Concurrent differential voltage measurement

[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 AIs 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