Hi Perk_Bud,
There are a few things you can try to narrow down the source of the problem. Please provide the following details so that we can help you further:
Is the signal you are trying to measure a DC signal?
Does the voltage difference (between the USB-6009 and the voltmeter) remain fairly constant at 1.5V or does it fluctuate over time?
Are you measuring a differential signal, referenced single ended signal, or a non-referenced single ended signal?
Is the source of your signal grounded to earth ground or is it a floating source?
Try to connect an analog output channel from the USB-6009 back into an analog input channel. Output a constant voltage and check if it the analog input channel is reading the correct voltage. If it is not correct, then try a different input channel. If it is correct, then you might have a problem with grounding. Refer to the DeveloperZone article:
Field Wiring and Noise Considerations for Analog Signals for details on how to properly ground your signal.
Best Regards
Hani R.
Applications Engineer
National Instruments