Differential impedance reference plane or planes?

[ricko_uk]

Hi,
when you need to calculate the track/gap widths for a controlled differential impedance, if you have a GND plane above the signal layer and a VDD below the signal layer, do you consider both planes or only ground? i.e. would it be an Edge-Coupled Surface Microstrip or an Edge-Coupled Surface Stripline?

If I understand correctly, only GND should be used as reference but due to capacitive coupling of the GND and VDD planes (and the very many decoupling caps around the PCB) then the VDD also has some effect. Is that correct or VDD always has an effect the same as GND and needs to always be treated as a GND plane?


I am asking because I am thinking of using this 6 layer PCB stackup fro a new PCB:

1) Signal
2) GND
3) Signal
4) Signal
5) POWER
6) Signal

Thank you

[Manul]

Well, It's all about signal return. If the power plane in question is the one powering the driver and also a receiver, then there is no difference.

Simple example. If we have an IC powered by 3V3, we might as well call it's VDD "ground" and it's GND as "-3V3".

In practice we preffer GND to be our return path, because usually it's common for everything which might be on the board and outside the board and thus is a universal return path. What you don't want is return path having to make extra jumps or taking extra paths.

[ricko_uk]

Thank you Manul