AD7731BRUZ Datasheet, PDF (39/44 Pages) Analog Devices – Low Noise, High Throughput 24-Bit Sigma-Delta ADC

AD7731
APPLICATIONS
The on-chip PGA allows the AD7731 to handle analog input
voltage ranges from 20 mV to 1.28 V. This makes the AD7731
suitable for a range of application areas from handling signals
directly from a transducer to processing fully-conditioned full-
scale inputs. Some of these applications are discussed in the
following sections.
The AD7731 offers both unipolar and bipolar input ranges. In
many cases, the application is single supply with the bipolar
input voltages referenced to a biased-up differential voltage.
Some applications will, however, require the flexibility of han-
dling true bipolar inputs. Figure 25 shows how to configure the
AD7731 to handle this type of signal.
It should be noted in multiplexed applications that an input
overvoltage (either >AVDD + 0.3 V or <AGND – 0.3 V) on an
unselected channel can affect the conversion result on the se-
lected channel. The system design should ensure that the input
voltage on channels where input leads may be unconnected or
broken be kept within the above limits.
The AD7731 has a variety of different modes aimed at optimiz-
ing the AD7731’s performance across differing application require-
ments. The issue of filtering and settling time and throughput rates
in multichannel applications has previously been discussed in the
Filter Architecture section.
Data Acquisition
The AD7731 with its three differential channels (or five pseudo-
differential channels) is suited to low bandwidth, high resolution
data acquisition systems. In addition, the three-wire digital
interface allows this data acquisition front end to be isolated
with just three optoisolators. The entire system can be operated
from a single +5 V supply provided that the input signals to the
AD7731’s analog inputs are all of positive polarity. Figure 21
shows the AD7731 in an isolated three-channel data acquisition
system.
Programmable Logic Controllers
The AD7731 is also suited to programmable logic controller
applications. In such applications, the ADC is required to
handle signals from a variety of different transducers. The
AD7731’s programmable gain front end allows the part to either
handle low level signals directly from a transducer or full-scale
signals which have already been conditioned. The fast through-
put rate and settling-time of the part is also an important feature
in these applications where loop response time is often critical.
The configuration of the AD7731 in PLC applications is similar
to that outlined in Figure 21 for the data acquisition system.
IN1+
IN1–
IN2+
IN2–
IN3+
IN3–
+5V
+V_IN
AD780 ^VOUT
GND
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
REF IN (+)
REF IN (–)
AGND
+5V
AV_DD
AV_DD
100nA
DV_DD
BUFFER
PGA
AD7731
SIGMA-DELTA A/D
CONVERTER
SIGMA-
DELTA
MODULATOR
DIGITAL
FILTER
100nA
AGND
SERIAL INTERFACE
AND CONTROL LOGIC
CLOCK
GENERATION
REGISTER BANK
DGND
DOUT DIN
CS
SCLK
STANDBY
SYNC
MCLK IN
MCLK OUT
RESET
RDY
POL
MICROCONTROLLER
OPTO-ISOLATORS
Figure 21. Data Acquisition Using the AD7731
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