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DP83620 AC coupled only on TD pair?

A Griffith
Prodigy 20 points
Other Parts Discussed in Thread: DP83620

Hi,

I'm using the DP83620 with the AFBR-5803 Avago transceiver.  The datasheets for both devices indicates that they are PECL compatible.  The reference schematic in the DP83620 datasheet (page 35) includes AC coupling for the TD+/- pair.

Why isn't it necessary to include AC coupling for the RD+/- pair?

I read the "DP83620 AC coupled?" question (http://e2e.ti.com/support/interface/industrial_interface/f/142/t/194096.aspx), and in that case, I can see AC coupling is needed due to the different logic levels, but could somebody please explain why it is needed for the AFBR-5803 (same transceiver on the Eval board) and if so, why only on the TD+/- pair?

Thanks,

Alex

  • 0
    Mastermind 7395 points

    When you go to a connector, in many cases you prefer to go out without a DC component. That is why the transmit is AC coupled. If the transmitter in the other side wants to filter the outgoing DC, it will also have AC coupling on its transmitting side.

  • 0 in reply to Avi Chami1
    Prodigy 20 points

    Then how about the RD+/- pair?  Do I need to AC couple those lines too?  The reference schematic does not suggest so.

    Thanks,

     

  • 0 in reply to A Griffith
    Mastermind 7395 points

    Normally you expect that the coupling is done on the Tx side, so you dont couple again on the Rx side.

  • 0 in reply to Avi Chami1
    TI__Mastermind 31600 points

    Alex,

    Similar to the post that you referenced and consistent with Avi's comments, the coupling is related to the necessary voltage levels for both components.  In this case, the Avago transceiver supports LVPECL levels for transmit, receive and signal detect.  The DP83620 supports LVPECL inputs for the receive and signal detect signals, but does not provide LVPECL output levels from the transmitter.  Therefore, only the transmit outputs from the DP83620 need to be AC coupled.

    Patrick

  • 0 in reply to Patrick OFarrell
    Prodigy 130 points

    Hi,

    AC coupling is neccesary ,  but I want to know why the pullup resistor is 130Ω and pulldown resistor is 82Ω? When the power  supply is 3.3V,the LVPECL common-mode voltage should be 2V,  according to the value of resistors,the common-mode voltage is  3.3*82/(82+130) =1.3, Why?

    I think the  pullup resistor should be 82Ω and pulldown resistor 130Ω, the DP83620 datasheet or demo board is wrong. Is it?

    thanks!

    zchong

  • 0 in reply to rtos
    TI__Mastermind 31600 points

    I have moved your post to the Ethernet forum so that it can get visibility from the experts.

    This post is continued on a separate thread at:

    http://e2e.ti.com/support/interface/ethernet/f/903/p/343445/1200558.aspx

    Patrick