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Effect of reference ground on differential reading

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Posted 03 October 2014 - 12:22 PM

We are taking a differential reading between an analog signal (AIN0) and machine ground (AIN1).  If machine ground fluctuates it apparently affects the differential reading.  Am I correct?  If so, is there a way to filter the ground signal?   Our set-up is written below.  0.001V differential reading is equal to 1 micron.  We can see spikes of greater than 100 micron; i.e. 0.1V


We have one U6 Labjack integrated into each of our labatory instruments.  A laser distance scanner analog output is connected to AIN0 and the laser scanner reference ground is connected to AIN1.  We take a differential reading between ref ground AIN1 and the analog signal AIN0. 

Labjack SGND is tied to the machine ground which is the facility ground via main power feed ground to the machine.  0VDC of the 24VDC supply that powers the distance scanner is connected also to machine ground.


AIN1 ref ground signal logging is attached.   Any thoughts on this are appreciated.



#2 LabJack Support

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Posted 03 October 2014 - 02:26 PM

To what the U6 is capable of with your particular settings (Range and ResolutionIndex), remove your signals and instead jumper AIN0 & AIN1 to GND, and you should see noise levels that match Appendix B:





So machine ground is tied to SGND and AIN1?  Seems okay, but I to try something different remove the connection from SGND and instead just add a 100k resistor from AIN1 to GND to provide a weak ground reference yet keep the signal pretty much differential.


Or don't connect machine ground at all, but instead just connect Laser+ to AIN0 and Laser- to AIN1 with 100k to GND.  Or connect Laser- to GND and do a single-ended reading of AIN0.



Perhaps get a USB isolator to isolate the U6 from host ground and further break any ground loops:



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