At 03:44 PM 6/3/2003 -0400, you wrote:
> I thought that the following stackup was prefered because then every
> signal is one layer from a ground plane.
>
>  1    signal
>  2    gnd
>  3    signal
>  4    pwr
>  5    pwr
>  6    signal
>  7    gnd
>  8    signal

But then you don't have as good decoupling between your pwr and gnd planes,
since they are farther apart.

My stackup (as mentioned in an earlier post, and repeated below) gives you
copper balance, better decoupling, and your signals are still only 1 layer
away from a pwr or gnd plane.  And since the pwr and gnd planes are
effectively the same thing to high frequencies, a signal being next to a pwr
plane is the same as that signal being next to a gnd plane.

sig
gnd1
pwr1
sig
sig
gnd2
pwr2
sig

While we are on this subject, I like to use 0.01 uF caps for decoupling, not
the 0.1 uF caps you frequently see on digital circuits.  The reason is that
0.01 uF caps have a higher self-resonance frequency than 0.1 uF caps, which
makes them better able to decouple the high-speed transients that are so
common in today's circuits.  Also, 0.01 uF caps are less expensive and take
up less space (0805 vs. 1206).

First, my experience regarding layout is minimal at best since I seem to exhaust all the wrong ways first. I once took a class in multilayer layout and was told that the capacitance between the power/ground planes themselves was sufficient for decoupling high frequencies and that adding capacitors could cause tuned circuits and troublesome resonances. I'm simply asking, any views on that theory?


Ray



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