[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

  • From: Fred Balistreri <fred@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 11 Aug 2003 15:21:16 -0700

I believe the condradictions start when we generalize. Every design
is different and the solutions for one do not necessarily apply to
another. But we should be able to agree on the basic concepts and
come to understand what the assumptions are when statements are made.

Best Regards,

Peter Arnold wrote:
> 
> All,
> 
> I am not sure there should be so much controversy over this placement thing.
> Among the conclusion in the UMR paper that I have a copy of [1] is this
> assertion: "...all decoupling capacitors are shared in the frequency range
> where they are effective (typically below 200-300MHz), and the location of a
> decoupling capcitor on the board is relatively unimportant."
> 
> The "unimportant placement" claim is made dependent on the condition that
> the caps, as mounted, are ineffective above 200MHz. At this frequency an
> 8"x9" test board is electrically small with respect to 200MHz wavelength in
> FR4 (30") and the paper reports placement not to be crucial in determining
> impedance.
> 
> I think this electrical-smallness, and taking capacitor mounting inductance
> at 2-10nH to be >> plane inductance between parts, is what prompts them to
> simplify the power delivery model to a network connected between two spice
> nodes as if all components were physically at the same place. Analyzing this
> the authors identify a frequency "fa" lying between the last zero and the
> last pole of the impedance plot above which the decouplers are ineffective.
> If this frequency is low as in the examples given, then cap placement might
> well not matter.
> 
> But designs today do not necessarily support the simplification given above.
> Modern techniques like via-in-pad, multiple fanouts, "broadside" case
> connections etc. reduce mounted inductance to 1nH or less per part. This
> should allow mounted capacitors to be effective at higher frequencies where
> wavelengths are only a few inches and the board is no longer electrically
> small. At this point the everything-between-two-nodes and
> no-inductance-in-series-with-plane-capacitance simplification may not hold
> and a bedspring-type model would be needed. Decoupler placement may then
> become important (as experience appears to confirm!) This does not
> contradict the UMR authors' statement, which relates to a
> high-mounted-inductance regime and frequencies below 200MHz on a small
> board.
> 
> Can anyone comment?
> 
> [1] Hubing et al., "Power Bus decoupling on Multilayer Printed Circuit
> Boards," in IEEE trans. EMC vol 37, no. 2, May 1995.
> 
> Regards,
> peter arnold.
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ray Anderson
> Sent: Monday, August 11, 2003 9:36 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
> 
> Lee Ritchey wrote:
> > If the location of decoupling capacitors matters, perhaps some technical
> > demonstration would prove that.  Short of such a demonstration, this is
> > speculation and not the sort of thing that should be used to make design
> > choices.
> >
> > It's time to do some good engineering on this subject and do away with
> > opinion.  The UMR paper is good engineering.  Anyone who chooses to
> > disagree with it has the burden of showing where it is wrong by using some
> > good science.
> >
> > Lee
> >
> > Lee Ritchey
> > leeritchey@xxxxxxxxxxxxx
> > Why Wait?  Move to EarthLink.
> 
> Or conversely, show that it doesn't matter.
> 
> Take a large system board (say about 24" square) put a bunch of high current
> processors and ASICS on the left side of the board. Put all your decaps
> on the right side of the board (since you maintain position doesn't matter).
> 
> In this hypothetical case I can just about guarantee the board won't
> function
> properly or pass EMI. Admittedly, it is a contrived case, but I think it
> illustrates the point.
> 
> As far as the technical demonstration goes, we have indeed demonstrated to
> ourselves that position does matter.
> 
> I'm not attempting to further an argument, but I do feel those who have
> responded with  viewpoints other than yours shouldn't be chastised and
> accused
> of spreading unfounded opinion. Don't discount the chance that an opinion
> that
> differs from yours just might be correct.
> 
> -Ray Anderson
> Sun Microsystems Inc.
> 
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-- 
Fred Balistreri
fred@xxxxxxxxxxxxx

http://www.apsimtech.com
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