6704 Analogue Output produces a small positive voltage when I ask for zero

[Gordon W]

Hey NI Forums,

I have a PXI 6704 connected to a servo valve and I notice that its producing a small voltage when I command it to produce zero volts.  I can confirm this by disconnecting the 6704 from my servo valve and I can see that the valve is returns to its neutral state.  I will go ahead with verifying & calibrating since this is probably the logical thing to do.  Does anybody have any other ideas?

[Chris_K_]

Hi Gordon

Thank you for using our discussion forums. 

The problem you are seeing is most likely due to the fact that the analog output ground is at a different level than that of your servo valve.  However the first thing I think you should verify is that you are actually outputting 0 Volts.  If you have an analog input card, you should verify that you are actually outputting 0V. 

Next if you are seeing 0 volts, then the problem is most likely a grounding issue.  I would suggest trying to ensure that both your analog output card’s ground is connected to the same ground as your valve.  This may be the simplest.  In addition to calibrating your card,   checking your cabling is a good idea. Try to verify that you have a common ground. 

Another option is attempt to output a slightly lower voltage, whatever is needed to put the valve into its neutral state, and use that voltage value as an offset to scale your voltage output so that when you output 0V you are actually outputting a slightly different voltage, but your device sees 0V.

The latter solution is not as ideal as ensuring that your grounds are the same but it may be a simple work around.

Chris_K

[Gordon W]

Thanks Chris,

I will try your suggestions.  They seem to make a lot of sense and I'm definitely a big fan of that!