Voltage creeping up during load cell measurement

[Chris_P_SD]

I am measuring a load cell through an instrument amplifier that gives -10V to +10V to AI1 on a PCI-6052E through my SCXI-1303 break-out mudule.  I have checked the voltage at my breakout box with a digital multi meter and it is reasonable.  When I look at the voltage in MAX, I will get ~ -1.21V when the mult-meter shows .091V.  Furthermore, this voltage displayed in MAX slowly creeps to a larger negative value when I allow it to sample the signal in continuous mode.  I am pretty new at LabVIEW and am not sure what is going on.  Any advice?  Thanks in advance.

[Rob B]

Cpesko,

When you load the load cell are you getting reasonable voltage values?  Have you looked into Single Ended vs. Differential?

If you amplifier is outputting a SE signal and is wired as SE to the 6052, but the 6052 is configured for Diff, you will get bad readings as only one input terminal is connected to your signal and the other one is floating.  Check and try different 'Input Configuration' settings in MAX when you use the TestPanel.

Hope this helps

Rob Benjamin

[Brooks_C]

Hello Cpesko,

Rob has offered some solid advice about the input configuration.  Since it sounds like you're using an SCXI system it would be good to get more information about what hardware you're using.  Here are a couple good SCXI resources for you to look over:

Getting Started with SCXI

I would double check to make sure the software configuration in MAX matches the hardware.  In particular make sure you configure the properties tab of your SCXI chassis in MAX with the correct accessory.  The SCXI-1303 is a terminal block for an SCXI module and not just a break out module for the PCI-6052E incase that's the problem.  Since your reading seems to be floating I would guess that you are probably not measuring the channel you think you are, that could be because you're trying to read one of the PCI-6052 channels directly or your SCXI chassis is not correctly configured in hardware or software.  If you're still having trouble it would be helpful to know the following: 

1.  What SCXI module are you using?

2.  What SCXI cabling assembly do you have?

3.  What reading rate are you using?

4.  What accessory?  (I'm guessing SCXI 1303)

I hope this helps and have a great day!

Cheers,

[Chris_P_SD]

Thanks guys - Rob had it with the single ended setting.  I changed it to RSE in MAX and got the same voltage as the volt meter.  I checked the NRSE setting too and that one didn't work.  Anyone know what the R or NR before the SE stand for?  Thanks again folks.

[AnalogKid2DigitalMan]

R stands for refereced, in context as in a signal that is referenced to ground. I'd put in some links but my browser is slow today. You can search the Developer Zone for some in depth explanation of different analog input topologies

[griffindore]

and NR stands for non-referenced. I think the DevZone AnalogKid2DigitalMan would like to include is the Field Wiring and Noise Considerations for Analog Signals. This is a great DevZone to help you in connecting your signals properly.

[AnalogKid2DigitalMan]

Mike: Bingo, that's the link I was shooting for. Thanks for the follow-up.

-AK2DM