Voltage input in USB-6008 DAQ is unstable

[marcnol]

I am trying to control the position of a piezoelectric stage with a DAQ USB 6008. I am using the AO1 to send to the signal input (0-5V) in the piezo controller and reading the output signal (sensor position proportional to the position of the piezo, 0-5V) from AI0. This voltage should correspond to the same value as that of the input signal. The problem I have is that the voltage at AI0 jitters with an amplitude of ~ 0.003V (usually single readings jumps), which in terms of the the position translates into tens of nanomers (much larger than the resolution of the stage which is under a nanometer). When I read this signal from a multimeter, it is highly stable. When I use a second PC to provide the input signal to the piezo and just read the sensor position using the AI0 of USB-6008 I don't get fluctuations in the voltage anymore.

So this seems to be happening when asking the DAQ to send an output and read an input in the same while loop.

Does anyone has any idea of how to solve this? The precision of this DAQ should be 14bits, so the precision at 10V should be 0.0006 V not 0.003V, so I am not sure what is going on.

Thanks for any help!
marcelo 

[AnalogKid2DigitalMan]

marcelo:

Are you using single ended or differential inputs? If differential, then what input range are you using. The resolution of the 6008 is 11 or 12 bits for the input, here is a link to the manual and then recheck your resolution calculations. You probably could do better with a higher performance card.

http://www.ni.com/pdf/manuals/371303l.pdf

A DMM may not show the jitter due to internal filtering. An oscilloscope may be a better choice for monitoring the signal. Do you have proper ground reference connections for the input and output signals?

-AK2DM

[marcnol]

Hey AK2DM,

The 11-12 bits explains it all. I am using differencial input and all grounds are common so that should be ok. I will try reducing the input range but I guess the solution is, as you recommend, to go to a higher performace card.

Thanks for your help,

best

marcelo