AD7731 Analog Devices, AD7731 Datasheet - Page 40

• Manufacturer Part Number: AD7731
• Description: Low Noise, High Throughput 24-Bit Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7731

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,(Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

Pressure Measurement
One typical application of the AD7731 where it is connected
directly to a transducer is in pressure measurement. Figure 22
shows the AD7731 with a pressure transducer in a bridge
arrangement. The differential output from the transducer is
connected directly to the AIN1/AIN2 input channel. The entire
circuit is powered from a single +5 V supply that generates the
excitation voltage for the transducer and the power supply, and
reference voltage for the AD7731. The application is ratiometric
and variations in the excitation voltage do not introduce errors
in the measurement.
Temperature Measurement
Another application area where the transducer can be connected
directly to the AD7731 is in temperature measurement. Figure
23 outlines a connection between a thermocouple and the
AD7731. In order to place the differential voltage from the
AD7731 on a suitable common-mode voltage, the AIN2 input
of the AD7731 is biased up at the reference voltage, +2.5 V.
AD7731
OUT–
IN–
IN+
EXCITATION VOLTAGE = +5V
OUT+
REF IN (+)
REF IN (–)
Figure 22. Pressure Measurement Using the AD7731
AGND
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AGND
AV
DD
DGND
100nA
100nA
AV
BUFFER
DD
AND CONTROL LOGIC
PGA
SERIAL INTERFACE
–40–
DV
DOUT
DD
Figure 24 shows another temperature measurement application
for the AD7731. In this case, the temperature transducer is an
RTD (Resistive Temperature Device), a PT100. The arrange-
ment is a four-lead RTD configuration. There are voltage drops
across lead resistances RL1 and RL4 and across resistor R2 but
these simply shift the common-mode voltage. Resistor R2 is
required to set the common-mode voltage within the allowable
range for the AD7731. The voltage differential caused by RL2
and RL3 and the AD7731’s offset current is negligible.
In the application shown, the external 400 A current source
provides the excitation current for the PT100 and it also gener-
ates the reference voltage for the AD7731 via resistor R1. Varia-
tions in the excitation current do not affect the circuit as the
input voltage and the reference voltage vary ratiometrically with
the excitation current. Resistor R1, however, must have a low
temperature coefficient to avoid errors in the reference voltage
over temperature.
REGISTER BANK
MODULATOR
DIN
SIGMA-
DELTA
SIGMA-DELTA A/D
AD7731
CONVERTER
CS
GENERATION
CLOCK
DIGITAL
FILTER
SCLK
STANDBY
SYNC
MCLK IN
MCLK OUT
RESET
RDY
POL
REV. A
REV. 0