Difference between Ground reference and differential mode of acquisition

[Jhareesh Akella]

Hi,

I'm using NI PXIe-6251 module to acquire analog inputs. I've attached the code that I'm using. The code is working fine for Ground as reference. When I replaced AREF_GROUND with AREF_DIFF, the card is giving faulty readings below -5 V. (-5 V to -10 V). 

I'd like to know if I missed something.

Regards

Jhareesh

[Gilles Detillieux]

Hi, Jhareesh.

You didn't mention anything about how you wired things up to the
PXIe-6251 module for the two different modes of operation, so perhaps
that's what you missed? NI modules actually have three modes of
"terminal configuration", and each one has its own unique needs for
cabling the signals. These are differential (AREF_DIFF in Comedi), NRSE
or non-referenced single-ended (AREF_COMMON), and RSE or referenced
single-ended (AREF_GROUND). The latter case is also commonly known
simply as single-ended. Some A/D boards don't implement NRSE as NI
boards do, only differential and (ground-referenced) single-ended.

The biggest difference between the differential input mode and
single-ended modes is that differential uses two A/D inputs for each
signal, as there is no common ground or reference. In some ways,
differential is easier because you don't have to wire up a common ground
or reference, but the tricky part is you must be sure you're using the
correct pair of input channels for each signal.

National Instruments has a few very helpful documents online to guide
you in the proper wiring for each mode of operation:

Grounding Considerations for Improved Measurements
http://www.ni.com/white-paper/3394/en/

How To Measure Voltage
http://www.ni.com/tutorial/7113/en/

Field Wiring and Noise Considerations for Analog Signals
http://www.ni.com/white-paper/3344/en/

Also check the wiring guide specific to your DAQ module, and to the
patch panel you're using with it. Some patch panels, like the BNC-2090A,
have slide switches for each pair of channels to select between SE and
DIFF, and another switch to select between RSE and NRSE when using SE,
so make sure these are all set correctly for the mode you're using.

Good luck,
Gilles

--
Gilles R. Detillieux E-mail: <grd...@scrc.umanitoba.ca>
Spinal Cord Research Centre WWW: http://www.scrc.umanitoba.ca/
Dept. of Physiology and Pathophysiology, Rady Faculty of Health Sciences,
Univ. of Manitoba Winnipeg, MB R3E 0J9 (Canada)