# Re: [SI-LIST] : Parallel Plate Capacitance for Bypass

From: Jerry Johnson ([email protected])
Date: Wed Mar 29 2000 - 17:34:38 PST

Ron,

Could be we're missing each other somehow.

What I saw was Steve say that 5 mils of FR4 would yield ~200 pF/square
inch.

Then I thought you said "So, by extension, 2.2 mil dielectric. . .
should be
about 1000pf/in sq for FR4 . . ."

That looked to me like you had taken [(5^2)/(2.2)^2] * 200 = 1033
pF/square inch.

I commented that capacitance wasn't an inverse square relationship with
distance
between plates. I calculated more like [5/2.2] * 200 = 455 pF/square
inch.

Perhaps I completely misunderstood your reasoning. If so, I'm sorry.

Jerry Johnson

Ron Miller wrote:
>
> Jerry
>
> maybe I am missing something or maybe we are not communicating
> well. The capacitance I am talking about would be an IC or a discrete
>
> device connected to power and ground. The capacitance would be
> somewhat limited by the quarterwave distance from that pin for any
> frequency of intrest, because above that distance the delay is too
> great for the capacitance to be of any effect.
>
> So, the 1 inch concern would roughly describe a circle with a 1/2 inch
>
> radius, which for FR4(velocity 170 ps/inch) or 85 ps.
>
> The frequency which has a quarterwavelength of 85 ps is 1/340ps=~3Ghz
> At 3 Ghz and below we get 200 pf per square inch, or about .27 ohms.
>
> Scaling from 3 Ghz, down in frequency we get higher Xc, but the
> effective
> capacitive area goes up. The limits are that several devices will be
> sharing
> the same capacitance and the board has dimensional limits so we must
> add capacitances. Scaling up in frequency, the effective capacitance
> goes
> down but so does the Xc.
>
> TRACES across a plane is an entirely different matter, and PLANE
> capacitance for the entire board is again a different matter.
>
> BOTTOM LINE:
> Capacitors below 200 pf in value are of little value for bypassing
> if you have 5 mil dielectric spacing power to ground.
>
> Ron Miller
>
>
> Gerald Johnson wrote:
>
> > Ron,
> >
> > Capacitance of planes is NOT a function of inverse square of
> > distance.
> > See thread in this list about a week ago. Well covered there.
> >
> > Valid to use inverse square distance for point source, not for
> > planes.
> >
> > Result, capacitance would be about (5/2.2)*200 = 454.6 pF if the
> > 100 pF per in^2 at 5 mils is correct.
> >
> > Jerry Johnson
> >
> > >Date: Mon, 27 Mar 2000 10:39:10 -0800
> > >From: Ron Miller <[email protected]>
> > >To: [email protected]
> > >Subject: Re: [SI-LIST] : Parallel Plate Capacitance for Bypass
> > >
> > >Thanks Steve, I knew it was somewhere around there.
> > >
> > >So, by extension, 2.2 mil dielectric, available at very little if
> > any
> > >extra cost should be about 1000pf/in sq for FR4
> > >and eliminatee the need for discrete capacitors below 1000 pf.
> > >
> > >ron
> > >
> > >"S. Weir" wrote:
> > >
> > >> Ron,
> > >>
> > >> 5 mils should yield about 200pF / in sq, before taking away for
> > vias.
> > >>
> > >> Regards,
> > >>
> > >>
> > >> Steve.
> > >> At 02:30 PM 3/24/2000 -0800, you wrote:
> > >>
> >
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> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
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