Re: [SI-LIST] : Decoupling caps selection

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From: Chris Padilla (cpad@cisco.com)
Date: Fri Oct 27 2000 - 16:04:08 PDT


Fourier analysis. It takes a lot of sine/cosine waves to make up a square
wave and the faster the risetime, the more frequency content the digital
signal has.

Lots of 1st order equations are used but I like: 0.5/tr as the 3 dB cut off
for determining the overall bandwidth of the digital signal. So the
frequency versus impedance charts are important for the cap but you also
must consider the inductance associated with mounting pads, the capacitor
geometry, and vias. This will cause an anti-resonance peak in the Z vs. f
plot and then the cap looks more inductive and impedance increases with f.

If your circuit operates with a pure AC signal, then you are correct and
only need to worry about that particular frequency.

----->Chris

>Hi all,
> I was wondering if the selection of the decoupling capacitor of a
> circuit is MAINLY governed by the edge rates of the switching signals or
> by the frequency of their switching. I believe the answer is later cause
> all the documentation that I have seen on the dcoupling cap selection so
> far do not have any mention of the edge rates. Most of the capacitor
> manufacturer have "Impedance Vs Frequency" charts in their
> specs/catalogs. I am assuming that the Frequency that they are referring
> to is the frequency of operation of the circuit. Any ideas/comments are
> greatly appreciated.
>
>Dilpreet
>
>
>
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