a
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
0.0015% 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
AV
DD
LC
2
MOS
Signal Conditioning ADC
AD7712*
FUNCTIONAL BLOCK DIAGRAM
REF REF
AV
DD
DV
DD
IN (–) IN (+)
V
BIAS
REF OUT
2.5V REFERENCE
4.5 A
CHARGE-BALANCING A/D
CONVERTER
PGA
A = 1 – 128
AIN2
VOLTAGE
ATTENUATION
CLOCK
GENERATION
SERIAL INTERFACE
MCLK
IN
MCLK
OUT
AUTO-ZEROED
–
MODULATOR
DIGITAL
FILTER
SYNC
STANDBY
AIN1(+)
AIN1(–)
M
U
X
TP
AD7712
CONTROL
REGISTER
OUTPUT
REGISTER
AGND DGND V
SS
RFS TFS
MODE SDATA SCLK
DRDY
A0
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
transducer 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 pro-
grammed 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 also 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 avail-
able in a 24-lead, 0.3 inch wide, plastic and hermetic dual-in-
line package (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.
*Protected
by U.S. Patent No. 5,134,401.
REV. E
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998
AD [ ANALOG DEVICES ]
