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

From: Michael Vrbanac ([email protected])
Date: Thu May 11 2000 - 21:29:55 PDT

Barry,

The moving charge on the conductor has to obey the laws of physics. The resultant
electric field set up by the moving charge is affected by the permittivity of the medium
around it. In fact, it is in much the same way that we look at the propagation delay on transmission lines. The 170-180 pS/in number that we estimate for typical delay in a
stripline in FR-4 comes solely from the dielectric's characteristics. The speed by which
the charge moves is governed by the permittivity or the dielectric constant which is
normalized against the permittivity of free space.

... and it doesn't matter if its a signal on a transmission line in a PCB or a decoupling
capacitor on the same board.

Regards,

Michael E. Vrbanac

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.
>
> (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.
>
> Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. ......
>
> Regards,
> Barry Ma
> [email protected]
> Morgan Hill, CA 95037
> Tel. 408-778-2000
>
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