LC2MOS Signal Conditioning ADC
with RTD Excitation Currents
AD7711
FEATURES
Charge-Balancing ADC
24 Bits, No Missing Codes
؎0.0015% Nonlinearity
2-Channel Programmable Gain Front End
Gains from 1 to 128
1 Differential Input
1 Single-Ended Input
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
RTD Excitation Current Sources
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)
APPLICATIONS
RTD Transducers
Process Control
Smart Transmitters
Portable Industrial Instruments
GENERAL DESCRIPTION
The AD7711 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 ⌺-⌬ 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 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.
The part features one differential analog input and one singleended analog input as well as a differential reference input.
Normally, one of the input channels will be used as the main
channel with the second channel used as an auxiliary 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 provides two current
sources that can be used to provide excitation in 3-wire and 4-wire
RTD configurations. The AD7711 thus performs all signal
conditioning and conversion for a single- or dual-channel system.
REV. G
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 that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
FUNCTIONAL BLOCK DIAGRAM
AVDD
REF
IN (–)
DVDD
REF
IN (+)
VBIAS
REF OUT
AVDD
2.5V REFERENCE
4.5A
CHARGE-BALANCING A/D
CONVERTER
AIN1(+)
M
U
X
AIN1(–)
AUTO-ZEROED
⌺-⌬
MODULATOR
PGA
DIGITAL
FILTER
SYNC
A = 1–128
AIN2
200A
CLOCK
GENERATION
AVDD
RTD1
MCLK
IN
MCLK
OUT
SERIAL INTERFACE
200A
CONTROL
REGISTER
RTD2
OUTPUT
REGISTER
AD7711
AGND DGND
VSS
RFS TFS MODE SDATA SCLK DRDY A0
The AD7711 is ideal for use in smart, microcontroller based
systems. Gain settings, signal polarity, input channel selection,
and RTD current control can be configured in software using
the bidirectional serial port. The AD7711 contains selfcalibration, system calibration, and background calibration
options, and also allows the user to read and write the on-chip
calibration registers.
CMOS construction ensures low power dissipation, and a software
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, plastic and hermetic dual-in-line package (DIP) as well as a 24-lead small outline (SOIC) package.
PRODUCT HIGHLIGHTS
1. The programmable gain front end allows the AD7711 to
accept input signals directly from an RTD transducer,
removing a considerable amount of signal conditioning.
On-chip current sources provide excitation for 3-wire and
4-wire RTD configurations.
2. No missing codes ensure 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.
3. The AD7711 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.
4. The AD7711 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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© 2004 Analog Devices, Inc. All rights reserved.
