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

From: Ron Miller ([email protected])
Date: Mon Mar 27 2000 - 19:32:34 PST

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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