----- Original Message -----
From: David Cary <[EMAIL PROTECTED]>
To: Multiple recipients of list proteledausers
<[EMAIL PROTECTED]>
Greetings folks,


> Although I suspect this
> "interlayer capacitance" idea is a red herring -- what's really important
is the
> impedance of the power planes, which doesn't much depend on where they are
in
> the stackup.


No way !

The earth/power planes form an excellent "low-ESR" (effective series
resistance) bypass capacitor.
At high frequencies it is this shunt capacitance which primarily decides the
impedance of the power supply.

At first glance it seems that the best approach is to have the earth and
power planes on the outside, in order to shield the internal layers.

However from an EMC perspective, it usually works out best to have the power
planes on the inside.
The main reason is that the SM footprints take up so much room (on the top
layer) that there is little room left for copper.

The primary concern for EMC is not so much that the noisy lines are
shielded, but that they form a very small area radiating loop.
By placing them directly over a continuous sheet of copper, they form the
smallest area loop possible.
For this to occur, the copper sheet must be unbroken.


Regards ..................... Graeme Zimmer



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