signal offset on analog inputs PXIe-6361

[mcwilcr]

Hi,

I am using a PXIe-6361 and am experiencing some offset in the analog channels. I am making a multi-channel measurement (AI 0-15) in RSE mode with a sampling rate of 125K and 12.5K samples. I have one signal that is a 3.5Vsquare wave on AI15 and signals with amplitudes of hundreds of milivolts on the ramaining channels. I have reviewed the "Field wiring and noise consideration for analog signals" document and from what I can tell, I seem to have everything sufficiently grounded.

Now for the issues. the individual channels when measured indepindently have some offset on the order of hundreds of miliamps and I can not seem to find a way to eliminate these other than in the software (LabVIEW 8.2). Additionaly, when all of the channels are on and sampling, the 3.5V square wave on AI15 shows up on all of the other channels???. Again, I am able to filter this out in the software but the problem is that by filtering out some of these unwanted signal artifacts, I filter out necessary parts of the signals as well. Can someone suggest posible solutions I may be missing here?

Thanks,

Chris

[HeatherBaker]

Hi Chris,

Can you elaborate more on what measurements you are taking?  How are you determining this offset?  Do you see this behavior with a known value?  Does the offset exist in Test Panels?  I think you are seeing ghosting between channels.  See the Multichannel Scanning Considerations section of the X Series manual.  Hopefully this helps!

[mcwilcr]

Hi h_baker,

Thank you for the link, this was tramindusly informative. From what  I can tell, the issue is infact Ghosting. Unfortunately, I dont think there is anything I can do about it. the signal that is being ghosted to the other signals is the signal that I need to trigger on and thus must be the first channel. because I am using all analog channels, I do not see a way to sample a grounded channel after it before sampling the other signals. I am already taking special care to shorten and shield any cabling being used here so there is not much room for improvement there either. are there any other steps that I might take to remove this ghosting? at this point, I think I will just need to keep filtering it out in software. The attached images show the trigger signal and the output signals. In the image containing the output signals, the noise on the square wave is actually the signal I am measuring.

Regards,

Chris

[HeatherBaker]

Hi Chris,

What is generating the square wave?  Is this something we can use as a digital trigger rather than analog?  What sensors are you acquiring from on the rest of your channels?  Are they all the same?  What is the signal range for all of the sensors?  We may be able to rearrange them so that less of an effect is seen.  

[mcwilcr]

Hi h_baker,

the square wave is generated on the same board that generates the input signals for the sensors (actually, pixels in an array). The square wave has typical TTL voltage levels of ~0V low and 3.5V high and was designed with being used as a digital trigger in mind. A sensor board was designed to handle all the signal IO connections and the square wave can easily be connected to AI15 or P0.0 (currently AI15). Is it posible to use channel P0.0 as a digital trigger? if so this could posibly fix the problem. all of the remaining channels are connected to individual collumns in a 14 x 14 pixel array and the signal levels on all of the channels will be roughly the same at +- 200-300mV depending on the actual size of the pixels.

Regards,

Chris

[HeatherBaker]

Are you unable to use a PFI line?  This would enable you to create either a trigger to start the acquisition or use the pulse as a retriggerable solution.  In the LabVIEW Example Finder, take a look at Hardware Input and Output<<DAQmx<<Analog Measurements  1)  Acq & Graph Voltage- Int Clk-Dig Start.vi  2)  Acq & Graph Voltage- Int Clk-Retriggerable