Analog input not working

[Pagoda]

Recently I have been unable to read an analog signal. I am using a TBX-68 connected to a PCI-MIO-16XE-50 board.  The signal will max out at whatever I set the maximum voltage to read when I hook anything up to it.  It will read zero volts with noise when the wires are not connected to anything and it will read somewhere around 6+/- 2 volts when the two wires are hooked up to each other, well away from the 0 volts that I would expect.  I have tried replacing the board with another PCI-MIO-16XE-50 without luck.  I have hooked up the wires to different analog inputs too.  I know its reading from inputs that I have hooked up because it will react to what I am doing only to the inputs that I specified (if it's hooked up to AI 1 and I connect a battery to 2, the reading is unaffected).  Analog Outputs and Digital are working fine.

Recently I have gone into DAQ Express changed some things to allow me to specify channels from my front panel, yet it doesn't work when I create a new one too.  Could I have permanently changed the DAQ Express?  It was working before I messed with any of that but It may have also been working after.  I cant recall the last time it worked vs. when I first opened the front panel of the a DAQ Express.

I've attached a simply VI that I would expect to read voltage inputs (even with nothing connected, it is reading -10 volts).

[EBL · Accepted solution]

Hi, Pagoda.

It sounds like you may be in differential configuration and may not be biasing the input lines. In differential configuration, both input lines of a channel must receive bias from ground somehow; it can be through huge resistances, but something has to tether the voltages to keep them from floating away. Connecting the + and - lines together is not enough, since that doesn't provide a path to ground.

Try connecting both + and - directly to analog ground, and see if you read a quiet 0 V. If that works, then hook up the inputs to whatever you want to measure. As long as both + and - have a DC path to ground, however circuitous, everything should work fine.

Normally, if your source is floating, you'll want to tie the - input to your device and also directly to analog ground. If your source is grounded, you shouldn't connect the - to ground, since it's probably already connected.

Hope this helps,

EBL

[Pagoda]

Yeah it did.  I thought I was in single ended, I didn't see the option for differential.  I switched it to single ended and it's working fine.

Thanks for your help