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Incorrect voltages measured by PCI-6120 when using "Warp Mode"

I've implemented "warp mode" as shown in this article to sample at 1 MS/s with the PCI-6120:

 

http://zone.ni.com/devzone/cda/epd/p/id/3987 

 

However, measuring DC voltages in "warp mode" gives an incorrect value (due to a ~4 mV offset and slightly different gain, depending on the channel as shown in the attached figure).  I have verified that the voltage measured at sampling rates <= 800 kS/s, agrees with that measured by a digital multimeter, and that it is the warp mode value for the voltage which is incorrect.  This behavior occurs both in code written using the NIDAQmx C API and using the LabView VI from the above link.

 

Any suggestions would be appreciated.

 

Thanks,

Dave

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Hi Dave,

 

I was wondering if you could provide a little more information about how you are testing the DC voltages. Is the value plotted just a single sample or is an average of multiple samples? In the article that you linked, it mentions that the first point acquired in warp mode is not accurate and should be discarded. The more details you can give me on how you conducted this test, the better I will be able to try and recreate the problem over here.

 

Regards,

Kent

Applications Engineer

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Hi Kent,

 

Thanks for getting back to me -- sorry to not be very detailed in my initial post.  The plot posted above was created by setting the analog output to a voltage from 5-150 mV, and connecting this to each analog input channel and reading 1s of data using warp mode (sample rate 1 MS/s) and normal mode (sample rate 800 kS/s) in succession.  The voltages in the plot are the median of the data recorded, so they are not affected by the first few samples.  I'm attaching the matlab code used to take the data (wm_test.m in the archive code.zip).  

 

As you point out, there is some strange behavior with the first few samples in warp mode, although it is not the behavior documented in the linked article.  The attached plot (warp_matlab.png) shows the time stream data for warp mode and normal mode taken immediately in succession, with the same 100 mV signal from the analog output channel (verified to be 100.2 mV with a DMM).  In fact, the first point is the only one that is correct!  After the first point, the warp mode measurement fluctuates over ~5 samples and comes to a steady state value with an offset ~4 mV from the true value.  This is the offset seen in the plot I posted in my initial post.  This same behavior with the correct value at the first point and then a steady state value offset from the true value was seen at each voltage I looked at.

 

I don't think it's a problem with my Matlab code, since this behavior is reproduced in the VI linked from your site above (modified not to discard the first sample).  A screen shot of the same behavior in the VI is shown in warp_labview.png.

 

-Dave 

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Hi Dave, 

 

I'm currently working on getting a test system set up to see if I could reproduce this issue. I had a few more questions about the test that you ran. Would it be ok if I got your contact information from the forum administrator?

 

Regards,

Kent

Applications Engineer

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Hi Kent,

 

Yes, please feel free to get my information and contact me directly if you have any questions. 

 

Thanks,

Dave 

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Hi Kent,

 

Were you ever able to reproduce this issue?  Any ideas what the problem could be?  Please let me know if I can provide any additional information.

 

Thanks,

Dave 

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