# [SI-LIST] : Decoupling Caps and Return Currents

Mark Nass ([email protected])
Wed, 29 Oct 1997 09:55:45 -0800

I have a board that is 6 layers.
Top
GND
sig
sig
split power
bottom

This will have 100Mhz clocks and bus speeds.
Because I have a split power plane I am concerned about the
high speed signals crossing the planes and their return currents. What I want
to do is place small caps, .001uf, around the board so that there is at
least 1
per square inch. The theory being that with the caps present all signals that
have their return currents on the power plane will have an AC path to the
GND plane and then the GND plane will provide every signal with an AC return
path.
Is this a valid solution? Would I be better off putting the caps between
the power planes that two are split(say +5v to +3.3V) instead of +5v to GND
and +3.3V to GND? If I use a formula to calculate resonant
frequency, for proper capacitor selection, do I use 100mhZ. How would
I calculate the frequency of the AC currents from the edge rate?

Thanks,
mark Nass