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Coc and AC Coupling capacitor selection for TLK6201EA

SwebAL
Prodigy 80 points
Other Parts Discussed in Thread: TLK6201EA

Can somebody point me in the right direction in selecting values for the AC Coupling capacitors and Coc for the TLK6201EA? 

  • 0
    TI__Prodigy 510 points

    The coc pins determine the bandwidth of the offset cancellation, which determines the low frequency cutoff.

    For non pathological signals which don't contain thousands of consecutive identical bits the pins can be left open, since the lf cutoff is already smaller than the cutoff of the external ac coupling for the differential lines which is usually up to 100nF.

    The low frequency cutoff can be moved further down  by adding a 100nF capacitor to the coc pins.  Shorting the coc pins disabled the offset cancellation and moves the lf cutoff to DC.

     

     

  • 0 in reply to Karl Muth
    Prodigy 80 points

    Thanks for the guidance.  Do you have any recommendations as to the AC coupling caps?

  • 0 in reply to SwebAL
    TI__Prodigy 510 points

    Sorry, for some reason I did not see this question earlier.

    The ac coupling caps are depending on the low frequency content of the data stream for a given data rate, i.e. on the coding used. The goal is to avoid DC wander because this would reduce the bit error rate. On the other hand the cap shouldn't be too big not to represent a considerable high frequency loss and mismatch because of its size. In most communication systems up to 10Gbps I'd recommend 100nF (64b/66b coding). AC coupling caps for 8b/10b coding can be smaller for higher speeds (e.g. 10nF for 8GFC).

    There are systems like PCIe which require a certain ac coupling cap range (receiver detection). Numbers can be taken from the standard.

    There are extremes like in professional video applications (SMPTE) where pathological patterns can occur rquiring ac coupling in the range 5-10uF at about 3Gbps speeds.