Decoupling caps must respond to transients. The typical PIC output pin has a
rise/fall time of a few nanoseconds, so in order to stuff current into and
source current out of a decoupling cap in, say 10nS, requires a very good
high frequency response up into the 100 MHz plus range.



Jack

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jamie Jensen
Sent: Thursday, November 06, 2003 8:25 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]:Capacitors (basic question)


Not being an EE by education.....

I always thought capacitors as simple bulk storage devices, use some big
ones to filter out the ripple on power supplies and bunches of lil ones on
chips for adding the last "umph" when they switch and draw more current.

But ive seen and read stuff about using a mix of big and small for better
frequency response.  But can someone explain exactly what that means?  If
you have a circuit running at 20 MHz do you need to worry about a cap that
responds at 100MHz?  I just am not so clear on this (frequency response of a
cap).  Sorry if this is too basic of a question.

-JJ


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