion). As the conversion
rate scales, the converter always spends the same percentage
of time in power down. Since less power is drawn by the
digital logic, there is a small decrease in power consump-
tion, but it is very slight. This effect can be seen in the
typical performance curve “Supply Current vs Sample Rate.”
tCVL
tCVCK
CONV
tCKCS
tCKCH
14
15
16
1
2
3
4
11
12
13
14
15
16
1
CLK
(1)
tCKDE
tCKDD
D11
(MSB)
D0
(LSB)
DATA
D10
D9
D2
D1
tACQ
tCVHD
tCKSP
SAMPLE/HOLD
MODE
SAMPLE
HOLD
tCONV
SAMPLE
HOLD
(2)
INTERNAL
CONVERSION
STATE
(
3)
IDLE
IDLE
CONVERSION IN PROGRESS
NOTES: (1) Clock periods 14 and 15 are shown for clarity, but are not required for proper operation of the ADS7818, provided that the
minimum tACQ time is met. The CLK input may remain HIGH or LOW during this period. (2) The transition from sample mode to hold
mode occurs on the falling edge of CONV. This transition is not dependent on CLK. (3) The device remains fully powered when
operated as shown. If the sample time is longer than 3 clock periods, power consumption can be reduced by allowing the device to
enter a power down mode. See the power down timing for more information.
FIGURE 3. Basic Conversion Timing.
®
9
ADS7818
CONV
CLK
1
2
3
12
13
D11
(MSB)
D0
(LSB)
DATA
D10
D1
tCVSP
tACQ
SAMPLE/HOLD
MODE
SAMPLE
HOLD
SAMPLE
HOLD
(3)
INTERNAL
CONVERSION
STATE
IDLE
CONVERSION IN PROGRESS
IDLE
tCVPU
tCKPD
POWER MODE
FULL POWER
LOW POWER
FULL POWER
(1)
(2)
NOTES: (1) The low power mode (“power-down”) is entered when CONV remains LOW during the conversion and is still LOW at the
start of the 13th clock cycle. (2) The low power mode is exited when CONV goes HIGH. (3) When in power-down, the transition from
hold mode to sample mode is initiated by CONV going HIGH.
FIGURE 4. Power-down Timing.
tCVH
CONV
tCKCH
1
2
3
12
13
14
23
24
CLK
tCKCS
tCVDD
D11
(MSB)
D0
(LSB)
D11
(MSB)
DATA
D10
D1
D1
D10
LOW...
(1)
(2)
SAMPLE/HOLD
MODE
SAMPLE
HOLD
INTERNAL
CONVERSION
STATE
IDLE
CONVERSION IN PROGRESS
tCVPD
IDLE
POWER MODE
FULL POWER
LOW POWER
(3)
NOTES: (1) The serial data can be transmitted LSB first by pulling CONV LOW during the 13th clock cycle. (2) After the MSB has been
transmitted, the DATA output pin will remain LOW until CONV goes HIGH. (3) When CONV is taken LOW to initiate the LSB first transfer,
the converter enters the power-down mode.
FIGURE 5. Serial Data “LSB-First” Timing.
In contrast, the second method (clocking at a fixed rate)
means that each conversion takes X clock cycles. As the
time between conversions get longer, the converter remains
in power-down an increasing percentage of time. This re-
duces total power consumption by a considerable amount.
For example, a 50kHz conversion rate results in roughly
1/10 of the power (minus the reference) that is used at a
500kHz conversion rate.
®
ADS7818
10
Table II offers a look at the two different modes of operation
and the difference in power consumption.
the conversion will terminate immediately, before all 12-bits
have been decided. This can be a very useful feature when
a resolution of 12-bits is not needed. An example would be
when the converter is being used to monitor an input voltage
until some condition is met. At that time, the full resolution
of the converter would then be used. Short-cycling the
conversion can result in a faster conversion rate or lower
power dissipation.
POWER WITH
POWER WITH
CLK = 8MHz
fSAMPLE
CLK = 16 • fSAMPLE
500kHz
250kHz
100kHz
11mW
10mW
9mW
11mW
7mW
4mW
There are several very important items shown in Figure 6.
The conversion currently in progress is terminated when
CONV is taken HIGH during the conversion and then taken
LOW prior to tCKCH before the start of the 13th clock cycle.
Note that if CONV goes LOW during the 13th clock cycle,
then the LSB first mode will be entered (Figure 5). Also,
when CONV goes LOW, the DATA output immediately
transitions to high impedance. If the output bit that is present
during that clock period is needed, CONV must not go LOW
until the bit has been properly latched into the receiving
logic.
TABLE II. Power Consumption versus CLK Input.
LSB FIRST DATA TIMING
Figure 5 shows a method to transmit the digital result in a
least-significant bit (LSB) format. This mode is entered
when CONV is pulled HIGH during the conversion (before
the end of the 12th clock) and then pulled LOW during the
13th clock (when D0, the LSB, is being transmitted). The
next 11 clocks then repeat the serial data, but in an LSB first
format. The converter enters the power-down mode during
the 13th clock and resumes normal operation when CONV
goes HIGH.
DATA FORMAT
The ADS7818 output data is in straight binary format as
shown in Figure 7. This figure shows the ideal output code
for the given input voltage and does not include the effects
of offset, gain, or noise.
SHORT-CYCLE TIMING
The conversion currently in progress can be “short-cycled”
with the technique shown in Figure 6. This term means that
tCVL
(1)
CONV
tCVH
1
2
3
4
5
6
7
CLK
tCVDD
D11
(MSB)
DATA
D10
D9
D8
D7
D6
SAMPLE/HOLD
MODE
SAMPLE
HOLD
INTERNAL
CONVERSION
STATE
IDLE
CONVERSION IN PROGRESS
FULL POWER
IDLE
tCVPD
POWER MODE
LOW POWER
NOTE: (1) The conversion currently in progress can be stopped by pulling CONV LOW during the conversion. This must occur at
least tCKCS prior to the start of the 13th clock cycle. The DATA output pin will tri-state and the device will enter the power-down
mode when CONV is pulled LOW.
FIGURE 6. Short-cycle Timing.
®
11
ADS7818
microcontrollers form various manufacturers. CONV would
be tied to a general purpose I/O pin (SPI) or to a PCX pin
(QSPI), CLK would be tied to the serial clock, and DATA
would be tied to the serial input data pin such as MISO
(master in slave out).
FS = Full-Scale Voltage = 2 • VREF
1 LSB = FS/4096
1 LSB
11...111
11...110
11...101
Note the time tDRP shown in Figure 9. This represents the
maximum amount of time between CONV going LOW and
the start of the conversion clock. Since CONV going LOW
places the sample and hold in the hold mode and because the
hold capacitor looses charge over time, there is a require-
ment that time tDRP be met as well as the maximum clock
period (tCKP).
00...010
00...001
00...000
LAYOUT
(1)
0V
4.999V
(2)
For optimum performance, care should be taken with the
physical layout of the ADS7818 circuitry. This is particu-
larly true if the CLK input is approaching the maximum
input rate.
Input Voltage (V)
NOTES: (1) For external reference, value is 2 • VREF – 1 LSB. (2) Voltage
at converter input: +IN–(–IN).
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connec-
tions, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “win-
dows” in which large external transient voltages can easily
affect the conversion result. Such glitches might originate
from switching power supplies, nearby digital logic, and
high power devices. The degree of error in the digital output
depends on the reference voltage, layout, and the exact
timing of the external event. The error can change if the
external event changes in time with respect to the CLK
input.
FIGURE 7. Ideal Input Voltages and Output Codes.
DSP INTERFACING
Figure 8 shows a timing diagram that might be used with a
typical digital signal processor such as a TI DSP. For the
buffered serial port (BSP) on the TMS320C54X family,
CONV would tied to BFSX, CLK would be tied to BCLKX,
and DATA would be tied to BDR.
SPI/QSPI INTERFACING
Figure 9 shows the timing diagram for a typical serial
peripheral interface (SPI) or queued serial peripheral inter-
face (QSPI). Such interfaces are found on a number of
CONV
15
16
1
2
3
12
13
14
15
16
1
2
3
4
CLK
D11
(MSB)
D0
(LSB)
D11
(MSB)
DATA
D10
D1
D10
D9
FIGURE 8. Typical DSP Interface Timing.
tDRP
tACQ
CONV
1
2
3
4
13
14
15
16
1
2
3
CLK
D11
(MSB)
D0
(LSB)
D11
(MSB)
DATA
D10
D1
FIGURE 9. Typical SPI/QSPI Interface Timing.
®
ADS7818
12
With this in mind, power to the ADS7818 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the device as possible. In addition, a
1µF to 10µF capacitor is recommended. If needed, an even
larger capacitor and a 5Ω or 10Ω series resistor my be used
to lowpass filter a noisy supply.
capacitor. An additional larger capacitor may also be used,
if desired. If the reference voltage is external and originates
from an op-amp, make sure that it can drive the bypass
capacitor or capacitors without oscillation.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power supply
entry point. The ideal layout will include an analog ground
plane dedicated to the converter and associated analog
circuitry.
The ADS7818 draws very little current from an external
reference on average as the reference voltage is internally
buffered. However, glitches from the conversion process
appear at the VREF input and the reference source must be
able to handle this. Whether the reference is internal or
external, the VREF pin should be bypassed with a 0.1µF
®
13
ADS7818
PACKAGE OPTION ADDENDUM
www.ti.com
16-Aug-2012
PACKAGING INFORMATION
Status (1)
Eco Plan (2)
MSL Peak Temp (3)
Samples
Orderable Device
Package Type Package
Drawing
Pins
Package Qty
Lead/
Ball Finish
(Requires Login)
ADS7818E/250
ADS7818E/250G4
ADS7818E/2K5
ADS7818E/2K5G4
ADS7818EB/250
ADS7818EB/250G4
ADS7818EB/2K5
ADS7818EB/2K5G4
ADS7818P
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
VSSOP
VSSOP
VSSOP
VSSOP
VSSOP
VSSOP
VSSOP
VSSOP
PDIP
DGK
DGK
DGK
DGK
DGK
DGK
DGK
DGK
P
8
8
8
8
8
8
8
8
8
8
8
8
250
250
2500
2500
250
250
2500
2500
50
Green (RoHS
& no Sb/Br)
CU NIPDAUAGLevel-2-260C-1 YEAR
Green (RoHS
& no Sb/Br)
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAUAGLevel-2-260C-1 YEAR
CU NIPDAU N / A for Pkg Type
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
Green (RoHS
& no Sb/Br)
ADS7818PB
PDIP
P
50
Green (RoHS
& no Sb/Br)
CU NIPDAU N / A for Pkg Type
ADS7818PBG4
ADS7818PG4
PDIP
P
50
Green (RoHS
& no Sb/Br)
CU NIPDAU N / A for Pkg Type
PDIP
P
50
Green (RoHS
& no Sb/Br)
CU NIPDAU N / A for Pkg Type
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Addendum-Page 1
PACKAGE OPTION ADDENDUM
www.ti.com
16-Aug-2012
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
16-Aug-2012
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
B0
K0
P1
W
Pin1
Diameter Width (mm) (mm) (mm) (mm) (mm) Quadrant
(mm) W1 (mm)
ADS7818E/250
ADS7818E/2K5
ADS7818EB/250
ADS7818EB/2K5
VSSOP
VSSOP
VSSOP
VSSOP
DGK
DGK
DGK
DGK
8
8
8
8
250
2500
250
180.0
330.0
180.0
330.0
12.4
12.4
12.4
12.4
5.3
5.3
5.3
5.3
3.4
3.4
3.4
3.4
1.4
1.4
1.4
1.4
8.0
8.0
8.0
8.0
12.0
12.0
12.0
12.0
Q1
Q1
Q1
Q1
2500
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
16-Aug-2012
*All dimensions are nominal
Device
Package Type Package Drawing Pins
SPQ
Length (mm) Width (mm) Height (mm)
ADS7818E/250
ADS7818E/2K5
ADS7818EB/250
ADS7818EB/2K5
VSSOP
VSSOP
VSSOP
VSSOP
DGK
DGK
DGK
DGK
8
8
8
8
250
2500
250
210.0
367.0
210.0
367.0
185.0
367.0
185.0
367.0
35.0
35.0
35.0
35.0
2500
Pack Materials-Page 2
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