Interference in data acquired by SCB-68

[Elijah_K]

Hey Varun,

I think Brian was on the right track when he mentioned the possibility of exceeding the common mode voltage rating.  If I understand your circuit, your controller is essentially providing a potential which is divided across the motor and the resistor.  The common mode voltage for this card is 12V, which means that the difference from the lowest potential of a measurement to the highest potential of a second measurement cannot exceed 12V (with respect to the same ground). 

Be sure that you're not exceeding the common mode voltage by checking the voltage output from the controller.  I may have misunderstood your circuit, so if this doesn't solve your problem, please attach a diagram and we'll take it from there!  (to attach diagrams, check out the forum help)

Happy new year!

[Varun Agrawal]

Hi Elijah,

Your suggestions have indeed been helpful. It looks like the problem indeed is with common mode voltage. Since the Differential Mode automatically eliminates that problem, I didn't look into that aspect earlier. It turns out that the voltage of each end of the resistor terminal with respect to the SCB-68 block GRND terminal is quite high. I tried using the NRSE mode but it still is showing the problem. And I'm not quite sure how to get rid of that Common Mode Voltage.

Thanks,
Varun Agrawal

[AnalogKid2DigitalMan]

How about moving the sense resistor to the common (negative) end of the supply?

(-) _/\/\/\/\______(Motor)_______(+)

Or use a hall effect current transducer for total electrical isolation

[Varun Agrawal]

Thanks Kiddo..

I missed to mention in my previous post that the way the voltage is being supplied to the motor (and the resistor) by the controller card is, though the voltage drop across the motor is very low, the voltage level with respect to the SCB-68 block GRND is very high (of the order of 47 V). So even changing the position of the resistor will not help here.

And unfortunately I don't have any current transducer to use in this case.

Looking forward for any possible suggestions,
Thanks,
Varun