AD7792/AD7793
Rev. B | Page 28 of 32
APPLICATIONS INFORMATION
The AD7792/AD7793 provide a low cost, high resolution
analog-to-digital function. Because the analog-to-digital
function is provided by a ∑-Δ architecture, the parts are more
immune to noisy environments, making them ideal for use in
sensor measurement and industrial and process control
applications.
TEMPERATURE MEASUREMENT USING A
THERMOCOUPLE
Figure 20 outlines a connection from a thermocouple to the
AD7792/AD7793. In a thermocouple application, the voltage
generated by the thermocouple is measured with respect to an
absolute reference, so the internal reference is used for this
conversion. The cold junction measurement uses a ratiometric
configuration, so the reference is provided externally.
Because the signal from the thermocouple is small, the
AD7792/AD7793 are operated with the in-amp enabled to
amplify the signal from the thermocouple. As the input channel
is buffered, large decoupling capacitors can be placed on the
front end to eliminate any noise pickup that may be present in
the thermocouple leads. The AD7792/AD7793 have a reduced
common-mode range with the in-amp enabled, so the bias
voltage generator provides a common-mode voltage so that the
voltage generated by the thermocouple is biased up to AV
DD
/2.
The cold junction compensation is performed using a thermis-
tor in the diagram. The on-chip excitation current supplies the
thermistor. In addition, the reference voltage for the cold
junction measurement is derived from a precision resistor in
series with the thermistor. This allows a ratiometric measure-
ment so that variation of the excitation current has no effect on
the measurement (it is the ratio of the precision reference
resistance to the thermistor resistance that is measured).
04855-012
DOUT/RD
DIN
SCLK
CS
DV
DD
SERIAL
INTERFACE
AND
CONTROL
LOGIC
Σ-Δ
ADC
AD7792/AD7793
AIN2(+)
REFIN(+)
REFIN(–)
AIN2(–)
AV
DD
GND
MUX
BAND GAP
REFERENCE
INTERNAL
CLOCK
CLK
GND
GND
DD
AV
DD
IN-AMPBUF
REFIN(+) REFIN(–)
V
BIAS
AIN1(+)
AIN1(–)
R
R
HERMOCOUPLE
JUNCTION
C
R
REF
IOUT2
Figure 20. Thermocouple Measurement Using the AD7792/AD7793
