Re: [SI-LIST] : Decoupling capacitor resonance

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From: Ron Miller (rmiller@Brocade.COM)
Date: Tue Jan 25 2000 - 18:10:12 PST


RIGHT ON CHRIS

Chris Bobek wrote:

> Hi,
>
> There's been a lot of discussion about selecting the right decoupling
> capacitor(s) for an IC. Howard Johnson's philosophy seems to say "pick
> the largest value cap(s) in the smallest smt package that you can
> reliably purchase". I tend to agree with that. However, I have an
> appnote for a PLL (f~=50Mhz) that says to use a 22uF and a 10pF to
> properly decouple the device. Instead, I'm using a 22uF and a 0.1uF in
> the smallest package I'm allowed to procure (0805). FOR THE SAME SIZE
> PACKAGE, should I change the 0.1uF to 10pF? If so, why?
>
> My understanding is that the inductance of the 10pF and 0.1uF is almost
> equal (because they are in the same size package). Inductance being
> equal, the larger capacitance is better because it provides a lower
> impedance to ground. Therefore, using a 10pF would not gain us anything
> except an extra part to procure in addition to our ubiquitous 0.1uF's.
>
> Thank you for your help/insight,
>
> Chris
>
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Ronald B. Miller  _\\|//_  Signal Integrity Engineer
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