AD7711 Datasheet, PDF (1/28 Pages) Analog Devices – LC2MOS Signal Conditioning ADC with RTD Excitation Currents

a
LC²MOS Signal Conditioning ADC
with RTD Excitation Currents
AD7711*
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
؎0.0015% Nonlinearity
Two-Channel Programmable Gain Front End
Gains from 1 to 128
One Differential Input
One 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 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.
The part features one differential analog input and one single
ended 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 V_SS pin to AGND) provided that the
input signals on the analog inputs are more positive than
–30 mV. By taking the V_SS pin negative, the part can convert
signals down to –V_REF on its inputs. The part provides two
current sources that can be used to provide excitation in three-
wire and four-wire RTD configurations. The AD7711 thus
performs all signal conditioning and conversion for a single or
dual channel system.
The AD7711 is ideal for use in smart, microcontroller based
systems. Gain settings, signal polarity, input channel selection
*Protected by U.S. Patent No. 5,134,401.
REV. F
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.
FUNCTIONAL BLOCK DIAGRAM
AV_DD
REF REF
DV_{DD IN (–)} IN (+)
V_BIAS
REF OUT
AV_DD
AIN1(+)
AIN1(–)
AIN2
4.5␮A
2.5V REFERENCE
CHARGE-BALANCING A/D
CONVERTER
M
PGA
U
AUTO-ZEROED
MODULATOR
DIGITAL
FILTER
X
A = 1 – 128
RTD1
RTD2
200␮A AV_DD
200␮A
CLOCK
GENERATION
SERIAL INTERFACE
CONTROL
REGISTER
OUTPUT
REGISTER
AD7711
SYNC
MCLK
IN
MCLK
OUT
AGND DGND V_SS RFS TFS MODE SDATA SCLK DRDY A0
and RTD current control can be configured in software using
the bidirectional serial port. The AD7711 contains self-
calibration, 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 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, 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 three-wire and
four-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 which 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.
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© Analog Devices, Inc., 1998