High speed signal return path in a stack-up with multiple ground planes

Question

Consider the following 6-layer stack-up for a mixed signal board.

 1: Top --- analog signals, components ---------------
    ================================================== (0.20 mm)
 2: GND --- return path for analog signals -----------
    ================================================== (0.36 mm)
 3: Pwr --- supply 1, supply 2, supply 3 -------------
    ================================================== (0.28 mm)
 4: Sig --- high-speed digital signals ---------------
    ================================================== (0.36 mm)
 5: GND --- return path for high-speed signals ??? ---
    ================================================== (0.20 mm)
 6: Bot --- control signals --------------------------

Layer 2 and 5 are solid ground planes, without splits in the copper. Layer 3, the power plane is split up into several regions (AVDD, DVDD, VCLK).

What I am trying to achieve, is to keep the layer 2 GND "clean".

My guess is that the high-speed return current should flow trough layer 5. Since this plane is directly underneath the high-speed signal trace, it should offer the lowest impedance return path. Am I right? Does this stack-up make sense at all?

Answer 1

In this design, the return current of high speed digital signals in layer 4 will be almost equally distributed between layers 3 and 5. This is OK until layer 3 has no discontinuities (isolation gaps) along any of digital lines. Careful routing may help to achieve this goal. Then your planes 1 and 2 stay "clean" of digital signals.

If you can not avoid gaps in layer 3 across the digital lines, you have to place them in Bottom layer.