RE: [SI-LIST] : Decoupling capacitor resonance

About this list Date view Thread view Subject view Author view

From: Chris Cheng (hycheng@3pardata.com)
Date: Wed Jan 26 2000 - 11:58:01 PST


buried in all the discussion was the original question.
for pll applications, the most important frequency is
the loop filter bandwidth which typically sets around
two to three orders below the reference frequency. in
this case, if you have a 50mhz clock, the loop bandwidth
will most likely be in the 10-100'skhz. any noise above
the filter bandwidth can be corrected by the pll itself
(after all the filter is a 2nd order low pass filter).
any noise below the bandwidth will need external
help through filtering. i suspect that's the reason for
the 22uf. the 10pf seems to be a typo or afterthought to
provide a flat response at high speed.
chris

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Chris Bobek
Sent: Tuesday, January 25, 2000 5:42 PM
To: Si-list
Subject: [SI-LIST] : Decoupling capacitor resonance

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

**** To unsubscribe from si-list: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Thu Apr 20 2000 - 11:34:50 PDT