BNC/TC-2095 Rack-Mount Adapter Installation Guide
16
ni.com
Errors Due to Open-Thermocouple Detection Circuitry
Open-thermocouple detection circuitry can cause two types of
measurement errors. These errors are the results of common-mode voltage
at the input of the SCXI module and current leakage into the signal leads.
Common-Mode Voltage at the Input of the
SCXI Module
With 10 M
Ω
pull-up and bias resistors, a common-mode voltage of
2.5 VDC develops if the thermocouple is floating. At a gain of 100, the
common-mode rejection of the SCXI-1102/B/C module is sufficiently high
that the resulting offset error is negligible.
If the application demands extremely high accuracy, you can eliminate this
offset error by calibrating the system with both the pull-up and
ground-referencing resistor switches ON. You also can turn off the pull-up
resistor switch, which eliminates the open-thermocouple detection feature,
or use the 10
Ω
ground-referencing resistor networks, which brings the
common-mode voltage down to nearly 0 VDC.
Current Leakage
The open-thermocouple detection circuitry results in a small current
leakage into the thermocouple. With the 10 M
Ω
bias and pull-up resistor
networks, the current leakage results in a negligible error. With the
10
Ω
bias resistor, the 10 M
Ω
pull-up resistor connected to 5 VDC causes
a current leakage of approximately 0.5
µ
A (5 VDC
/
10 M
Ω
) to flow into the
unbroken thermocouple.
If the thermocouple is lengthy, a voltage drop develops in the thermocouple
because of lead resistance. For example, if you have a 24 AWG J-type
thermocouple that is 20 feet long, a voltage drop of approximately 8
µ
V
can develop in the thermocouple, which corresponds to an error of 0.18 °C
according to the following equation:
Voltage drop value = (0.145
Ω
/ft + 0.658
Ω
/ft)
×
20 ft
×
0.5
µ
A
If the application demands high accuracy, you can eliminate the voltage
drop inaccuracy error by turning off the appropriate pull-up resistor
network or by calibrating the system offset. For the location of these
switches, refer to Figure 3.
