Digital Multimeters (DMMs) and Precision DC Sources

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Multiple measurements with DMM


I'm using SCXI 1127 with PCI-4060 DMM for measuring two wire resistance. I'm reading the measured data using the multi-fetch VI of the NI-DMM in synchronous mode with SCXI-1127. The sample trigger is interval. I have issues with the accuracy of the measurements. If I have a slightly loose but settled contact in the electrical path, the measurements vary by a large factor, if i re-run my VI. e.g if i get 250 ohms in the first run, then i may get 400 in the next run. What i am trying to do is to take multiple measurements from the same connection once a switch is closed, so that i can take median or the best of all. How can I do it? I can repeat the measurements in the loop, but that causes the switches to switch again. I configure
the switches in the scan list, intiate the DMM and fetch/read the readings. I want DMM to sample the values once a switch in the scan list is closed. I should be able to take atleast three readings and then take a median of it, and then move to next entry in the scan list.
Any comments?

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From what I can tell, a good example to base this off of would be niSwitch DMM Switch Handshaking.VI. Assuming you are using this example, you could modify the number of samples per trigger by changing the "Sample Count" constant going into "niDMM Configure Multi Point.VI" (VI #5 in the DMM VI chain in section B of the block diagram) to the number of points you want per measurement. You may also need to adjust the "Samples to Fetch at a Time" control on the front panel. Once you get the points back, you can do averaging on the data to get the desired measurement data.

I hope this helps you get started on this one, post here if you have further questions.


Ross C
Applications Engineer
National Instruments
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I have a follow-up question related to this DMM-Switch set up.


I have multiple routes in my scan list, for example, 4 routes. Each route has two connections (plus terminal and minus terminal of a resistor). I'm taking resistance measurements. 


The scan list is something like this:

r0->c0 & r1->c1;
r0->c2 & r1->c3;
r0->c4 & r1->c5;
r0->c6 & r1->c7;


How do I ouput the measured resistance value of each route? I expect 4 resistance values from this scan list.


Thank you!

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The "Switch - Scanning with a DMM Handshaking (NI-SWITCH).vi" will be the best example to follow. This example is set up to cycle through your scan list continuously and take a DMM measurement after the switch finishes closing/opening (after each ';'). You change modify the example to only take 4 measurements by changing the scan list boolean to false, telling the DMM to only accept 4 triggers, tell the DMM to only read one sample each trigger, tell the DMM to fetch 4 samples, and than stop. There may be other minor edits you have to make, but that should get you in a good place if you are only wanting to take 4 samples.


Hope this helps!

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