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  • Connecting PHY ports inside board

    Hello,
    Normally PHY pins of IC are isolated from connectors by magnetics or capacitors (if there is no place).
    What about internal connection, i.e. if transceiver (e.g. KSZ8081) pins TXp, TXn, RXp, RXn not go to connector, but to the switch (e.g. Realtek), located on the same board ?
    Should I place capacitors between TXp, TXn, RXp, RXn of transceiver and TXp, TXn, RXp, RXn of switch ?
    Thanks ?
  • Answer selected by Pavel_47 at 12-01-2022, 06:29 AM.

    I always put in 0402 0 ohm resistors inline.. this way if you need serial resistors or capacitors you do not need to redo your layout but can just update the bom

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    • #2
      I think you need to be careful about if they are modulated on DC or not. Some PHY do that, some not.

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      • #3
        Thanks Robert,
        In KSZ8081 datasheet the DC level on TX isn't specified, only differential voltage after transformer is provided
        Click image for larger version

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        The Realtek doesn't provide any documentation, so, probably, the wise solution is to keep capacitors ?​

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        • #4
          I don't know which version you have, but I believe, this is important (so you may need the capacitors):
          Click image for larger version

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          • #5
            Yes, I use this chip.
            In the fragment you provided they talk about connection to the "external world" featuring transformers.
            My goal is to wire transceiver internally, i.e. other IC (i.e. Realtek switch) us located in the same board.
            So, I guess, the only problem is to know if DC level of TX on Microchip and Realtek is Ok for corresponding RX, isn't it ?
            The Microchip datasheet doesn't specify DC level on TX outputs.
            If we assume that these TX oscillate between ... let's say 1V and 2V (so DC = 1.5) and this DC is Ok for Realtek RX, perhaps capacitors are useless ?

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            • #6
              I don't know - it may need some testing, measuring and datasheet and specification reading. But if you would like to connect them together, it may be useful to have some components on PCB so you can experiment with the connection if something is not working properly.

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              • #7
                I always put in 0402 0 ohm resistors inline.. this way if you need serial resistors or capacitors you do not need to redo your layout but can just update the bom

                Comment


                • #8
                  Ok, thanks

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