Why does my card have a 12 volt dc offset

[neurostu]

I recently got my card working under linux and I've been running a few of the example programs written in c.

I hooked up a signal generator to my card, with the signal going into AD0 and the ground plugged into AD GND.  I also have the same signal going into an oscilloscope and a volt meter (for sanity checking).  I have found that the voltage being measured in my program is coming out around 12 volts more negative then what I'm putting in.  

At first I thought it might be the signal generator that was producing the 12 Volt DC offset but this doesn't make sense for 2 reasons. The volt meter is measuring the correct voltage across the input leads into my NIDAQmx - 6254. 

I've attached the code I'm using...

[neurostu]

I've tried calibrating the card using both the internal and external calibration function calls:

DAQmxErrChk (DAQmxSelfCal("Dev1"));
DAQmxErrChk (DAQmxInitExtCal("Dev1", "NI", &calHandle));

 But the voltages I'm reading out still have the DC offset.

[Efrain_G]

Dear nerostu,

That is a quite interesting behavior. What if you measure a known voltage source such as a battery to see what kind of reading do you get on your M series card. What kind of range did you choose? Do you have any custom scaling activated?

[neurostu · Accepted solution]

Thanks for your reply, I actually just figured out what was wrong, I feel pretty stupid but the example script was referencing AI0 against AI1 and not against ground, which is what I assumed it did. When I fixed this the DC offset went away.