©
National Instruments Corporation
15
TBX-1303 32-Channel Isothermal Terminal Block
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
so that the resulting offset voltage is negligible.
If the application demands extremely high accuracy, you can eliminate this
offset error by calibrating the system. Refer to the module documentation
for more information on calibration. You can also remove the pull-up
resistor, which eliminates the open-thermocouple detection feature, or use
the 10
Ω
bias resistor networks, which bring 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 µA (5 V
÷
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 µV can
develop in the thermocouple, which corresponds to an error of 0.18 °C.
The following equation shows how to arrive at the voltage drop value:
(0.145
Ω
/ft + 0.658
Ω
/ft)
×
20 ft
×
0.5 µA
If the application demands high accuracy, you can eliminate this error by
removing the appropriate pull-up resistor network or by calibrating the
system offset.
