REV. F
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AD7712
LC
2
MOS
Signal Conditioning ADC
FUNCTIONAL BLOCK DIAGRAM
CLOCK
GENERATION
SERIAL INTERFACE
OUTPUT
REGISTER
CHARGE-BALANCING A/D
CONVERTER
AUTO-ZEROED
⌺–⌬
MODULATOR
DIGITAL
FILTER
AD7712
AGND
DGND
MODE SDATA SCLK
A0
MCLK
OUT
MCLK
IN
AIN1(–)
REF
IN (–)
REF
IN (+)
SYNC
4.5A
A = 1 – 128
DRDY
TFSRFS
REF OUT
V
BIAS
VOLTAGE
ATTENUATION
AIN2
TP
STANDBY
CONTROL
REGISTER
V
SS
2.5V REFERENCE
DV
DD
AV
DD
AV
DD
M
U
X
AIN1(+)
PGA
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
ⴞ0.0015% Nonlinearity
High Level and Low Level Analog Input Channels
Programmable Gain for Both Inputs
Gains from 1 to 128
Differential Input for Low Level Channel
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
Bidirectional Microcontroller Serial Interface
Internal/External Reference Option
Single- or Dual-Supply Operation
Low Power (25 mW typ) with Power-Down Mode
(100 W typ)
APPLICATIONS
Process Control
Smart Transmitters
Portable Industrial Instruments
GENERAL DESCRIPTION
The AD7712 is a complete analog front end for low frequency
measurement applications. The device has two analog input
channels and accepts either low level signals directly from a trans-
ducer or high level (±4 ⫻ V
REF
) signals, and outputs a serial
digital word. It employs a sigma-delta conversion technique to
realize up to 24 bits of no missing codes performance. The low
level input signal is applied to a proprietary programmable gain
front end based around an analog modulator. The high level
analog input is attenuated before being applied to the same
modulator. The modulator output is processed by an on-chip
digital filter. The first notch of this digital filter can be programmed
via the on-chip control register, allowing adjustment of the filter
cutoff and settling time.
Normally, one of the channels will be used as the main channel
with the second channel used as an auxiliary input to periodi-
cally measure a second voltage. The part can be operated from a
single supply (by tying the V
SS
pin to AGND), provided that the
input signals on the low level analog input are more positive
than –30 mV. By taking the V
SS
pin negative, the part can con-
vert signals down to –V
REF
on this low level input. This low level
input, as well as the reference input, features differential input
capability.
The AD7712 is ideal for use in smart, microcontroller based
systems. Input channel selection, gain settings, and signal polar-
ity can be configured in software using the bidirectional serial
port. The AD7712 also contains self-calibration, system calibra-
tion, and background calibration options, and allows the user to
read and to write the on-chip calibration registers.
CMOS construction ensures low power dissipation, and a hard-
ware programmable power-down mode reduces the standby power
consumption to only 100 µW typical. The part is available in a
24-lead, 0.3 inch wide, plastic and hermetic dual-in-line pack-
age (DIP), as well as a 24-lead small outline (SOIC) package.
PRODUCT HIGHLIGHTS
1. The low level analog input channel allows the AD7712 to
accept input signals directly from a strain gage or transducer,
removing a considerable amount of signal conditioning. To
maximize the flexibility of the part, the high level analog
input accepts signals of ±4 ⫻ V
REF
/GAIN.
2. The AD7712 is ideal for microcontroller or DSP processor
applications with an on-chip control register that allows
control over filter cutoff, input gain, channel selection, signal
polarity, and calibration modes.
3. The AD7712 allows the user to read and to write the on-chip
calibration registers. This means that the microcontroller has
much greater control over the calibration procedure.
4. No missing codes ensures true, usable, 23-bit dynamic range
coupled with excellent ±0.0015% accuracy. The effects of
temperature drift are eliminated by on-chip self-calibration,
which removes zero-scale and full-scale errors.
