AD7707 Analog Devices, AD7707 Datasheet - Page 45

• Manufacturer Part Number: AD7707
• Manufacturer: Analog Devices

Specifications of AD7707

• Resolution (bits): 16bit
• # Chan: 3
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Bip
• Ain Range: Bip (Vref)/(PGA Gain),Bip 10V,Bip 5.0V,Uni (Vref)/(PGA Gain),Uni 10V,Uni 5.0V
• Adc Architecture: Sigma-Delta
• Pkg Type: SOIC,SOP

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.
OUT(–)
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.
EXCITATION VOLTAGE = 5V
IN(–)
IN(+)
Figure 28. Pressure Measurement Using the AD7707
OUT(+)
15kΩ
24kΩ
5V
AIN1
LOCOM
REF IN(+)
REF IN(–)
AGND
DGND
AV
AD7707
DD
DV
DOUT
DRDY
SCLK
DD
DIN
CS
MCLK OUT
MCLK IN
CONTROLLER
Rev. B | Page 45 of 52
THERMOCOUPLE
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, R
mode voltage. There is no voltage drop across lead resistances
R
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.
L2
and R
RTD
JUNCTION
Figure 29. Thermocouple Measurement Using the AD7707
REF192
6.25kΩ
L3
5V
R
R
R
R
because the input current to the AD7707 is very low.
L1
L2
L3
L4
Figure 30. RTD Measurement Using the AD7707
GND
L1
CJC
and R
400µA
REF IN(+)
REF IN(–)
5V
OUT
AD590
L4
8.2kΩ
5V
, but these simply shift the common-
REF IN(+)
REF IN(–)
AIN1
LOCOM
AGND
DGND
AV
5V
AD7707
DD
LOCOM
AIN2
REF IN(+)
REF IN(–)
AGND
DGND
AIN1
AV
AD7707
DV
DD
DRDY
DOUT
SCLK
DD
DIN
CS
DV
DOUT
DRDY
SCLK
DD
DIN
MCLK OUT
CS
MCLK IN
CONTROLLER
MCLK OUT
MCLK IN
CONTROLLER
AD7707