[SI-LIST] Re: Using Planar Capacitors
- From: Ege Engin <engin@xxxxxxxxxx>
- To: michael@xxxxxxxxxx
- Date: Wed, 14 Aug 2002 10:02:03 +0200
Hi Michael,
One of the reasons to prefer thin dielectrics in power/ground pairs would be
the return path discontinuity that you mentioned. Higher power plane
capacitance would be seen as a lower impedance discontinuity for the return
current. You can find some info on this subject in the book "High-Speed
Digital System Design".
I think the effect of changing the reference planes would be like as if
there were no vias but slots in the reference plane (in place of the vias).
The parasitic capacitance of the slot resembles the plane capacitance of the
power/ground pair.
Another point with the vias is that, that they can excite the power/ground
plane similar to a radial transmission line.
I think a quantitative treatment of the impedance discontinuity caused by
changing the reference planes is not straightforward. Using thin dielectrics
and connecting each power (and ground) plane with sufficient vias looks
reasonable to lower the effect of this discontinuity.
Regards
Ege
Michael Smith schrieb:
> Hi,
>
> I am new to this list and am noticing a lot of discussion on using power
> and ground plane layers sandwiched together to form a capacitor. This
> seems like a fantastic way to provide a low impedance high frequency
> capacitor. But I am wondering how this will adversely affect a signal
> which is routed on either side of this capacitor. How significant will
> the impedance discontinuity seen by the ground return current be as the
> signal changes coupling planes from power to ground. I am designing a
> board with 2 power and 2 ground planes. If I form these planes into
> capacitor pairs, then every time a signal changes layers, it will couple
> to a different plane causing an impedance discontinuity in the return
> current. Can someone give me a feel for how significant this effect is?
> How can I calculate the impedance discontinuity as the signal passes
> through a via and the return current has to pass through a bypass/planar
> caps when the signal changes layers?
>
> Thanks for your help,
>
> Michael Smith
> iZ Technology Corp.
>
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