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Original question:

DP83822H: Looking for Documents that Outline PCB Design for Boards with Ethernet Circuitry

DP83822H: Coupling Component and MDI Trace Questions

Michael V
Prodigy 221 points

Part Number: DP83822H
Other Parts Discussed in Thread: DP83822EVM

Hello,

I have a customer who is considering the DP83822H in one of their designs. For reference on ethernet circuit/PCB designing, they have been reviewing the following documents:

  • snlc071 DP83822EVM
  • snlr048 Layout Guideline
  • snlr053 Schematic Checklist

 

After reviewing those documents, they have a few questions on information in snlr053.

Those questions are below:

  1. What is the purpose of the coupling 1MΩ resistor and 4700pF capacitor components between RJ45 Earth (RJ45_ERTH) and digital ground (GND), and what would be the effect of removing those components?  
  2. Is there a recommended MDI trace width?
  3. The total length of each MDI trace (PHY to magnetics) should be less than 2 inches, but in the case of an RJ45 with internal magnetics, what should the length of the MDI trace (PHY to RJ45 with internalized magnetics) be?

I look forward to your assistance.

Thank you,

Michael

  • +1
    TI__Mastermind 23141 points

    Hi Michael,

    1. The purpose of those resistor and capacitor is to ensure earth and digital ground are in a similar ground reference but to ensure the noise from high and low frequency are filtered out. We typically recommend customers to keep this ground type of separation but customers also like to keep two grounds float to each other. There may be no effect for keeping it floating but in some cases it may have increased EMI performance depending on the layout design.

    2. For trace width, there is no specific recommendation but I recommend to refer to the EVM designs and follow its trace length.

    3. For integrated magjack, please try to keep within 2in trace also.

    Best,

    J