4-4
Cabling and Wiring
Cold Junction Compensation (CJC) Temperature Sensor
Both the STC-TC/B and STA-TC/B screw terminal accessories include a
CJC temperature sensor. The CJC sensor and the screw-terminal blocks
of the STC-TC/B are both mounted on an isothermal bar; a metal plate
that keeps the screw terminals and the CJC sensor at the same tempera-
ture. Figure 4-3 shows the schematic for the CJC temperature sensor.
Figure 4-3. Schematic for CJC Temperature Sensor Circuit
The CJC sensor consists of an Analog Devices AD592CN integrated
circuit in series with a 20k
Ω
,
±
0.01% resistor. The output of the CJC
sensor, measured across the 20k
Ω
resistor, is 5.964V at 25
°
C. This output
changes at the rate of 20mV/
°
C. The CJC sensor requires no calibration.
The CJC is needed for accurate thermocouple temperature measurements.
A temperature-dependent voltage, called the reference junction voltage
(or cold junction voltage), is always generated between a thermocouple
and the terminals to which the thermocouple is connected. Therefore,
when you connect a thermocouple to a DAS-TC/B board via an
STC-TC/B or STA-TC/B screw terminal accessory, a reference-junction
voltage is generated between the thermocouple leads and the screw
terminals. The reference junction voltage is defined as follows:
Reference
junction
voltage
=
(Voltage generated by your
thermocouple at reference-
junction temperature—the
temperature of the screw
terminals)
-
(Voltage generated by your
thermocouple at 0
°
C, which
is defined to be 0V)
CJC Sensor In
(to pin 20)
20k
Ω
0.01%
CJC Sensor
AD592CN
+11 to +15V
(to pin 19)
Ground
(to pin 16)
