Using Cold Junction Compensation (CJC) on the SCB-68 or SCB-68A

Updated Jul 31, 2018

Reported In

Hardware

  • SCB-68
  • SCB-68A

Software

  • LabVIEW

Driver

  • NI-DAQmx

Issue Details

I am seeing variable, offset, incorrect or other unexpected errors in my temperature readings. How do I configure and use the cold junction compensation (CJC) on the SCB-68 or SCB-68A to perform a differential measurement?

Solution

Hardware Settings:
  1. Configure the switches on the SCB-68 and SCB-68A as shown in Figures 1 and 2 below. This enables the temperature sensor on differential channel 0, making both AI 0 and AI 8 unavailable for other connections. To make this information easily accessible, tape the correct quick reference label for your device (Related Links) to the SCB-68 SCB-68A. This label also shows how to configure the switches to either enable the temperature sensor on single-ended channel 0 or disable the temperature sensor
  2. Input your temperature sensor into AI 1 and AI 9. 

Figure 1: SCB-68


Figure 2: SCB-68A

Using NI MAX:
  1. Open Measurement & Automation Explorer (NI MAX).
  2. Expand Devices and Interfaces NI-DAQmx Devices.
  3. Right click on your device and select Configure.
  4. Click on the Accessory tab.
  5. Select the SCB-68 or SCB-68A as your Accessory and select Configure.
  6. Check the box for Temperature Reference Enabled.
  7. Click the OK buttons to save the changes. 
 
  1. Expand Data Neighborhood.
  2. Right click on NI-DAQmx Tasks and select Create New NI-DAQmx Task.
  3. Create a task for the correct temperature sensor.
  4. Select Built In as the CJC Source.
  5. Save the task and test it. 
 
Using LabVIEW:
  1. Use the Software CJC.vi inside the softwarecjc.llb provided inside the link in the Related Links section to perform the measurement.
  2. According to the hardware configuration mentioned previously, the front panel configuration is shown in the screenshot:
 

Additional Information

  • These steps will also work with M Series boards with two connectors. Just add two accessories instead of one and input the signal into AI 17 and AI 25 instead of AI 1 and AI 9.
  • When using the PCI/PXI-6225 and PCI/PXI-6255 which are two connector M Series devices, connector 1 will not have access to the CJC sensor on the SCB-68 or SCB-68A.  Please check related links for additional information.
  • If you configure the switches on the SCB-68 or SCB-68A to enable the temperature sensor on single-ended channel 0 (AI 0) rather than differential channel 0 (AI 0 and AI 8), you cannot use a DAQmx temperature task. Instead, you will have to use an analog input task to read both AI 0 (the CJC) and your temperature channel, and then scale the raw voltage in software. This procedure is only necessary if you need to use every analog input channel on your DAQ board and cannot afford to use two channels to read from the CJC in differential mode.
  • These steps will not work for the mass termination version of the USB-6251 and USB-6259 prior to DAQmx 8.8. This is a known issue stated in the DAQmx Readme.  This issue has been resolved in DAQmx 8.9 and later.

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