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ADS1256: Data Acquisition Returns Static Values

Abel John
Prodigy 20 points
Part Number: ADS1256

Hello,

My ADS1256 board is able to sample at rates adequate for my needs. The issue I have is that the data returned by the ADS does not vary as you would expect it to (I am feeding a sine wave into the board). Instead of capturing the wave, it instead will only capture the initial offset of the wave with minimal variation afterward. Here is an example of the data returned: (left column is voltage, right column is microsecond count - for wave w/ a 0.5 V offset and 0.5 V amplitude).

0.478901 1562

0.479181 2742

0.479105 3934

0.478935 5078

0.479025 6832

The data continues to hover at ~0.479 for the remaining thousand samples. For reference, the code I am using can be found at this github link (https://github.com/jurebartol/Rpi2_ads1256/blob/master/C/ads1256.c#L759), and I have modified it so that I am taking a single-ended read of the data for a certain count. I am wondering if someone has encountered this issue before and/or knows how to fix it.

Thanks in advance,

Abel

  • +1
    TI__Guru** 113765 points

    Hi Abel,

    Welcome to the E2E forum!  Can you tell us the register settings you are using?  Do you have a schematic that you can share?  What sine wave frequency are you providing?

    It would also be helpful if you provided the entire set of data as raw codes from the ADC instead of converting it to volts.  I also noticed that the time is not consistent between captures.  It is not clear whether this is the actual time of the collection and if the all the data is being captured following the end of each conversion.

    By Nyquist theorem you will be limited to 1/2 of the data output rate for your maximum input signal.  Also note that the digital filter is not a flat passband filter, but rather a sinc filter which will affect the ability to measure certain frequencies.  If there is an analog input filter this will also limit the frequency range that can be measured depending on the roll-off of the filter.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hey Bob,

    Thanks for getting back to me on this. Indeed the issue had to do with the Nyquist Theorem. Needless to say its been resolved and I am now able to sample at a rate adequate for my needs. 

    If you have the time, could you help me understand how the digital sinc filter affects the measurement of frequencies? I'm not sure I fully understand its implications.

    Thanks,

    Abel

  • 0 in reply to Abel John
    TI__Guru 52336 points

    Hi Abel,

    What is the frequency of the input signal you are trying to measure, and what is the data rate you are using to try to measure this signal?

    This might help us break down why this isn't working and (potentially) how to fix it

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Hi Bryan,

    No further issues! Bob helped solve my problem. I was just curious about an aspect of the ads and that lead to my reply.

  • 0 in reply to Abel John
    TI__Guru 52336 points

    Ok, sounds good, please let us know if you need anything else. If you have new or unrelated questions, please start a new thread and we will support you there.

    -Bryan