PXI 4065 - resistance depends upon range

[Joe in the Know]

Hello!

I am using a PXI 4065 DMM to measure 2-wire resistance through the soft front panel. The strange thing is that when I measure the resistance, it depends upon the range! For Instance, if I set the range to 10M, I read 0.9M, if I set the range to 1M, I read 0.15M, if I set it to 100k, I read 93k. How do I know which is correct? Autorange gives me 1.5M and a fluke 40Mohm dmm gives me an open.

Also when I use labview to collect the resistance data, and I set the range to auto, it does not give the same value as when I measure with the soft front panel or a fluke DMM.  In fact the fluke DMM and the PXI often differ by an order of magnitude or more.

How do I use the PXI DMM to read the correct resistance value?

Thank you,

Joe

[johnsold]

Joe,

This is not uncommon with many ohmmeters.  If you look at the specifications for the 4065, you will see that the test current changes with range. 10 M => 500 nA, 1 M => 5 uA, 100 k => 10 uA. (20:1 range of current)  If the device you are measuring has any non-linearity or energy storage, the reading may be different on each range.

Probably ALL of the measurements are within the accuracy specification of the meter (~100 to 500 ppm of reading).  You are just measuring something different on each range.

To decide what is the "correct" reading, you need to carefully define the measurement conditions. In particular the applied voltage or current at which the measurement is made.  If the device being measured has energy storage or non-linearity, you must also specify the history of applied voltage or current over time to accurately define the measurement.

Lynn

[Steve_B]

Hi Joe, 

I was hoping you could provide some more detail regarding your connections and your load. As Lynn mentioned the load can affect the measurements. You say you are using 2 wire ohms, do you have anything connected to the HI and LO Sense inputs? If so, can you remove them?

[Joe in the Know]

Hi Lynn, Steve,

I do not have any connections to the HI and LO sense. The resistances I want to measure are nonlinear and have capacitance, so Lynn is probably right.

Thanks,

Joe

[johnsold]

Joe,

If you are not sure how to define exactly what you want to measure, please ask more questions here.  Several different techniques can be used to describe the impedance of such devices.

Lynn

[Joe in the Know]

Thanks,

It seems that using the Increase DC Resolution example VI gives me much more reasonable results - I am seeing 27Megs using that. I will procede using that model.

Joe