[SI-LIST] Re: placement of decoupling caps (again)
- From: "Paul Mobbs" <paul@xxxxxxxxxxxxx>
- To: <Raymond.Anderson@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 14 Apr 2004 16:33:10 -0700
I read a bit of the paper posted and it looks like it compares "global"
decoupling (spread all over) to "local" decoupling (right near the pin).
Although there is a lot of reference to "lumped" capacitance, it doesn't
look like they stuck it all in one place in their tests.
I think another interesting experiment would be to use the recommended
amount of decoupling and then start taking it away until you see a problem.
I wonder what those results would look like and under what circumstances the
results would be the most dramatic.
BTW Is there anything that can be done to eliminate all those "Out of
office" replies when I post?
Regards,
Paul Mobbs
Mobbs Engineering
Email: paul@xxxxxxxxxxxxx
Voice: (415) 563-1454
----- Original Message -----
From: "Ray Anderson" <Raymond.Anderson@xxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, April 14, 2004 4:19 PM
Subject: [SI-LIST] Re: placement of decoupling caps (again)
> Lee Ritchey wrote:
> > UMR did a paper in 1995 that determined location is not important as
long
> > as you don't cut up the power planes. Others have repeated those same
> > tests, as have I, and come up with the same answers.
> >
> > The proponents of location matters need to do some testing to show that
> > their position is valid.
> >
> > Lee
> >
> > Lee W. Ritchey
> > Speeding Edge
> > P. O. Box 2194
> > Glen Ellen, CA 95442
> > Phone- 707-568-3983
> > FAX- 707-568-3504
> >
>
> OK, how about a thought experiment.
>
> Scenario one:
> --------------
>
> You've got a rather large PCB (say 24" x 24") with no cuts in the power
> planes. The board has a fast, current hungry chip (say 400 MHz clock
> rate, 80psec rise time, 50 amps of transient current) in one corner
> clocking away. Some number of appropriate decaps are located within an
> inch of the chip. Everything works OK (whatever that means...).
>
> Scenario two:
> -------------
>
> You've got a rather large PCB (say 24" x 24") with no cuts in the power
> planes. The board has the same fast, current hungry chip in one corner
> clocking away. The same decaps as were used in scenario one are now
> located at the opposite corner of the board (about 34 inches away). Does
> everything still work OK?
>
>
> Without actually doing the experiment, I'll go out on a limb and venture
> to say no. Granted, this is an extreme contrived example, but I really
> don't think you can make a blanket statement and say that the location
> of the decoupling capacitors on a PCB is a "don't care" decision.
>
>
> -Ray Anderson
> Sun Microsystems Inc.
>
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