[SI-LIST] : Decoupling capacitor resonance

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From: Chris Bobek (cbobek@cadence.com)
Date: Tue Jan 25 2000 - 17:42:02 PST


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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