PXI-2530 defies the laws of physics when trying to quantify the relay resistance of specific relay pairs.

[GJM]

I have an issue with a PXI-2530 switch card running in 4X32 Matrix mode. I need to determine the resistance of specific relay pairs within the matrix. I have a PXI-4130 and a PXI-4071 in the same chassis so I am taking 4-wire measurements in a configuration like this...  

         The resistor symbols represent relays in the switch. Here is a matrix-style representation of the switch routes I am using.(This should look more like the switch soft front panel interface)  

 So both diagrams represent how I am taking my measurement. I Have externally shorted Columns C8 &C9 (shown in first diagram), I am sourcing 500 microamps of current, and toggling the current source for one positive measurement and one negative measurement, I am measuring the voltage with the DMM . For the measurement above I am measuring .2619 ohms. That should of course be my resistance of relays K8 and K41, as well as terminal block and wiring.

  Here is another diagram. This should be measuring the same two resistors....  

   All I have changed is the routing from the source meter. Here is the matrix view...

    With this measurement I am measuring .0873 ohms. I am measuring the same resistance as the first example except I get a very different measurement.

  It gets even more interesting. If I were to take 4 pairs of wires, as I am using here, there are 16 possible measurement configurations. I took every one of these measurements and half of them gave me about .25 ohms while the other half gave me about .09 ohms. I tried this on a 2nd chassis and got the same result. My data is in the attached spreadsheet. (My examples above are rows 1 & 5.)  

  By taking the measurement the way we are, all of these measurements should be about the same. I have a constant current source, (I even tried a Keithley in place of the 4130 and got the same result.) and the meter is high impedance.    If I had a schematic for the PXI-2530 I could do a little more analysis to find out why I get different measurements. Is there some clamping diodes on the rows, or columns within the matrix? Something external must be acting apon this circuit. If I can find out what that is, I could possibly determine my relay resistance to a quantifiable level of uncertainty. 

Any help would be greatly appreciated.

Greg

[GJM]

Sorry, This is my first post. I thought I was posting to the PXI board, but it seems I did not.

I will repost to the PXI board and remove this one.

My apologies.

[GJM · Accepted solution]

It seems I cannot delete. The correct post has been placed here...

http://forums.ni.com/t5/PXI/PXI-2530-defies-the-laws-of-physics-when-trying-to-quantify-the/td-p/122...

Again, my apologies for the incorrect placement of the post.