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AD7707 Datasheet(PDF) 46 Page - Analog Devices

Part No. AD7707
Description  3-Channel 16-Bit, Sigma-Delta ADC
Download  53 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7707 Datasheet(HTML) 46 Page - Analog Devices

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AD7707
Rev. B | Page 45 of 52
PRESSURE MEASUREMENT
Other typical applications for the AD7707 include temperature
and pressure measurement. Figure 28 shows the AD7707 used
with a pressure transducer, the BP01 from Sensym. The pressure
transducer is arranged in a bridge network and gives a differential
output voltage between its OUT(+) and OUT(−) terminals.
With rated full-scale pressure (in this case 300 mmHg) on the
transducer, the differential output voltage is 3 mV/V of the input
voltage (that is, the voltage between its IN(+) and IN(−) termin-
als). Assuming a 5 V excitation voltage, the full-scale output
range from the transducer is 15 mV. The low level input channels
are ideal for this type of low signal measurement application.
The excitation voltage for the bridge is also used to generate the
reference voltage for the AD7707. Therefore, variations in the
excitation voltage do not introduce errors in the system. Choosing
resistor values of 24 kΩ and 15 kΩ, as per Figure 28, gives a 1.92 V
reference voltage for the AD7707 when the excitation voltage is 5 V.
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7707 being 15 mV, which
corresponds with the output span from the transducer.
AD7707
DVDD
5V
REF IN(–)
AGND
MCLK IN
MCLK OUT
DRDY
REF IN(+)
24kΩ
AVDD
AIN1
LOCOM
DGND
OUT(–)
SCLK
DIN
DOUT
CS
CONTROLLER
15kΩ
OUT(+)
EXCITATION VOLTAGE = 5V
IN(–)
IN(+)
Figure 28. Pressure Measurement Using the AD7707
THERMOCOUPLE MEASUREMENT
Another application area for the AD7707 is in temperature
measurement. Figure 29 outlines a connection from a thermo-
couple to the AD7707. In this application, the AD7707 is operated
in its unbuffered mode to accommodate signals of ±100 mV on
the front end. Cold conjunction compensation is implemented
using the AD590 temperature transducer that produces an
output current proportional to absolute temperature.
AD7707
DVDD
5V
AIN2
REF IN(–)
AGND
MCLK IN
MCLK OUT
DRDY
REF IN(+)
8.2kΩ
CJC
AVDD
AD590
5V
AIN1
LOCOM
DGND
OUT
SCLK
DIN
DOUT
THERMOCOUPLE
JUNCTION
CS
CONTROLLER
REF192
5V
GND
Figure 29. Thermocouple Measurement Using the AD7707
RTD MEASUREMENT
Figure 30 shows another temperature measurement application
for the AD7707. In this case, the transducer is an RTD (resistive
temperature device), a PT100. The arrangement is a 4-lead RTD
configuration. There are voltage drops across the lead
resistances, RL1 and RL4, but these simply shift the common-
mode voltage. There is no voltage drop across lead resistances
RL2 and RL3 because the input current to the AD7707 is very low.
The lead resistances present a small source impedance so it is
generally not necessary to turn on the buffer on the AD7707.
If the buffer is required, the common-mode voltage should be
set accordingly by inserting a small resistance between the bottom
end of the RTD and GND of the AD7707. In the application
shown, an external 400 μA current source provides the excitation
current for the PT100 and generates the reference voltage
the AD7707 via the 6.25 kΩ resistor. Variations in the excitation
current do not affect the circuit because both the input voltage
and the reference voltage vary radiometrically with the
excitation current. However, the 6.25 kΩ resistor must have a low
temperature coefficient to avoid errors in the reference voltage over
temperature.
AD7707
DVDD
5V
REF IN(–)
AGND
MCLK IN
MCLK OUT
DRDY
RTD
AVDD
AIN1
LOCOM
DGND
SCLK
DIN
DOUT
CS
CONTROLLER
RL3
6.25kΩ
RL2
RL4
RL1
REF IN(+)
400µA
REF IN(+)
REF IN(–)
Figure 30. RTD Measurement Using the AD7707


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