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TLK110: different GND connection

Wiky Liao
Intellectual 2000 points
Part Number: TLK110


Hi Team,

We are looking into TLK1XX Design and Layout Guide file, for MDI connection, the left GND and right GND are connected directly, see below capture:

but at figure 4, these 2 GND are isolated by capacitor, is there any concern about these two GND? Could you help to guide us the root cause? And, which one is common used, thanks.

BR.

Wiky

  • 0
    TI__Mastermind 23675 points
    Hi Wiky,

    It is common to isolate the RJ45 chassis ground from the system ground to eliminate ground loops and also for safety.
    The final implementation is up to the customer though. We mostly see isolation used with a 4700pf cap and a parallel 1M ohm bleed resistor.
  • 0 in reply to Ross Pimentel
    TI__Intellectual 2000 points

    Hi Ross,

    Thanks for your reply.

    Agree with you that normally use cap isolation for safety. But I am still curious why MDI don't require the cap for isolation, could you share more knowledge? thanks.

    BR.

    Wiky

  • 0 in reply to Wiky Liao
    TI__Mastermind 23675 points
    Hi Wiky,

    For 10BASE-T and 100BASE-TX PHYs (TLK110 for instance) CAT5 cables are used. These are unshielded and only have data lines.
    Since you have a transformer, the interface is isolated. The RJ45 in this case can be connected to the system ground because there is no possibility for ground to be shared between one PHY to the other. If you are using a shielded cable though, the RJ45 casing is shorted to the shield and you could have a ground loop if the system ground is not isolated from the RJ45.