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ADS54J54 Spurs related to JESD204b K value

David Hargrave
Prodigy 10 points
Other Parts Discussed in Thread: ADS54J54, LMK04828

I've got a custom board with an quad channel ADS54J54.  Captures from the data stream show spurious responses at frequencies and harmonics of the LMFC frequency.  I can change the value of K and these spurs move a predictable value.  What would cause this?

These spurs show up when the input to the ADC is terminated.  If an input signal is applied the spur remain in the same location.  The signal input into the system appears as expected and without any unexpected harmonics.  This removes the possibility of misinterpreting the data format.

Thanks.

  • 0
    TI__Mastermind 29220 points

    Hi,

    what are you using for your source for the clock to the ADC and for SYSREF?  Is SYREF continuously running?  On our EVM, we use the LMK04828 to generate the sample clock to the ADC and the SYSREF, and after we establish the JESD204b link we turn off SYSREF.  I am told that there is some contamination of the sample clock by SYSREF, and after I no longer need SYSREF to be running I turn it off and that quiets down the resulting FFT spectrum quite a bit.  Most of the FFT plots that I have handy are with the SYSREF off, so I don't have plots at the moment comparing SYSREF on vx SYSREF off.   Are you able to turn off SYSREF and compare the results?

    Regards,

    Richard P.