Re: [SI-LIST] : How do you use buried capacitance?

Steve Weir (weirsp@a.crl.com)
Fri, 11 Jun 1999 14:05:04 -0700

Chris,

The theory is pretty straightforward, this is just a parallel plate
capacitor. The thrill is that Zycon offers a fairly thin dielectric, so
the capacitance is much better than what is possible with typical
fiberglass. For 10mil separation between planes, the approximate
capacitance is 100pF per square inch. With Zycon, the separation is 2mils,
so we get 25X the capacitance per square inch = 2.5nF/"^2. Because this is
a plate capacitor, it has very high bandwidth. Because it is distributed
across the PWB assembly, we get very good load sharing. This also helps
the return current coupling as the signal trace moves from one plane to
another.

You will still need to use bulk capacitors for mid-frequency decoupling, ie
some number of 100nF, and multi-microfarad caps.

Regards
At 10:17 AM 6/11/99 -0700, you wrote:
>I understand the priciples behind buried capacitance. What I don't
>understand is how you use it when designing a board.
>
>Is it only used for decoupling?
>
>How do you figure out how much decoupling capacitance it is equivalent to
>for a particular IC?
>
>Do you use it along with your standard decoupling capacitors (for added
>effect)?
>
>Thanks,
>
>Chris
>
>
>
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