PXI 4071 getting negative resistance readings

[Gryphon]

Hi All,

 We are working on a system where we are measuring leakage currents and resistances using a PXI-4071 with some other gear as well such as an SMU and 2530B relay card.

The main issue is that for some reistance measurements we are seeing negative values like -200k or -300k.  The DMM has been set to the 10M range. My understanding is that in this mode, the applied voltage is 10V and the test current is 1uA. 

For me to be seeing a negative resistance across some paths, the voltage must be opposite to the 10V and higher than 10V.  Is this an accurate way of thinking?  The SMU is off and disconnected from the circuit when the resistance measurements are made.

It is not all DUTs that show this negative voltage.  The DUTs are all passive devices (connectors) and so they don't have any large capacitances that could store a voltage that I can see.  I am taking 5 readings  with auto zero done once at the start of the 5 readings.

What is causing these negative readings?  The connectors are submerged in saline and there should be an open circuit between the points being measured but when they start to fail a resistance can be seen.

Any thoughts would be much appreciated.  Please let me know if you'd like more details.

cheers

Peter

[Brian_P]

Peter,

Thank you using the forums.  I have been able to do some research on your issue and I have found a few resources that will probably be helpful in your application. The first is a knowledgebase article titled Getting a Negative Reading When Making a Resistance Measurement and the second is another discussion forum where someone was experiencing the same issue as you are when trying to take a resistance measurement of an open circuit.

The way that DMM makes it's measurement in 10M mode is by putting 1 uA through the resistor and then reading the voltage back and it can read up to 10V max.  So with Ohms Law V/I=R so that is why it can read 10M Ohms max. What is the voltage that you are reading accross the DUT?  If there is voltage accross the DUT that could be what is messing up your readings.

I hope that helps,

Brian P. 

[Gryphon]

Hi Brian,

thanks for your response.

I had seen the first knowledge base article you pointed out.  

I did initially have more instances of weird readings when the DMM was set up with autoranging.  Autoranging was used because the DMM is being used to alternatively measure paths that should have continuity followed by paths that should be open.  With autoranging, readings were more  random in nature particularly for high resistance paths.  After reading the knowledge base article I set the ranges for all readings explicitly.  1k range for the paths that should have continuity and 10M for the paths that should be open circuits.

With the ranges explicitly set, the readings are much more consistent but after some time running, negative values start to appear on some channels as opposed to others.  

The forum post you pointed out was interesting but I think alot of what was being seen was because of the use of 4 wire mode for high resistance readings.  I may be wrong but I had always understood that 4 wire mode was more appropriate for low resistance readings and not high resistance measurements.

It is difficult for me to jump in and measure the voltages as the signal paths are through a 2530B relay and in an automated test configuration so probing the voltages is tricky at present.  

I will try measuring the voltages across the paths when the current test run has completed.  

Is it fair to say that the only way for the DMM to return a reading of -200k is that there is an opposing voltage?  Is the negative reading caused by the current flow being in the opposite direction to what would normally be seen (with a passive resistor for example) and so a

negative resistance is being returned?

What does the high value of the negative readings and the fact that they can be fairly constant for some signal paths suggest.

Thanks and regards,

Peter

[Brian_P · Accepted solution]

Peter,

To answer your questions, yes, if the DMM is reading -200k  then there is an opposing Voltage on the DMM.  If you are using 10M Ohms then the opposing voltage would be .2 V and if you were using the 1M Ohms then the opposing voltage would 2V.  Also the negative reading is also caused by the current flow being in the wrong direction, because if there is a negative voltage then the current will flow the opposite direction.

You can verify the voltage accross the resistor while taking the resistance measurement by using another DMM, such as a handheld DMM.  This will be more than accurate enough to verify the voltage accross the resistor.

I hope that helps,

Brian P.

[Gryphon]

Thanks Brian,

those confirmations help. I will try and get some measurements across the DUTs to see if these voltages are visible.

cheers

Peter