AD7711A_15 Datasheet, PDF (1/28 Pages) Analog Devices – LC MOS Signal Conditioning ADC

a
LC²MOS Signal Conditioning ADC
with RTD Current Source
AD7711A^*
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
؎0.0015% Nonlinearity
2-Channel Programmable Gain Front End
Gains from 1 to 128
Differential Inputs
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)
APPLICATIONS
RTD Transducers
GENERAL DESCRIPTION
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.
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 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 also
provides a 400 mA 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-
channel system.
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 by using the bidirectional serial
port. The AD7711A contains self-calibration, system calibra-
tion, and background calibration options and allows the user to
read and write the on-chip calibration registers.
*Protected by U.S. Patent No. 5,134,401.
FUNCTIONAL BLOCK DIAGRAM
AV_DD
REF REF
DV_{DD IN (–)} IN (+)
V_BIAS
REF OUT
AV_DD
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
4.5␮A
2.5V REFERENCE
CHARGE-BALANCING A/D
CONVERTER
M
PGA
U
AUTO-ZEROED
⌺-⌬
MODULATOR
DIGITAL
FILTER
X
A = 1 – 128
AV_DD
CLOCK
GENERATION
400␮A
RTD
CURRENT
AD7711A
SERIAL INTERFACE
CONTROL
REGISTER
OUTPUT
REGISTER
SYNC
MCLK
IN
MCLK
OUT
AGND DGND V_SS RFS TFS MODE SDATA SCLK DRDY A0
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
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
options, 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.
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.
REV. D
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