Reference
Analog Terminal Board
Thermocouple Cold Junction Compensation, CJC.
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Much of the following information was gleaned from the
Analog Devices "Transducer Interfacing Handbook - A Guide to
Signal Conditioning." Edited by Daniel H. Sheingold ISBN 0-
916550-05-2, see especially Chapter 7 "Thermoswitches and
Thermocouples". This is a tremendously good reference book
about all sorts of signal acquistion and conditioning.
The circuitry to support thermocouples attached to the
AD1200 series boards consists of two parts:- the cold junction
compensation, CJC, circuitry and the open circuit detection for
each individual thermocouple attached. Thermocouples are
inherently differential, so the AD1200 card should be configured
for 8 differential inputs, DI, and so the 10k Ohm Differential
return path resistors should be fitted.
Open Circuit Detection.
Thermocouples are used to measure temperatures and are so
subject to heating and cooling stress, in addition they are usually
in HOT places and so not available for easy inspection. If a
break occurs in the thermocouple or its connecting lead it may
be difficult to tell. The open circuit detection is a simple but
effective method of telling that the thermocouple circuit has a
break somewhere. A large pull up resistor is attached to one end
of the thermocouple. The other end of the resistor is attached to
a bias voltage, VBIAS of 2.4 Volts. A break in the line causes
the A/D input to go to full scale. When a full scale input is read
by the computer, the user knows that the thermocouple
connection has broken.
Open circuit detection is sometimes known as up line break
detection.
Resistor R20, 10k and Zener diode ZD1 of 2.4Volts
connected b12 and Analog ground generate the bias
voltage. Resistors R26, R27, R28, R29, R30, R31, R32 and R33,
one each for channels 0 to 7 respectively, are the 100 Meg Ohm
bias pull up resistors. See Figure 1-3. Only resistors for the
number of channels actually being used in this mode need be
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