analog values varying

[balajikasthuri]

Hi,

I have connected an unground(isolated) anolog output of a tachometer to ACH0 and ACH8(Chanel 1) of PCI6014 in differential mode. I have connected a 100 K bias resistors from ACH0 and ACH8 to AIGND. I am able to measure the analog values through labview (and MAX)correctely as long as I keep pressing the resistors across the terminal board with my hand . Once I remove my hand, I measure my values correctly for few seconds, but later my readings jump erratically before returning to normal. This cycle of normal readings and disturbance keeps continuing through out my experiment.
Could any one help.

[RIO-Todd]

I'm wandering if you are looking at the correct channel in LabVIEW and MAX. When in differential mode and connected to channels 0 and 8, that would correlate with channel 0 and not channel 1. So, if you're reading from channel 1 within MAX, you would be reading from a channel that nothing is connected to.

I would suggest to read from channel 0 within LabVIEW and MAX and see if you have the same results.

I hope this helps.

Regards,

Todd D.
NI Applications Engineer

[balajikasthuri]

Hi Todd,

Sorry, I had my signal correctly connected to channel0(ACH0 &ACH8)(not channel1). But I sitll have this problem. Please Help

[RIO-Todd]

What is the impedance of the tachometer? If the impedance is very low then I would try reducing the bias resistors to around 1 Kohm and see if that helps.

The size of the bias resistors should be ~100 times greater than the impedance of the source.

Regards,

Todd