high resistance measurement by 4071 and 2575

[faili]

I used NI 4071 to measure a high resistance. The resistance is 78.6MΩ.

When I use 4071 directly to measure it(2-wire), the result is 78.58MΩ.

The measurement is ok. But wnen I connect the resistance to NI Switch 2575,

the result become smaller, about 76.4MΩ。

It is strange, and how to deal with the problem?  Thanks

[chericks1]

Hi faili,

I have never witnessed this issue before, so I attempted to reproduce your problem.  I used the following hardware:

I PXI-4071 DMM
NI PXI-2575 Switch
LFH200 to 50-Pin DSub Switch Cable
NI TBX-50, 50 Pin Dsub Screw Terminal Block
Short 24-gauge wires between resistors and terminal block
Resistor Combination in breadboard (28 2.7MΩ resistors = 75.6MΩ)

I used the following procedure:

Opened Switch Soft Front Panel
Configured PXI-2575 in 2-wire 95x1 Topology
Connected ch94->com in Switch Soft Front Panel
Wired CH94+ (50-pin DSub pin 21) to one side of 75.6Mohm resistance, CH94- (50-pin DSub pin 22) to other side

I made the following measurements:

Opened DMM Soft Front Panel
With DMM, measured one side of resistance to other side (after settling) = 75.6MΩ
Wired COM+ (50-pin DSub pin 23) to DMM HI Lead, COM- (50-pin DSub pin 24) to DMM LO Lead
With DMM, measured COM+ to COM- (after settling) = 75.6MΩ

So, I wasn't able to reproduce your issue.  Perhaps the above will help.  Can you tell me more about your system and how it is configured?  Perhaps there is a hidden resistance to GND somewhere in your setup.  Looking at the equation, to receive an error of:

78.6MΩ -76.4MΩ = 2.2MΩ (or 2.8%)

you could have a hidden 2.73GΩ path to GND.  How are you making your connections to the PXI-2575?  Just a thought...

Let me know if you have further questions or if this does not resolve your issue!

Chad Erickson
Switch Product Support Engineer
NI - USA

[faili]

Hi,Chad
     Thank you for doing so many things for us. Recently I am bored with it.
The instruments below is used in my test.

     PXI-1036 6-slot
     PXI-PCI8331,MXI-4 kit with Copper Cable
     PXI-2530 and TB-2630 (In Slot 4)
     PXI-4071 DMM
     PXI-2575 and LFH200 to 50-Pin DSub Switch Cable (In Slot 4)

     My test process is below.
Step 1: I measured the resistance directly by DMM. No cable from Switch was connected.
Opened Switch Soft Front Panel, I got 78.6MΩ.    
Step 2: I used a male 50-Pin DSub to connect with LFH200 to 50-Pin DSub Switch Cable,
and welded some cables with male DSub directly. I used  2-wire 98x1 Topology, and
wired Ch84 to COM. CH84 is wired to the resistance, COM is wired to the DMM. The result
is 76.4MΩ, sometime is 77.4MΩ.
There is another phenomenon in Step 2. When I disconnected the resistance to CH84, I used
Switch Soft Front Panel to measure and sometime I got a 4GΩ result, sometime there was
no result. Does it mean that there is a parallel resistance between the CH84+ and CH84-?

About the GND, I just connected the GND of PXI1036 which is at back of the PXI1036.
I don't know how to connect the PXI 2575 to GND. Is it necessary to set the settling time
or aperture time?

I have a question of your test. When you measured one side of resistance to other side
with DMM directly, whether you still connect the CH94 to the resistance? If you did like
that, I think the switch still has effect on the measurement of resistance.

Looking forward to your answer. Thanks a lot.

faili

ATS Engineer

ESPEC 

帖子被faili在06-20-2006 02:18 AM时编辑过了

[chericks1]

Hi faili,

Thanks for the detail.  I'm very interested in the following comments:

"Step 2: I used a male 50-Pin DSub to connect with LFH200 to 50-Pin DSub Switch Cable, and welded some cables with male DSub directly."

This is the one large difference between your system and mine.  When you welded, what steps did you take to clean the flux residue from the connection?  Is it possible a thin film could be providing the parallel resistance affecting your measurements?  Is there anywhere your wiring/cabling could be providing a leakage current between CH84+ and CH84-?

"When I disconnected the resistance to CH84, I used Switch Soft Front Panel to measure and sometime I got a 4GΩ result, sometime there was no result. Does it mean that there is a parallel resistance between the CH84+ and CH84-?"

Yes, probably.  I can't think of another reason the resistance value would decrease.  I measured the leakage current between two open channels on the PXI-2575 by supplying a voltage and measuring with the PXI-4071.  I measured a value in the pA, giving me a resistance of 100s of GΩ.  I never received anywhere close to nA, which is necessary to read 4GΩ.  Hmmm... Do you know if CH84+ and CH84- are the only channels to exhibit this behavior?  Do other channels get different readings?  Are they off by the same factor, or is the offset different from channel to channel?

"About the GND, I just connected the GND of PXI1036 which is at back of the PXI1036.  I don't know how to connect the PXI 2575 to GND."

Whoops.  Typo.  Great catch.  Both instances I mentioned "to GND" above you can switch out with "between CH84+ and CH84-".  Sorry for the misunderstanding.

"Is it necessary to set the settling time or aperture time?"

You can add settling time to the DMM reading or increase measurement aperture time.  However, above "(after settling)" meant I waited for the DMM value displayed on the DMM Soft Front Panel to reach repeatable measurements. 

"When you measured one side of resistance to other side with DMM directly, whether you still connect the CH94 to the resistance?"

Great question.  I didn't remember, so I repeated the measurement making sure not to connect CH94 to the resistance.  I still did not reproduce your issue.  The values I measured were within 0.1MΩ.

Hope this helps!

Chad Erickson
Switch Product Support Engineer
NI - USA

[faili]

Hi,Chad

The problem have not been resolved.

How can I measure the leakage current between two open channels.

Can you explain for me?

If the leakage current  in the nA, does it means the quality of switch is not good?

[chericks1]

Hi faili,

It's been a while since your last post.  Are you still having this issue?

Thanks!

Chad Erickson
Switch Product Support Engineer
NI - USA

[psgeorge]

Chad,

I came across this thread while searching for programming information using the pxi-2575 switch and a DMM (such as the pxi-4071).  I am going to be building a test application for our QC inspectors to test an electrical unit.  This will involve measuring voltages and resistences from many different points on the unit.  I have cable harnesses which will bring the various measurement points into specific channels of the 2575 for measurement.  I have been looking for some sample code (preferably vb.net or c# along with NI's Measurement Studio) to individually address specific channels and then make a measurement.  From your involvement in this thread, I thought that perhaps you could point me to some code which I could use as a guide for my task.

Also, the pxi-2575 can be configured either as 196x1 or 98x2.  For my application where I will simply be measuring either a voltage or a resistence (great accuracy not necessary), I have decided to configure the pxi-2575 as a 98x2 and then bring in each side of the measurement on a seperate channel.  Does this make any sense?

Anyway, if you are able to point me to some sample code, I would be most appreciative!

Thanks!