RE: [SI-LIST] : Adding inductors to ground?

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From: Ray Anderson (raymonda@radium.eng.sun.com)
Date: Fri Mar 03 2000 - 12:22:47 PST


One quick additional comment to attach to Larry's earlier reply.
You may find the paper located at the following URL instructive:

ftp://ftp.qsl.net/pub/wb6tpu/si_documents/epep-98.pdf

On page 3 of the paper there is a chart that depicts the relative
contributions to mounting inductance made by the capacitor, mounting
pads, and board spreading inductance for a not so well done design,
a typical design, and a well done design. The following examples
are extracted from the chart in the paper.

For a well done design it breaks out approximately as
follows:
                Cap contribution: ~ 0.4 nH
                Spreading inductance: ~ 0.125 nH
                Pad/Escape Geometry: ~ 1.4 nH
                                TOTAL: ~ 1.925 nH
                
For a typical design it breaks out approximately as
follows:
                Cap contribution: ~ 0.4 nH
                Spreading inductance: ~ 0.125 nH
                Pad/Escape Geometry: ~ 1.6 nH
                                TOTAL: ~ 2.125 nH
                
For a not so good design it breaks out approximately as
follows:
                Cap contribution: ~ 0.4 nH
                Spreading inductance: ~ 0.4 nH
                Pad/Escape Geometry: ~ 4.5 nH
                                TOTAL: ~ 5.3 nH
                                
                                
All these numbers are approximate as the exact values depend on the
details of the design, but they are pretty close to what is typically
seen.
                                        
                                        
You can see from these numbers that if you aren't careful with the pad/escape
geometry and board stackup, the contributions from those elements can
increase by a factor of 4 to 5 on the Spreading inductance and a factor of 2
to 3 on the inductance contributed by the pad geometry.

So the moral of the story is that unless you do a good job on
your mounting geometries and board stackup, paying extra money
for 'exotic' capacitors won't reduce the total mounted inductance
by a great deal (or at least as much as you may have hoped for).

-Ray Anderson
Sun Microsystems Inc.

> Date: Fri, 3 Mar 2000 09:55:16 -0800 (PST)
> From: Larry Smith <ldsmith@lisboa.eng.sun.com>
> Subject: RE: [SI-LIST] : Adding inductors to ground?
> To: si-list@silab.eng.sun.com
> Cc: JohnGal2@aol.com
> MIME-Version: 1.0
> Content-MD5: 9gulA+SLjhR46MPGvFQ3SQ==
>
> Chris - The effective inductance associated with a discrete decoupling
> capacitor is an extremely important parameter in determining the
> effectiveness of a the capacitor. We find that most of the
> inductance associated with typical decoupling capacitors is associated
> with the pad and via layout which conducts current to the power
> planes. Reverse aspect capacitors can help but may not be address
> the dominant inductance. After the via and pad layout has been
> optimized, there is some value to using reverse aspect capacitors.
>

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