DMM IVI read measurement update

[Fardm]

Hi,

I am using Teststand DMM IVI step to measure voltage to ground via 100K ohm pull-up resistance to 20V supply. While monitoring this voltage using external DMM it shows 13.4V however after "DMM IVI Read" step is ran the external DMM voltage shows 19.7V!! Any idea why this is happening, I'm using:

NI DMM 4070

TestStand 4.1

Labview 2010

and Switch Executive 3.0

Many Thanks

Mehran

[RayFarmer]

What reading are you expecting?

Are you sure you are measuring the same points with respect to the same reference point?

Maybe you are left something connected.

[Fardm]

Hi Ray,

Thanks for your reply.

We expect to see 19.7V all the time but we only see the correct voltage (19.7V) after "DMM IVI Read" step is ran in TestStand the reading before that is 13.4V.

Also all previous routes are disconnected and the ground reference point is shared.

Thanks and Regards

Mehran Fard

[Andy.G]

Hey Mehran,

This may be due to a calibration issue, as when the DMM read function is calling a driver which may be pre-programmed to calculate the voltage with respect to a reference value. Just to clarify you are getting the expected voltage reading after running the DMM IVI Read step?

[Fardm]

Hi Andrew,

Yes  I am geetting the expected voltage after "DMM IVI Read step".

The point you made with regards to calibration issue makes some  sense but how can I prove it. I connect DMM -ve to shared ground prior to the measurment.

This is causing me a bit of problem during validation where we compere the NI DMM reading against external DMM.

[Andy.G]

Hey Mehran,

You should ignore the reading of the DMM prior to that of the IVI call, for the reasons stated above. Are the readings different from that of the external DMM?

[RayFarmer]

Is it the external DMM reading the lower value and does this change to 19.7 after the IVI call?

[Fardm]

Hi Ray,

Yes the external DMM is the lower value and it does change to expected 19.7V after DMM IVI read.

We record an external DMM reading to validate our test software and these two readings do not match untill DMM IVI call.

I can get round this problem by having a dummy "DMM IVI read" prior to recording the external DMM value, but cann't explain why?

Regards

Mehran Fard

[Andy.G]

Hey Mehran,

Can we have a look at the sequence file?

[Fardm]

Hi Andy,

It is just an step with "DMM IVI" to measure voltage, attached is the image of the sequence I am using with simplified cct. 

Unfortunately I won't be able to post you the complete sequence.

Regards

Mehran fard