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Ethernet MDI trace impedance (DP83848C)

Colin Helliwell
Prodigy 150 points
Other Parts Discussed in Thread: DP83848C

I'm laying out a PCB for the DP83848C (to an RJ-45 with internal magnetics). The Apps Note guidelines say "MDI signal traces should have 50 ohm to ground or 100 ohm differential controlled impedance". I understand the differential impedance being crucial, but am unclear about the single-ended - I can get a lay-up/spacing/etc for 100 diff, but the single-ended comes out at ~66ohm. Is this going to cause a problem?

Also, the Apps Notes say "trace lengths should be kept as short as possible" but other info I've read (albeit for different Phy's) suggest that >20mm or so is advisable?

  • 0
    TI__Mastermind 19780 points
    Hi Colin,

    66 ohm single ended should be OK if 100 diff is met.

    Is your second question in regards to the MDI traces? We do not have a minimum length requirement. 20mm is OK but not necessary. Some PHYs may have issues with linking up on reflected pulses, or compliance violations if these traces are extremely short.