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ADS5560 jitter question

David Park
Expert 6060 points
Other Parts Discussed in Thread: ADS5560

Hi,

Customer thinks 0.5ps combined clock jitter is needed for ADS5560.

Aperture jitter of ADS5560 is 90~130fs.

Question is what jitter is needed in external clock for 0.5ps combined clock jitter.

external clock jitter has to be 0.5ps-(90~130fs)= around 0.4ps?

or is external clock jitter ok to be just 0.5ps?

Thanks.

  • 0
    TI__Mastermind 29220 points

    Hi,

    Assuming the clock jitter is gaussian (as opposed to something deterministic like duty cycle or some aggressor coupling in) then external clock jitter combines with aperture jitter in a square root of sum of squares fashion.

    So if we want 500 fs rms combined jitter and take the aperture jitter to be 130 fs rms, then the external jitter would need to be better than 483 fs rms.

    square root ( 130 ^2  + 483 ^2)  ~= 500

    So when external jitter gets to be much larger than aperture jitter then the combined jitter gets to be pretty close to just the external jitter.   Likewise if the external jitter gets to be much less than the aperture jitter then there isn't much point in trying to improve external jitter any more. 

    Regards,

    Richard P.