DMM read different values each time

[mama007]

Hi,

I have a small trouble reading from the DMM. I have conncted my DMM to the points where I want to measure the voltage difference(1.7mv). When I used the DMM soft front panel, I can read the values precisely,even at higher range.

But I tried to read the value from TestStand and Lab windows. the value keep changing every time I make the measurements. When I loop the measurement step, it returns ranging from 4.8mv to -0.5mv and its quite a lot of difference from the actuval value. i have even tried to use the recommended/default sample time -1, but of no use. The same thing happens in Labwindows interactive execution.

Is there anything I can make my code better to get accurate measurements.

Attached is my Labwindows code.

Thanks

mama007

Message Edited by mama007 on 07-10-2006 07:00 AM

[RobSims]

Hi mama007,

Thanks for posting on the NI forums!

I will have a look into the issue with you, however it would helpful if you could let me know the version of LabWindows/CVI you are using and also which DMM (and version of NI-DMM) you are wishing to use. Without this info it will be difficult to get a full picture of the situation.

As an aside there are some DMM examples that you may find useful. You can find these by navigating throught the start menu- 'Start>All Programs>National Instruments>NI-DMM>Examples>CVI'. Perhaps try running one of these continuous examples to see if you observe the correct values.

Thanks,

Rob

[mama007]

Thanks for the reply Rob,

Here is the information you asked:

Labwindows/CVI - V7.0

NI-DMM               - V1.6

Hardware            - NI4060,NI-4070

I couldn't find any examples menu in 'Start>All Programs>National Instruments>NI-DMM'. I hfeel it because of the old version of NI-DMM I am using. the problem I have I want my code compatible with NI-4060 too and I suspect if I upgrade there are some functions that might be obsolete and new functions might not work with NI-4060.

I have execute the example file in CVI, which has the front panel its also returning different values and I have done in the same way it was done in the example. My DMM is calibrated by NI only couple of months ago.

Thanks

mama007

[RobSims]

Hi mama007,

Thanks for getting back to me with that info. I would advise that you perhaps explore the option of upgrading your NI-DMM driver. The NI 4060 DMM is still supported, even in the most recent release of NI-DMM (v2.5), alongside the NI 4070, 4071 and 4072 DMMs. To my knowledge no functionality has been removed for the 4060, but some functions/VIs have been changed/revised. I would suggest taking a look at the readme for NI-DMM versions 2.0 and above as support for the 4070 is added at this stage, and also to see their differences. You will also have access to the CVI examples I noted to you in my last post.

You can browse/search for NI drivers at the below link:

http://digital.ni.com/softlib.nsf/MainPage?ReadForm&node=132010_US

Given the DMM returns 'correct' values via the NI-DMM Soft Front Panel (SFP) I would suspect that the issue perhaps resides in either your code, or the interface between CVI and NI-DMM 1.6. In this case I would suggest exploring some of the continuous measurement examples to check for differences.

If you have any questions or queries then please do let me know. Thanks,

Rob

[mama007]

Thanks Rob,

I'll consider upgrading NI-DMM to V2.5. Meanwhile

I have also used an example from C:\Program Files\National Instruments\CVI70\samples\niDMM\Single Point\DC Volts

I have created exe file from the project in the above folder and run it to take measurements and I have the same problem as like before. Every time I click on Read I get diffeent value, while soft front panel shows a constant value.

Any idea why this is happening...

Thanks

mama007

[RobSims]

Heya mama007,

Thanks for that. I'll take a look at the code on a machine here. May I enquire as to which DMM (4060 or 4070) you are using when you see this issue? and what are the settings within NI-DMM SFP that allow you to gain correct values? (a screen shot would be sufficient)

I'll let you know what I find. Cheers,

Rob

[mama007]

Hi Rob,

Actual NI-DMM version I am using is 2.1 not 1.6. I have referred from paper documentation before.

The front panel settings are

Range - 1V, Resolution - 6.5 digits, Power Line - 60Hz and Auto zero,Filter options set. Taking DC measurements.

Attached is the screen shot...

Thanks

mama007

[RobSims]

Hi mama007,

That's perfect! I'll take a look at the issue and I'll let you know what I find.

Thanks,

Rob

[mama007]

Hello Rob,

I have noticed the problem. The signal I am trying to measure has small noise, which is picked up, whenver I tried to take measurements. But When I open the front panel it does filter out that noise,even if I untick the filter option. So sometihing special function in the front panel that filter the noise or its just might be a slow acquisition, which can't see the noise.

When I connected my DMM to a PSU unit, both front panel and my function returns same measurements. So nothing wrong with DMM. Is there a way to sort out this problem..

Thanks

mama007

[Mark_S1]

Hello mama007,
   Looking at your program, you are requesting 5 1/2 digits, not 6 1/2 digits:  .001 V on the 50 V range.  This applies to both DC Volts and AC Volts.  This is why you are seeing the noise on the source.  When you ask for 6 1/2 digits, the aperture time is longer.

   You could instead use niDMM_ConfigureMeasurementDigits (available in NI-DMM 2.4 and later) to configure your measurement using digits instead of absolute units.  Or alternatively, change the requested resolution.

Good luck,
Mark S

NI-DMM Software