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DP83640: 100base-fx mode MDI terminations

Francisco Aguerre
Prodigy 80 points
Part Number: DP83640
Other Parts Discussed in Thread: DP83620

Hi everyone,

We are working on a design with the DP83649 PHY in 100BASE-FX mode and the Broadcom/Avago HFBR-5963ALZ which uses LVPECL interface.

I am a bit confused about the proper termination of the TD+/- pair. I understand the PHY transmitter is an open drain output, so it needs the 50ohm pull-ups and because the Avago transceiver is LVPECL we have to use AC-coupling and then properly bias the LVPECL side.

What has me confused is the fact that in the PHY datasheet it recommends to use 130ohm/82ohm pull-up/pull-down termination, but in the TI scaa059c app note when it explains terminations for an AC-coupled LVPECL interface it says it requires a 83/130 termination because LVPECL needs 2V common mode voltage.

Which one is right?

Regards and thanks in advance!

  • 0
    TI__Mastermind 26320 points
    Suggest you follow as mention in Phy datasheet as it takes care of the other aspects of internal temination etc.

    Regards,
    Geet
  • 0 in reply to Geet Modi
    Prodigy 80 points

    Geel,

    Thank you very much for your response. Unfortunately, I don't think it answers my question.

    I've been investigatin further and I found this application note by Broadcom/Avago where they recommend termination schemes for several of their optic transceivers. Among the example transceivers is the AFBR-5803Z. This transceiver has a LVPECL interface like the one we are using and it's the one that appears in the schematic of the DP86340-EVK.

    As you can see in the Avago app note, all the examples of this transceiver using AC-coupling on the Tx lines use a 82ohm pull-up and a 130ohm pull-down. This is the same as what the scaa059c TI app note says, but in both the PHY datasheet and schematic of the EVK they use 130ohm pull-up and 82ohm pull-up. Hence my confusion. I guess maybe it doesnt really matter which one you use.

    We are having some packet drop issues with our design, so I've been reviewing the whole desing thing making sure all components were chosen properlyl, as we are having problems pinpointing where the issue is. That is maybe a post for another day.

    Regards.

  • 0 in reply to Francisco Aguerre
    TI__Mastermind 23675 points
    Hi Francisco,

    I believe the EVM and the datasheet to have incorrect termination orientation. I recommend that you follow the suggested termination of the Avago LVPECL design.
  • 0 in reply to Ross Pimentel
    Prodigy 80 points
    Ross,

    Thank you very much. I've come to the same conclusion, that the correct one is probably the one suggested by Avago.

    Regards.
  • 0 in reply to Ross Pimentel
    Prodigy 185 points

    Dear Ross,

    We are very keen to get to the bottom of this as we have had 4 separate customer issues with exactly this problem. In our testing in the laboratory we have found that DC coupling provides the best solution for the fiber-optic transceivers. We found that setting a common mode voltage of approx 1.25 and then DC coupling resulted in a more stable solution compared to the LVPECL terminations set either side of AC coupling caps. Can you please ask your applications team to test these options and provide us with a final solution. It would be greatly appreciated to have the best case recommendation direct from TI.

    many thanks

    Michael

  • 0 in reply to Michael O'Gorman
    Prodigy 80 points
    Michael,

    Just out of curiosity, are you using the DP83640 too? When you say you are using DC-coupling, you mean in both the TD and RD pairs or just the RD ones?

    Hope you get the problem sorted out soon.

    Regards.
  • 0 in reply to Francisco Aguerre
    TI__Mastermind 23675 points
    Hi,

    We recommend AC coupling and establishing the correct biasing on fiber transceiver side as well as the Ethernet PHY side.
    LVPECL is only appropriate when you are interfacing with an LVPECL transceiver. There are transceivers that are VML, CML or LVPECL.
  • 0 in reply to Francisco Aguerre
    Prodigy 185 points

    Dear Francisco,

    We are using the DP83620. We AC coupled the TX with 50 Ohm pull up on the PHY side. We DC coupled the RX without using the LVPECL termination and instead used the common mode of the transceiver at 1.25 to force the common mode of the PHY. The transceiver had built in 100 ohm termination so we did not need any further termination.

    best regards

    Michael