NI PXI-2535 or PXI-2536 FET Switch Doesn't Close Hardware: Switches>>Multiplexer/Matrix>>PXI-2536, Switches>>Multiplexer/Matrix>>PXI-2535
Problem: 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 that 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 scenarios can result in triggering the FET to go into fault protection mode. Example: A DMM taking a resistance measurement can output approximately 12 V (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 to ground, either directly or through a 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. Related Links: Attachments:
Report Date: 03/05/2008 Last Updated: 03/05/2008 Document ID: 4J4DMPLE |
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