NI PXI-2535/6 FET Switch Doesn't Close With a DMM

Issue Details

I am using an NI switch module with fault protected FET switches (e.g. PXI-2535 or PXI-2536). The module typically works as it should, but sometimes I notice the signal path reads open even after a command is sent to close the switch. I've noted this can happen when using the fault protected FET switch with a DMM. Why does the switch not close?

Solution

The FET switch is likely not closing because its fault protection mode has been triggered. Fault protected FET modules like the PXI-2535 and PXI-2536 protect against damaging over-voltages by opening the signal path whenever the voltage on the signal line has a magnitude outside the allowed voltage range.

If you are confident the signal being switched has a magnitude that is within the allowed voltage range, but the switch is still exhibiting a fault behavior, then a likely cause is that the FET module is responding to the amplitude of the signal in relation to the FET module's ground reference. Verify the applied signal is not referenced to a different ground potential. Also, verify the applied signal cannot float to a potential that is above the fault protection circuitry's limit. Both of these situations can result in triggering the FET to go into fault protection mode. The following three examples show common ways these two situations can occur.
 

1. Using a DMM in 2-wire resistance mode: A DMM taking a 2-wire resistance measurement outputs a small precision current that can develop approximately 12 V across an open circuit (acceptable for a fault protected FET switch, such as the PXI-2535). However, the DMM is a floating source and can drift with respect to earth ground to a point where the potential is greater than the approximate 14 V that triggers fault protection mode and opens the signal path. To avoid the problem, reference the DMM INPUT LO to earth/chassis ground, either directly or through a resistor. The following figure shows the required connections with the optional resistor.  

 

2. Using a DMM in 4-wire resistance mode: A DMM taking a 4-wire resistance measurement outputs a small precision current that can develop voltages in excess of 14 V if there are switches in both the INPUT and SENSE paths. The DMM is a floating source and can drift with respect to earth ground and also uses means to drive the LO side of the device under test to a reference voltage. To prevent the fault protected switch from going into fault protection mode, a large resistor needs to be placed between the DMM's INPUT LO and the LO SENSE connections. The recommended resistor for this is a precision 1 mega-ohm resistor. Also, a reference to earth/chassis ground, either directly or through a resistor, is needed on the DMM's LO SENSE terminal. The recommended resistor for this is a 100 ohm resistor.
   
   Note: The purpose of doing 4-wire resistance measurements is to have a highly accurate measurement of resistance. The use of the 1MΩ resistor will typically add some measurement error to the value being measured. This is due to the amount of leakage current present on the PXI-2535 and PXI-2536 and is discussed in Why do I get Incorrect Measurements when using the PXI-2535 or PXI-2536? article.    

 

3. Using a floating signal source: Any floating signal source that is connected to the PXI-2535 or PXI-2536 may have issues with signals approaching at or above the voltage limit of 14 V. When using a floating signal source DMM, reference the negative side of the floating signal source to earth/chassis ground, either directly or through a resistor. The following figure shows the required connections with the optional resistor.

  Note: Other FET products can be designed with different means of over-voltage protection. Modules such as the PXI-2501 use high path resistance to protect against over-voltage conditions but at the cost of added path resistance of greater than 1kΩ.