Offset clipped due to NI 9263 Card

[samputhur]

I have a NI Compact RIO setup with NI 9263 Analog output card. I generated Pulse Width Modulation ( Vlow = 0v, Vhigh = 10 v) of frequency 100 HZ using LabVIEW 2012. I verfied the signal with the agilent scope. Iam seeing exactly 100 HZ ( Vlow = 0v, Vhigh = 10 v). But unfortunately, it did not work when I sent that signal to my speedometer. When verified with my agilent scope, I found out there has been an offset shift. Now my Vlow is 6v and my Vhigh is 10V. That means my pulse amplitude has reduced from 10v to 4V. The speedometer works OK when a square wave of same amplitude is applied from a function generator.

Do you know what is the reason for this?. Is it due to the internal pull up resister with in the 9263 card acting as a pottential divider between NI 9263 and Speedometer input?.the internal pull up resister of Speedometer circuitry is 1K ohm.

Let me know if there is a workaround.

[0utlaw]

Hello Samputhur,

At first glance, this sounds like a grounding issue- how is the speedometer powered and are your grounds isolated from one another?

Regards,

[billko]

{edit]

read the post incorrectly.

[/edit]

[billko]

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@0utlaw wrote:

Hello Samputhur,

At first glance, this sounds like a grounding issue- how is the speedometer powered and are your grounds isolated from one another?

Regards,

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I agree with the 0utlaw, especially if your speedometer is on a car or something that is isolated from your other equipment.  Maybe you tie the car chassis ground to your equipment ground?

[samputhur · Accepted solution]

Thanks Guys. After quite a bit of research, i'm convinced the issue is caused due to impedence mismatch. The real solution will be to buy a arbitrary waveform generating card which can match impedeance up to 10K OHm. Genarating a PWM from AO card is a very bad idea. Again it depends on the requirement of the instrument under test