Question regarding return currents for high-speed signals

Discussion in 'General Electronics Chat' started by nickagian, May 28, 2015.

  1. nickagian

    Thread Starter Active Member

    Mar 12, 2010
    Hi guys,

    I have this question: It is common knowledge that for high-speed signals the return currents are flowing directly beneath the signal PCB lines, over some power or ground plane. And for that reason one should pay attention when such signals cross over breaks in the planes. So assuming we have the following PCB stack-up:

    1 -> Signals and Ground
    2 -> Ground
    3 -> Signals
    4 -> Power supply planes
    (bigger distance than between the other layers)
    5 -> Power supply planes
    6 -> Signals
    7 -> Ground
    8 -> Signals and Ground

    So for signals on layers 1 and 8, the reference layer for return currents are layers 2 and 7.

    But what about signals on layers 3 and 6? Is it 4 and 5 or again 2 and 7?

    I have seen some PCB designs where it is implied that the return currents are flowing over 4 and 5.

    I hope it is clear what I am asking!
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Well the return must obviously be through ground, but if the distnace 2-3 is the same as 3-4 then there will also be induced AC currents in the power plane as well.
    nickagian likes this.
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    And, the currents in the power planes will run into breaks that are not there in the ground planes. So the signal trace will be a full stripline most of the time, with impedance bumps where there are breaks in one plane but not the other. Over the power plane breaks the signal will act as a microstripline for that short distance.

    nickagian likes this.
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    check out microstrip, stripline and microstrip sre a bit different, one uses two ground planes, one above and one below the signal line. also, for high speed data, any paralell data lines must be treated as transmission lines for poth impedance, and any curves must be treated like stripline or micorstrip to prevent reflections and differences in time.