From: Ron Miller (rmiller@Brocade.COM)
Date: Wed Jan 26 2000 - 10:56:26 PST
From an analytical view you are right. However, since we are dealing
with parasitic inductance in a primarily capacitive element:
1. the PCB parasitic are as important as the capacitor inductance.
2. analysis would require a field solver to take into account all the parasitics
which is not going to happen.
3. the XL is so low that the resonant frequency will be in the multi-
4. the parasitic losses(resistive) in the inductance de-Que the resonance
as well as PCB dielectric losses which smears the effect to the
point that the impedance never goes very high at any frequency.
> Those that advocate putting a few pF across uF might like to work out
> what the parallel resonant frequency of the pF and the inductance of the
> uF is. If that frequency, when the resultant impedance is almost
> infinite, is inside the band you want to decouple then it ain't gonna
> Dave Instone. Compliance Engineer
> Test Systems, MP24/22
> Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
> Tel: +44 (0)23-92-496862 (direct line)
> Fax: +44 (0)23-92-496014
> http://www.xyratex.com Tel: +44 (0)23-92-486363
> 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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