[sdiy] Nice info on decoupling caps

[Richie Burnett]

I mostly use this technique on high speed digital boards where Radiated
Emissions compliance can be a problem, particularly with the modern trend for
putting things in molded plastic enclosures!  I've used it to good effect in
Industrial, Instrumentation and Clinical Diagnostic applications.

As Harry rightly pointed out, it does have its downsides though.  It's a bugger
to fault find because you can't see the burried signal traces, and you need to
put thought into bringing out test points at the design stage.  It's also not
great for power electronics boards because modern surface mount components get
rid of their heat through conduction to the copper tracks leading away from the
device.  This conduction cooling mechanism is less effective through vias unless
you use a via cluster.

I wasn't suggesting that everyone design all their PCBs as four layer boards
using this technique, as much as using it as a teaching aid.  It shows how you
can get extremely low supply stray inductance, whilst simultaneously screening
sensitive circuit nodes and shielding noisey nodes against potential radiated
emmisions.  As with all things in engineering you do as little as you need to in
order to arrive at a satisfactory solution.

-Richie,


Richie, on a four layer board I've always seen power and ground on the internal
planes with the signals on the two outer layers.  Not the other way round. 
Where is the method you described used?

-Dave