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DS90UB954-Q1: AC coupling capacitor

Tomoaki Yoshida91
Genius 5255 points
Part Number: DS90UB954-Q1


Hi team,

We use a combination of 953-954 for differential.

Could you provide advice for data line design?

We have found that some documentation on the placement of AC coupling capacitors recommends putting them close to the connector.

But, the 954 datasheet says to place it close the RIN pin.

Please tell us why you should place it near the RIN pin and not near the connector.

I understand that the constants of the AC coupling capacitor are determined by the speed of forward channel communication and back channel communication by reading the datasheet.

But I want to understand how the constant is determined.

For example, could you derive with formula or equivalent circuit?

We will use CMC, but please let us know if there is a SPEC to be careful when selecting.

I would appriciate it if you could tell me recommended parts.

Best regards,

Tomoaki Yoshida

  • 0
    TI__Intellectual 1565 points

    Hi Tomoaki,

    Thank you for the questions.

      • I use the DLW21SZ900HQ2L CMC on EVMs.  To select a CMC:
        • Differential insertion loss low enough to meet the loss budget when lumped with PCB/connector/cable losses.
        • Common mode impedance as high as practical while still meeting differential insertion loss
        • Small footprint to minimize impedance discontinuity of each CMC pad
      • Part placement is based on function:
        • ESD protection closest to the connector to maximize board protection
        • CMC or PoC network in the middle where there is room
        • AC coupling caps closest to the FPD-Link Rin/Rout to match how the product was characterized
      • AC coupling cap values are selected then characterized in the lab to ensure:
        • Low insertion loss at the slowest BCC
        • Low insertion loss at the highest line rate
        • Increasing insertion loss above the highest line rate.  This helps with EMC immunity testing.
        • We only support the recommended AC coupling caps.  I don't have models to share.

    Best regards,

    Mike

  • 0 in reply to Mike Leister
    Genius 5255 points

    Hi Mike-san,

    Thank you for reply.

    I understand that.

    This is a personal interest.

    Should the AC coupling capacitor be placed close to the connector or close to the device when considering pure characteristics except for functions such as ESD protection and CMC?

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    TI__Intellectual 1565 points

    Hi Tomoaki,

    We choose to put the AC coupling caps next to the PFD-Link pins and it has always worked, so we stick with that.  If you want to move them closer to the connector it should be ok.  Lay it out both ways and see which is easier to get good routing and controlled impedance.

    CMC and ESD should be close to the connector.  This limits coupling of the undesired waveform elements.

    Mike