USB-6251 Input channel displaying incorrect waveform

[jchsu]

It does work, but this doesn't tell me a whole lot.  By removing the whole Analog Output sections, I have to sample a wave from the function generator.  However, as I tested and said in the original post, everything seems to be working fine when I sample the function generator.  The second element channel only seems to respond strangely when it's sampling the output generated by the board.

[Dennis_Knutson]

Well, what it tells me is that there is nothing wrong with the analog input. I would suggest you look again at how you are generating the analog output and maybe use a scope to view the signal.

[jchsu]

I've already viewed the output on a scope, and it looks exactly like how I want it to.  And it also looks exactly how I want it to on the analog output waveform, and on the 1st, 3rd, and 4th input waveforms.  Regarding how I'm generating it, the waveform is still being generated when I tested everything with the function generator, so I don't think the code for that is faulty either.

[Dennis_Knutson]

You've verified how the signal looks coming out of the DAQ card or you've verified how the signal comes out of whatever you are measuring? I would assume they are two different things.

[JÞB]

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jchsu wrote:

However, this problem isn't specific to channel 2, it's specific to the second element of the array I created.  If I switched the order of the array so that channel 1 is the second element and channel 2 is the first element, then channel 2 is fine and channel 1 shows the same properties of the error discussed earlier.   All 4 output waveforms are coded exactly the same way.

Does anyone have any clues or ideas what could possibly be the problem?

Thanks,

Jeremy 

Message Edited by jchsu on 06-12-2009 04:26 PM

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OOFDA! I spent a bit of time on this. Luckilly, I remembered the first rule of the technician- assume some assumption is incorrect.

When you change the order of your array of channels it has no effect on the output array order.  The output order remains AI(1,2,3,4) .  There is no scale that could be applied that is frequency sensitve so, you have a capacitor in parralel with the AI2 input!  This may be a broken ground (since you are RSE coupled) or it may indicate a broken DAQ.  the 6501 has 8 AI, try moving your signal to another available AI and test your cabling.

[jchsu]

Thanks, but unfortunately, I have already tried this.  Any aI does not work when it is the second element of the array (or list). 

Message Edited by jchsu on 06-15-2009 01:48 PM

[jchsu]

Another thing I just found out is that the problem only seems to happen when I'm sampling between ~347kHz to ~351kHz.  I am unsure of any other frequency ranges that could reproduce the problem though.  I could probably just get away with not sampling at those rates, but it really bothers me that it's still incorrect at those rates.