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DP83867IS: DP83867IS: SGMII input impedence

user5118069
Prodigy 55 points
Part Number: DP83867IS

Good day,

In support case "DP83867IS: output 50 ohm impedence" it is mentioned that SGMII inputs are high impedance.

I'm measuring SGMII pair, it looks like there is reflection coming from high impedance (dip) when measured between pads 27 and 28 in DP83867IS. Is this expected behaviour in SGMII SIN/SIP?

Do you have reference signal waveform from SGMII SIN/SIP pads (it depends on the driver, too)?

  • 0
    TI__Mastermind 18425 points
    Hello.

    We have a IBIS-AMI model for SGMII on DP83867 which might help answer your question. SGMII SIN/SIP are input pads so they would not transmit a reference waveform. For SGMII input, the traces need to be 100ohm differential (50 ohm single ended). The PHY will need to be correctly configured to SGMII mode.

    -Regards,
    Aniruddha
  • 0 in reply to Aniruddha Khadye
    Prodigy 55 points
    Thank you for the answer.

    My question did not give all the details. When the other end of the SGMII pair (processor in my case) drives signal to this pair, I can see reflection when I measure the waveform between the DP83867 input pins 27 and 28. The layout is made using differential 100 ohm pair. SGMII is working but the signal quality needs confirmation.

    What I meant to ask was: is it ok to see reflection in this input pair? Is the pair terminated to 100 ohm (50 ohm single ended) inside the DP83867?

    Additional questions: Is there recommendation , where to place the AC capacitors, should they be placed close to transmitter (processor) or receiver (DP83867)?

    Unfortunately I do not have access to IBIS simulation tool.
  • 0 in reply to user5118069
    TI__Mastermind 18425 points
    Hello,

    The receiver input differential impedance is centered around 100ohms. Its ok to see some reflection which can arise due to tolerances in active components and board fabrication. SGMII spec allows for receiver differential input impedance to be between 80ohms to 120ohms. We ask customers to keep it as close to 100ohms as possible to reduce signal integrity problems.

    There are no requirement on the ac coupling caps to be near either transmitter or receiver. If DC strap resistors are used on the SGMII pins then the ac coupling caps need to be placed after the strap resistors.

    -Regards,
    Aniruddha