National Instruments SCC-TC02 用户手册

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The SCC-TC0X contains a 10 M

Ω pull-up resistor that connects to 

+5 V to detect open thermocouples. To determine if you have an open 
thermocouple, check whether the corresponding E/M Series DAQ device 
channel is saturated. The pull-up and bias resistors saturate the channel by 
applying 

+2.5 V at the input of an open channel. This results in saturation 

to either the positive or negative rails of the EM Series DAQ device 
(

±10 V).

The open-thermocouple detection circuitry can cause measurement errors. 
These errors are the results of common-mode voltage at the input of the 
SCC and current leakage into the signal leads. The 10 M

Ω bias resistor in 

the SCC-TC0X causes this error to be negligible. With the 10 M

Ω bias 

resistor connected to ground and the 10 M

Ω pull-up resistor connected to 

+5 VDC, a current leakage of approximately 0.25 μA (5 V/20 MΩ) flows 
into the unbroken floating thermocouple. Long thermocouple leads result 
in larger voltage drops due to lead resistance.

For example, if you have a 24 AWG J-type thermocouple that is 20 ft long, 
a voltage drop of approximately

4 

μV = (0.145 Ω/ft + 0.658 Ω/ft) × 20 ft × 0.25 μA

can develop in the thermocouple, which corresponds to an error of 0.09 °C.

With 10 M

Ω pull-up and bias resistors, a common-mode voltage of 

+2.5 VDC develops if the thermocouple is floating. The common-mode 
rejection of the SCC-TC0X is sufficiently high, which results in the offset 
voltage being negligible in most applications.

If your application demands extremely high accuracy, you can eliminate 
these errors by calibrating the system. Refer to the 

 section for more 

information.