# Re: [SI-LIST] : Decoupling between non-ground power rails, yesor no??

From: Mike Hughes ([email protected])
Date: Tue Jan 23 2001 - 05:46:30 PST

My earlier reply was intended to verify the validity of using pwr-pwr planes,
not necessarily to justify using discrete decoupling between pwr supplies. My
impression now is that using pwr-pwr planes is something you would do when you
can't use pwr-gnd planes.

Hopefully I'm not getting off the subject but I'd like to clarify this. First
when I said "small valued discrete caps", I meant 10pF or 100pF, not 10000pF or
0.1uF. It's true that plane capacitance is distributed, but not all of that
capacitance is "available" to any particular point when you are in the nano to
pico-second range. So I would think there are some similarities between the two
schemes. The advantage of the planes seems to be in the inductance. The
inductance of discrete caps is in the 1nH range, and somewhat lower but not zero
for the planes. (This is all assuming that things are done correctly.)

Thanks,
Mike

>
> I think not because the planes create a distributed capacitance rather than
> a discrete value. This is where the difference lies. Think about charge
> density between the two and how it is flowing. In the distributed case, as
> the charge flows, it is relatively "smooth" but now stick a huge 10,000 pF
> capacitance at one point and now think about your charge flow.
>
> Remember that capacitance isn't everything, we also have inductance
> associated with those discrete caps--on the order of 1.5 nH per cap. The
> distributed cap has at least a magnitude less than that I would think
> (never measured nor calculated so I am guessing here).
>
> Your stack-up and needs of the hardware engineer normally determine whether
> or not you want/wish to do Pwr/Gnd or Pwr/Pwr sandwiches in your
> stack-up. I often like to put 2 pwr planes inside to outer gnd
> plane--especially if those power planes need hacked up for several
> different voltage levels. Then you don't care about traces crossing cuts
> in the plane.
>
> ----->Chris
>

```--
================================
Mike Hughes
Product/Test Engineer
Analog Devices, Inc.
Phone 781-937-2370
Fax 781-937-1011
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