SCB 68 rigged up with potentiometer, having problems...

[jhammer]

[I'm trying to set up a grad student's equipment in the lab I work in so I apologize in advance if I misuse some terminology, I'm not entirely familiar with it]

I have an SCB-68 block hooked up to a computer via a PCI-6289.  I can see the device in MAX and it claims to pass a self-test.

I am trying to wire two 5V linear potentiometers into the block.  Right now, I have the first one with the feed and return wires in screw pins 68 and 67, which the pin-out diagram tells me are AI0 and AIGND.  The potentiometer should vary from ~0V at full extension to 5V at full displacement.  Instead, running a test on AI0 results in massive random uncorrelated fluctuations.

Another question on here suggested switching it to A1, which I did, but to no further result so I moved them back.

Does anyone have any advice on how to properly wire this setup?  Thanks.

[AnalogKid2DigitalMan]

No mention of your 5V connection.

Wire one side of the pot to AI GND, other side to +5V out, wiper gets wired to AI0. What is the full-scale resistance of the pot? If it is too low you may draw too much current from the +5 supply from the DAQ card, they are typically rated for a few 100 milliamps.

-AK2DM

[jhammer]

Mea culpa.  Red and black wires from the pot are going to a 5V power supply (+5V and GND respectively), wiper and return are to AI0 and AI GND respectively.

Pot resistance is at least 0.5MΩ according to the specs sheet (http://www.pennyandgiles.com/docGallery/190.PDF).

I should also add that in this configuration, I've used a multimeter to measure voltage across the wiper and return screws in the box, and the multimeter correctly varies from +5 to 0 as I close and release the pot.

Message Edited by jhammer on 01-15-2010 08:06 AM

[AnalogKid2DigitalMan]

Odd, check to make sure the cable is firmly seated at both ends (the DAQ end and the SCB68).

Have you set the switches up on the SCB-68 to match you DAQ card?

What ground-reference settings do you have the AI channel set to?

-AK2DM

[jhammer · Accepted solution]

Problem solved.  It turns out the issue was that because I was in differential mode, I needed to have both wires from the pot in two seperate channels.  Hooking them up to AI1 and AI9, everything is working as it should.  Logic dictates that future pots can be added across channels 2-10, 3-11, and so forth.

Thanks for the help.

[AnalogKid2DigitalMan]

jhammer:

Glad to have helped, based on what info you provided, those were my only 3 hunches.

-AK2DM