NI USB-6008

[CGAJ]

Hi, I'm trying to read single-ended analog voltage input from an Instron materials tester console.  When the USB is off, the voltage at the input pins is accurate, but when it's powered on the values have an offset of about -0.04 V.  Why does this happen?  If it's because of low current in the console, will using the USB with it damage the internal circuitry?

The Instron console is model 4400, and I don't have any other information about it.  There is no voltage offset when the USB is reading from a battery.

Thanks for your help

[SnowMule]

Could be a ground loop.

Could also be the low input impedance of the 600x. 

Have you tried measuring the output with a DMM?  Does that have an offset too?

Put a buffer (op-amp voltage follower) inbetween the DAQ and the device.  Helps a lot.