Trouble with Analog Inputs

[mhellman]

Hello,

I am currently taking in 8 DIFF analog inputs. When I look at them in LabView all of the inputs mirror each other. For example. If I really only have ACH0 hooked up with 5V and the rest of them have nothing on them...all of them will show 5V. Does anyone know why? Thanks for your help!

Michael

[BillMe]

More information would be helpful:

1) what hardware you're using
2) version of LabVIEW
3) Posting the VI you're using to acquire the data

[Neil.Pate]

You probably have crosstalk from your actual connected channel. Try grounding your other inputs, as you said you are using differential channels then you can just connect the + and - of the other channels to each other.

[johnsold]

I agree that this sounds like the crosstalk which is common with interminated inputs.

With many boards you must also provide a resistive path to ground (or some other voltage within the common mode range of the inputs) for the input bias currents while in differential mode. See the instructions for your device for details.

Lynn

[Nick_F1]

Hi Michael,

This might be a ghosting issue, which is essentially the samething.  Here is a knowledgebase article that explaines how to reduce ghosting.

How Do I Eliminate Ghosting From My Measurements?

[Neil.Pate]

Thanks for the link Nick,

The hyperlink to the DAQ pinout diagram does not seem to work though:

http://ae.natinst.com/Operations/AE/public.nsf/fca7838c4500dc10862567a100753500/73cb0fb296814e2286256ffd00028ddf/$FILE/ghosting.png

Message Edited by nrp on 08-03-2007 08:41 AM

[mhellman]

Thanks for the link. Very helpful. Thanks again!!!!

[Nick_F1]

Thank you for letting me know about the link.  I just fixed and the fix should go live after it is verified.

Message Edited by Nick F on 08-03-2007 03:49 PM

[tbob]

You should ground all unused inputs.  If the channel is floating (unterminated), you can't depend on the reading.  Why do you want to read an unused channel anyway?