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ADS127L11EVM-PDK: How many bins are trimmed in FFT for SFDR calculation ?

TonyMCC
Intellectual 760 points
Part Number: ADS127L11EVM-PDK
Other Parts Discussed in Thread: ADS127L11

I am testing ADS127L11 evaluation board and noticed that SFDR reports 128db, however there are some low frequency (non-harmonic) spurs at 110hz and 240hz that do not seem to be accounted for in the SFDR result.

Can you explain why SFDR does not account for these spurs and how many low-frequency bins are normally trimmed in the FFT for the SFDR measurement result...?

     

Thanks,

Tony

  • 0
    TI__Mastermind 46860 points

    Hi Tony,

    I have not seen this behavior before and will need to do some research to get you an answer.  Typically, the spur's occur at a higher frequency than the fundamental, and the software correctly identifies the spur in these cases.

    I am not certain how long this will take, but I will provide an update or hopefully an answer in the next 48hrs.

    Thanks,

    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Mastermind 46860 points

    Hi Tony,

    The algorithm will find the largest spur NOT in the 1/f region.  I believe in your case, the software has identified this region as 1/f and is ignoring any local peaks. In the below example, the software correctly finds the spur at 1155Hz.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Intellectual 760 points

    Thanks Keith.

    So what does the software consider as the frequency range of the 1/f region ?

    In my plot I thought the 1/f area would be as shown below...

    Tony

  • 0 in reply to TonyMCC
    TI__Mastermind 46860 points

    Hi Tony,

    After discussing with the software engineer, we figured out that the default frequency range that the GUI uses for SFDR is the fundamental (1kHz in your case) up to 1/2 of the nyquist rate.  The software will ignore any spurs less than 1kHz with a 1kHz fundamental.

    You can change this default by modifying an initialization file.  Assuming you installed the software to the default directory, you will find it here:

    C:\Program Files (x86)\Texas Instruments\ADS127L11 EVM\Configuration Files

    Open the Operator Support_ADS127L11 EVM.ini file with a text editor (you may need to change the file permissions in Windows) and change the below line to the lower frequency that you want the software to look for spurs.  In this case, I set it to 100, which will look for spurs in the range of 100Hz up to 1/2 times the output data rate.

    SFDR_MinFreq(Hz) = 100

    I was able to verify that this correctly works with a 120Hz spur and a fundamental of 2kHz using the ADS127L11EVM.

    Regards,
    Keith