Multifunction DAQ

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USB-6255 not reading correct voltage

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I have the USB-6255 and I'm using the differential inputs to read in a voltage value.  This voltage value is provided by an external power supply, However, the DAQ does not read in the correct voltage.  I set a power supply to 5V and measured with the DAQ and a digital multi-meter.  The multi-meter will read 5V but the DAQ will read 6V.   I have tried other supplies and the same results will occur. I have also tried the other USB-6255 that I have. The only case when I have measured 5 volts correctly was when I used a power supply that has both ground and earth ground tied together.  Why does connecting ground and earthground cause the voltage to be correct?  All power supplies and DAQS are powered by the same outlet.

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The DAQs can not be used with floating signals.  If you did not connect the AI GND to the supply reference, then Earth GND will be used as a reference (from the USB connection).


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