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DP83867 line driver center tap connection

A. D.
Prodigy 50 points

SNLA246A page 4 is telling "Each center tap of the magnetics should be independently de-coupled to ground via a 0.1uF capacitor."

What happens if they are not independently decoupled, i.e. all connected together within the integrated RJ45 can? Would it work? 

  • 0
    TI__Mastermind 19780 points
    Hi AD,

    We recommend not tying the center taps together. With the center taps tied together, you may effect the resolution time of auto-mdix and auto-negotiation.

    Keeping the center taps isolated from each other offers the best performance.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 50 points

    Hi Rob,

    thanks for the reply.

    one more related question: How important are the capacitor values shown on the application note.

    Especially, the 10nF, 2kV capacitor on the line side does not seem to be the standard: Tens of magnetics integrated RJ45 parts that I have checked all have 1nF there. 

    On the PHY-side, would having 4x10nF instead of 4x100nF be so bad?

    Kind regards, Aziz

     

  • 0 in reply to A. D.
    TI__Mastermind 19780 points
    Hi Aziz,

    To point 1, the capacitor is often 1nF but 10nF is also used in the Bob Smith termination. In our evaluation of the device, we used 10nF, this has become our recommended value.

    Anything outside of these values may display issues with common mode immunity in corner cases or across large production runs.

    The same applies for point 2.

    Best Regards,
  • 0
    Prodigy 105 points
    Hi Bob,

    I use DP83867 and i did not connect 0.1 uF capacitor for each center tap of the magnetics. Each center tap of the magnetics is connected ground. Does this effect transmission between Phy and external switch through RJ45.
    When i connect of DP83867 phy to external switch, I did not see link of switch up. I set auto- negotiation enable at address 0x0000 and when i read address 0x0001, i saw auto- negotiation is not completed. Do you have any recommendation why i do not have connection between phy to external switch?
    Thank you.
    Best Regards,
    Seda Demirhan
  • 0 in reply to Seda Demirhan
    TI__Mastermind 23675 points
    Hi Seda,

    The 0.1uF capacitor is required. The center tap cannot be grounded.

    Kind regards,
    Ross