LC2MOS Signal Conditioning ADC
with RTD Current Source
a
AD7711A*
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
؎0.0015% Nonlinearity
Two-Channel Programmable Gain Front End
Gains from 1 to 128
REF
REF
IN (–)
AV
DV
DD
V
IN (+)
DD
REF OUT
BIAS
AV
DD
2.5V REFERENCE
4.5A
Differential Inputs
CHARGE-BALANCING A/D
CONVERTER
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
(7 mW typ)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
AUTO-ZEROED
DIGITAL
SYNC
M
U
X
PGA
A = 1 – 128
FILTER
MODULATOR
MCLK
IN
CLOCK
GENERATION
AV
DD
MCLK
OUT
400A
SERIAL INTERFACE
OUTPUT
APPLICATIONS
RTD Transducers
CONTROL
REGISTER
REGISTER
RTD
CURRENT
GENERAL DESCRIPTION
AD7711A
The AD7711A is a complete analog front end for low frequency
measurement applications. The device accepts low level signals
directly from a transducer 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 input signal is
applied to a proprietary programmable gain front end based
around an analog modulator. The modulator output is pro-
cessed by an on-chip digital filter. The first notch of this digital
filter can be programmed via the on-chip control register allow-
ing adjustment of the filter cutoff and settling time.
AGND DGND
MODE SDATA SCLK
A0
DRDY
V
RFS TFS
SS
CMOS construction ensures low power dissipation, and a soft-
ware programmable power-down mode reduces the standby
power consumption to only 7 mW typical. The part is available
in a 24-lead, 0.3 inch-wide, hermetic dual-in-line package
(cerdip) as well as a 24-lead small outline (SOIC) package.
PRODUCT HIGHLIGHTS
The part features two differential analog inputs and a differen-
tial reference input. Normally, one of the channels will be used
as the main channel with the second channel used as an auxil-
iary input to periodically measure a second voltage. It can be
operated from a single supply (by tying the VSS pin to AGND)
provided that the input signals on the analog inputs are more
positive than –30 mV. By taking the VSS pin negative, the part
can convert signals down to –VREF on its inputs. The part also
provides a 400 µA current source that can be used to provide
excitation for RTD transducers. The AD7711A thus performs
all signal conditioning and conversion for a single or dual chan-
nel system.
1. The programmable gain front end allows the AD7711A
to accept input signals directly from an RTD transducer,
removing a considerable amount of signal conditioning. An
on-chip current source provides the excitation current for
the RTD.
2. The part features excellent static performance specifications
with 24-bit no missing codes, ±0.0015% accuracy and low
rms noise (<250 nV). Endpoint errors and the effects of
temperature drift are eliminated by on-chip calibration op-
tions, which remove zero-scale and full-scale errors.
3. The AD7711A 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, RTD current control and calibration modes.
The AD7711A 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 AD7711A contains self-calibration, system calibration
and background calibration options and also allows the user to
read and write the on-chip calibration registers.
4. The AD7711A allows the user to read and to write the
on-chip calibration registers. This means that the micro-
controller has much greater control over the calibration
procedure.
*Protected by U.S. Patent No. 5,134,401.
REV. C
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
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1998
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