Lets say we have a double sided PCB. On the top we have 2 chips and a signal track between the 2.
Both chips are connected to GND and decoupled. The ground plane is on the bottom side.
Now how would the return signal go back to the first chip?
My guess was, the fastest way like this (via the GND plane of on the bottom of course)
Apparently it takes the route of less flux resistance (or something like that, please correct me on the terminology)
It follows the upper track EXACTLY but on the bottom GND plane like this:
What? So directly I got this question on why we were thought to make separate analog and digital planes and stuff like star grounds…
Maybe this might be neccesary for very high frequency signals, but for ‘normal’ signals it appears that the ground return can handle itself
So why is this important? Remember the previous post about the twisted wires? The traces on a pcb are also wires.
In this case a pair of wires is the trace on top and the return is the path the return current flows through the GND plane.
In order to have a minimal effect of interferance, you want both ‘wires’ to be placed in the same magnetic field.
So if the return current follows the exact same path as the trace above it, this is (almost) the case. Just the distance to the GND plane difference (1.6mm or if the GND plane is in the middle of the PCB 0.8mm)
If this is not the case we create a loop and therefore a difference of the wires in the magnetic field and it becomes a receptor for interferance (read antenna).
In my next post there I explain how this can happen…