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TIDA-00299: Earth-to-GND Connection

Shotaro Sakai
Intellectual 2430 points
Part Number: TIDA-00299

Hello team,

My customer has questions for TIDA-00299 of 2.2.1.2.3.1. Please support him and me.

  1. The document says "due to the potential difference being outside of the specification of the PHY". Where is the potential difference on the schematic shown?
  2. Why does RC-connection prevent this potential difference? He wants to know the mechanism.

Best regards,

Shotaro

  • 0
    TI__Expert 7840 points

    Hello Shotaro,

    What we wanted to say with this sentence is that you have the transformer which is isolating the cable from the PHY.

    Due to this connection of the transformer you now have the bob smith termination which is related to Earth and you have the PHY which is referenced to GND.

    Now if the Earth potential is getting too big or small compared to the GND potential you will have an issue were the common mode of the MDI signal will be out of specification as the transformer will not be able to remove the difference of potential between earth and gnd.

    By making the RC connection you ensure you can keep the noise on the earth plane but keep the reference between the two potentials within an acceptable range due to the RC filter.

    If the time constant of the RC filter is too low it can be seen that after a couple of noise pulses the communication fails as the capacitor keeps increasing the potential difference of between host and client system, but it is not discharged quick enough after being changed.

    Regards,

    Kristen