[SI-LIST] : Re: Charge moving from decoupling capacitors

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From: Doug McKean ([email protected])
Date: Thu May 11 2000 - 22:05:57 PDT

Barry Ma wrote:
> Hi,
> As the speed of digital signals gets faster and faster, people begin being concerned
> with the distance for electric charge to move on power and ground planes of multilayer
> PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves.
> I would like to raise two questions.
> (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and
> gnd planes. Please let me know why you have to use the propagation velocity in the
> dielectric, instead of that in the metal.

Simple. The power/ground plane construction is
a 2-dimensional transmission line. Just as you
determine the velocity of propagation in a
1-dimensional transmission line, the dielectric
is part of the equation for the capacitive
component. And so it follows with the planar

> (2) The second question is regarding distance between the cap and the chip. Do we
> really have to limit the distance letting the charge have enough time to move from
> the cap to the chip during the rise time interval? I doubt it.

Depends. Distance can mean inductance.

Since, v(t) = L di/dt and with L in nano-henries (10^-9)
and suppose dt in pico-seconds (10^-12), we're already
up in the 10^3 range ... - Doug McKean

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