variation in resistance measurement using DMM

[Brandon_G.]

Hey RV,

You can use the DMM in digitizer mode by using the attached VI in this DevZone article.

If I understand you correctly it sounds like when you added the OCO in your program, you essentially ended up with 0 ohm measurements?  You were expecting something around 80 -120 mohms but after applying OCO, you are seeing only a fraction of a mohm.  Can you post your code?

I would certainly recommend using auto-zero, ADC cal, setting your line frequency appropriately and allow for settling time after switching on the SMU.  Lastly, you can set the Voltage DMM to 7 1/2 digits to give more precision in your measurements.

After taking these steps I would expect we'll have more repeatable results that maybe at an acceptable level for your setup.  I will review your code and see if we can find out why you're now seeing 0.5 mohm measurements and also see what other improvements can be made.

Thanks RV

Brandon G

[R.V.]

i am attaching code of resistance measurement. please find it & review for correction.

[Brandon_G.]

Hey RV,

To simplify down the problem, I created a test vi that should help us see what type of repeatability we can get in our measurements.  I connected a 100 mohm resistor through a switch and then used a 4130 to source current while 2 4071s measured voltage and current.  As a point of reference, the 100 mohm DUT actually measured 99.67 mohms when taking a 4-wire resistance measurement with OCO on the 4071.  When I then took resistance measurements using the 4130+4071+4071+switch, I got readings that varied from  ~98 - 101 mohms.  I may have gotten a tighter measurement if I had used twisted paired wiring, but this seems to be a tighter distribution than we've seen before and the measurements center around the nominal value. I would like to see if you get similar results when you run the code.  Below are settings that I've changed from your code, along with some suggestions when running the vi.

1. The test vi doesn't set up the switch.  I connected the switch up with the Soft Front Panel before running my vi.  I actually used a different switch than you, but the vi will still cancel out the thermal voltages.

2. I changed your voltage digits from 6 1/2 to 7 1/2.

3. The filter you used in your code was a second-order filter, but I used the High-Order filter which is selected by default when taking 7 1/2 digit measurements.

4. The two test currents I used were 100 uA and 4 uA.  I did not disable the 4130 output to take 0 uA measurements because I did not want the output floating.  Instead, I left the output enabled and set it to 4 uA (minimum value in the 200 uA range).

5.  Be sure to change your power line frequency appropriately on the front panel.

6. The default number of averages for the 4071 is 4.  I created a control on the front panel so you can experiment to see if more averages give a tighter distribution.

7. I would highly recommend for troubleshooting to use a fixed test resistor of a known value to make diagnosing the problem more easier.

I've attached both the VI and the screenshot of my test from the front panel as a reference.  Hopefully we can get much better, more repeatable measurements so we can use the lower test currents and the switch on your setup.  Please let us know how everything turns out.

Thanks RV

Brandon G