Incorrect Resistance Measurements Using PXI-2535 or PXI-2536

Updated Feb 9, 2018

Reported In

Hardware

  • PXI-4071
  • PXIe-4081
  • PXI-2535
  • PXI-2536

Driver

  • NI-DMM

Issue Details

I am getting strange readings when using the PXI 2535 or 2536 for resistance measurements. I followed the article NI PXI-2535 or PXI-2536 FET Switch Doesn't Close and it appears that the readings I am getting are significantly different when using the switch modules versus when not using the switch module for testing. What is going on?

Solution

The recommended solution would be to use the Offset Compensated Ohms function to give a more accurate measurement. Without performing this adjustment you might receive inaccurate measurements caused by leakage currents. 

Additional Information

Without using Offset Compensated Ohms, the error is due to the fact that the relays used in the PXI-2535 and PXI-2536 have additional leakage current that is being applied to your system as shown in the image below.

The expected typical value for leakage current at 25C is: +/-2nA for every closed row/ column connection, +/-1nA for every open column, and +/-1nA for every open row connection. These leakage currents can cause incorrect measurements when using a DMM in the configuration mentioned in NI PXI-2535 or PXI-2536 FET Switch Doesn't Close. This is caused because of the voltage divider that is created when using a 10Mohm to stabilize the voltage levels on a PXI-2525 / PXI-2526 as shown below. 
The expected error that you might receive in your test setup would be system dependent. NI recommends that you use experimental data to calibrate the measurements appropriately. Listed below would be the worst case errors you may be seeing when operating at 25 C.

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