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

  • From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 11 Aug 2003 13:26:44 -0700

I'm with Ray on this one. In all my power distribution analysis and
correlated with measurements, die and package is a 2 1/2 D job while PCB
decoupling is at least a 1D model (i.e. radial distance from the center of
the die). The frequency involved on PCB is low enough to only needing 1D but
there is definitely a dependency on distance from the die. I am not talking
about couple hundred mils will make or break your system but you will see
the differences between a few inches.
This is particular true for external terminators such as SSTL-2 type for DDR
or GTL I/O.

 -----Original Message-----
From:   Ray Anderson [mailto:raymond.anderson@xxxxxxx] 
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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