Connection to the PXI 6259

[Mudda]

Hi,

         I measuring a voltage signal using PXI 6259, Labview 7.1 and Windows XP. I have two DC motors connected to two different power supplies. I am measuring current through this motor when they are running. For this I connected a shut (0.001ohm) on the ground wire. The positive side of the shut I connected to the Analog Input channel 0 and the ground side which is the negative end of the shunt I connected it to AISense. I did the same thing for the second motor and the positive side I connected it to AI channel 1 and the negative side I connected to another AI sense of the PXI 6259. My question is by doing so do I create a ground loop? Any small noise on the signals shall make a big difference cause I a measuring milli volts. I also attached a Schematic to explain how I made the connections.

Thank you,

Mudda.

[Michael.Phillips]

Mudda,

Thank you for contacting National Instruments.

It looks like your configuration is correct, with the exception that you need to measure the voltage differentially instead of single-ended. Instead of using AISENSE, use AI Channel 0 (+) and AI Channel 8 (-) to do a differential measurement of the signal.  This will prevent the possibility of a ground loop being introduced in your system, as well as remove common mode noise. Take a look at this (http://digital.ni.com/public.nsf/websearch/82508CD693197EA68625629700677B70?OpenDocument) KnowledgeBase article for more information about measuring current using the Analog Input of a DAQ board.

Here are a few other links that might help:

Field Wiring and Noise Considerations for Analog Signals

http://zone.ni.com/devzone/conceptd.nsf/webmain/01f147e156a1be15862568650057df15

How Can I Do Current Measurements with NI Products?

http://digital.ni.com/public.nsf/websearch/29534F94B158054486256B490059E3AD?OpenDocument