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Concurrent differential voltage measurement

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!

measurement_circuit_DC.png

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

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