Weird Analog Input voltage reading

[VeeJay]

Hi Guys,

This is a really weird problem I am not able to figure out. And it is with respect to Analog Input voltage reading into DAQ 6008. I have a voltage reading of 1.7into ai5 and GND (RSE). Reading the voltage directly from the sensor (the device that gives this 1.7V) I get a reading of 1.69V. When I connect it to the DAQ and read the voltage off the screw terminals (Using DMM) , I read a voltage of 1.55V, and offset of 0.14V.

Now, when I disconnect the DAQ USB cable from the DAQ board and then measure off the screw terminals, I get the right 1.69V. But, when I put the USB back on the DAQ card, I get the 0.15V offset again. What is going on here?

V

[Broken_Arrow]

Try reading it differential to see if the problem goes away. You may have a grounding issue.

[AnalogKid2DigitalMan]

What is the sensor exactly? Do you know how much current it can source? The reason I ask is because I think the analog inputs on the 6009 have an input impedance of only 140kohms.

-AK2DM

[JÞB]

analogKid is dead on!  so 0.15V/1.7V so you are loading your signal by about 9%  I guess the source impedaince would have to be about 15Kohms

Message Edited by Jeff Bohrer on 03-19-2010 05:27 PM