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LMH1297: 12G-SDI AC coupling capacitors

Renzo Posega
Prodigy 170 points
Part Number: LMH1297

For SDI interface ICs, AC coupling is needed at SDI inputs/outputs.

Generally, a 4.7uF ceramic capacitor in 0402 footprint is recommended.

Nevertheless, MLCC capacitors with high value exhibits a rather low (in the MHz range) SRF, with impedance + ESR going rather high on beyond 1GHz.

Here below, the performances graphs for a typical TDK reference commonly used in TI boards.

Is this a non-issue, or an unavoidable evil?

should we try to piggy-back a smaller capacitor on top of the 4.7uF one?

  • 0
    TI__Guru* 93670 points

    Greetings Renzo,

    Frequency of interest is at about 6GHz. This is because this is the nyquist frequency at 12G SDI. At this frequency impedance is about 5 ohms. This has not caused performance issue or side effects in two areas: Firstly, device passes return loss with margin. Secondly cable reach is as expected. We would have preferred to use a cap that has lower impedance at 6GHz however cost and availability caused us to use this cap. You are welcome to use a better cap or a lower impedance cap and compare return loss using either one of these two parts.

    Regards,, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 170 points

    Dear Nasser,

    thank you for your reply. As I read your answer, it seems to me that the impedance if this capacitor - given the required high value and small footprint - is an unavoidable evil, anyway not causing great issues.

    I was just puzzled that no one seems to have noticed this.

    Fine for me, I am reassured.

    Thank for your support, kind regards

    Renzo