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DS90UH947-Q1: AC Coupling Cap Selection

Jared Becker
Genius 11845 points
Part Number: DS90UH947-Q1

Hi Team,

For the AC coupling caps of any serializer or deserializer, the datasheet recommends specific values. For example, on page 73 of the 947 datasheet, C1 - C4 are recommended to be 0.1uF. However, shouldn't the value be somewhat dependent upon the frequency of the data transmission? I ask this because the target transmission line impedance is supposed to be 100 Ohm differential, and if the caps are operating at a higher frequency than their self resonant frequency, they will start to look inductive as impact the differential impedance.. 

For example, if you were to operate the link with a data transmission rate of 2.5Gpbs, that would be well above the self-resonant frequency of a 0.1uF capacitor, which would start to look inductive at that point.

I imagine the recommendation is the way it is because this change in impedance doesn't impact the differential impedance too much, but would like to hear your thoughts on the matter.

Thanks for your support!

Jared

  • 0
    TI__Guru** 103215 points

    Hello Jared,

    The cap sizing is selected to reduce the negative effects of PDJ (pattern dependent jitter) for both the forward and back channels. When there are consecutive 1's or zeros in an AC coupled link, the trailing bits will always experience vertical amplitude reduction based on the step response of the first-order high pass filter given by the cap and the internal termination impedance of the SERDES device. By selecting a larger cap (0.1uF) you can significantly reduce PDJ for the system. You don't need to worry about the inductance change which is extremely low. 

    Best Regards,

    Casey