Re: 47uf decoupling caps?!

a7yvm109gf5d1@xxxxxxxxxxx wrote:
rickman wrote:

If you really believe that the power planes are just "supply lines",

In most PCBs they are.

then you have missed a chapter in the engineer's design book. If you
have high speed circuits on your board, you had better have power
planes to provide low inductance decoupling for the high frequencies.

That only works if the ground and power planes are closer together than
5 mils. There are prepregs out there in the 1-2 mils range for this
reason.

Where did you get this info? If you just think about that claim, you
will realize that it is not very sound. Does a capacitor stop being a
capacitor if the plates are more than 5 mil apart? In fact I think you
will find that at 5 mil you are getting a *great* decoupling capacitor
and at wider spacings you still get very good capacitors with very good
high frequency response. Of course the value of the capacitance drops
as you widen the spacing, but an 8 mil spacing is not going to "ruin"
the effectiveness of the plane decoupling.

This is just the sort of general rule that Lee Ritchey debunks in his
class. That was one of the truely impressive aspects of the class, the
way he tears apart a lot of the myths that people have developed and
never chalenge or verify.

.

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