Re: [SI-LIST] : Decoupling capacitor resonance

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From: David Instone (david_instone@uk.xyratex.com)
Date: Wed Jan 26 2000 - 02:11:41 PST


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
work!

-- 
Regards

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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