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DP83867IR: Short in the Center tap routing

KS
Prodigy 150 points
Part Number: DP83867IR

Hi - In our design we have accidentally shorted all the CT pins on magnetics as shown below with red line and we are seeing an intermittent link. We are routing on the internal planes so cutting these traces out is not straightforward. We are wondering if this short will cause this link issue ?  If we remove the short this will remove the four 0.1uF CT caps out of the circuit, will this impact the circuit operation? What purpose CT 0.1uF serves in the PHY circuit?  We plan to fix this issue in next revision but for prototype testing  we would like to revive it for now. Any thoughts will be appreciated?

Thx

  • 0 in reply to KS
    TI__Genius 11725 points

    Hello,

    With the center taps shorted, we have seen link instability issues. The decoupling capacitors are used to reduce common-mode noise. I understand this routing is done on an internal plane making it difficult to cut traces, but my recommendation would be to configure the center taps such that they are separately decoupled to GND as shown in the datasheet.

    What RJ45 connector is used? One other solution might be to remove the magnetics, and replace the RJ45 with an RJ45 with integrated magnetics, with the center taps separately de-coupled to ground. Then the MDI output pairs can be routed directly to the RJ45 with integrated magnetics, bypassing the old magnetics. Do you think this setup may be possible before the next set of boards arrives? One major challenge with this setup is that MDI output pairs should be routed such that they have a 100ohm controlled differential impedance, and they must be length matched. This "blue wire" setup may still result in similar issues. 

    Thank you,

    Nikhil