piclist 2000\06\06\115838a >
Thread: Filter Capacitors
www.piclist.com/techref/logic/dsps.htm?key=filter
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face BY : David VanHorn email (remove spam text)



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At 07:58 AM 6/6/00 -0500, Mark Peterson wrote:
>In many products, I see a large electrolytic capacitor in parallel with a
>small disc or dipped capacitor.  Why is that done?  Obviously, the pair
>does not act as a single large capacitor, or aonly a single one would be
>used.  Just how does that arrangement work?

They are telling you that they expect to draw large current pulses
(probably driving something connected to the I/O pins) in addition to the
switching currents for the uP.
The large cap is good at providing a lot of current over a relatively long
time, but it's internal impedance is too high to provide a lot of current
over very short time intervals, so they add the small cap.

Warning: Bypassing is very often done wrong, so don't take what you see as
gospel.
Believe instead, what you measure, or read Art of Electronics :)

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