Concurrent differential voltage measurement failing on M series DAQ

[trubilo]

Hi,

I'm using NI M series 6251 USB DAQ and SignalExpress.

With these I'm trying to measure voltages from the circuit depicted below - but unsuccessfully thus far!

The voltmeters in the circuit are two AI channels on the DAQ board and they are connected each via a BNC cable to the corresponding resistors: one to the 100Ohm resistor (for current measurement) and the other one on the first of the voltage divider resistors. The two AI channels are configured for differential measurement.

Now if I measure only voltage over the 1MOhm resistor I get a correct value. The same is true for the 100Ohm resistor (I get correct current). But If I try to measure both voltages concurrently I can only measure the voltage over the 100Ohm resistor while the 1MOhm resistor takes the voltage value of the source.

If I measure this with two separate voltmeters without the DAQ board - the measurements work.

So I have something misconfigured in my DAQ settings I just don't know what it is!

Please help.

Kind regards,

David

[VA.KI]

Hello,

NI 6251 has multiplexed AI.

The wrong measurements you see may be caused by ghosting effect. See articles below about ghosting effect and how to eliminate it:

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YHy6CAG&l=en-US

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P84RSAS&l=en-US

There are more articles bout this effect if you search more but the idea is the same.

There should be settling time added in order to exclude ghosting effect from the measurement.

In the NI 6251device specification document on page 2 table 2 you may find suggested values depending on range and accuracy needed.

More information about settling time with example can be found in the following article.