[R-390] Two questions

[Jeff Adams]

> To answer the second question.  More than one capacitor is placed on
> parallel in a bypass circuit to bypass different frequencies.  In the
> larger capacitor, there is a lot if inductance due to the way it was
> made and its physical size. This cap is effective for low frequencies
> but not even seen by the higher frequencies.  The smaller cap is used to
> bypass the higher frequencies.  Sometimes you might see another one or
> two capacitors in parallel, again to bypass different frequencies.  
Glenn nailed it on the head.

A third (non-topic) reason has to do with modern day power supply 
design.  Often times I place smaller value capacitors in parallel in 
my power supply designs.   Smaller Tantalum capacitors on average 
have lower ESR (effective series resistance), verses one large 
tantalum capacitor.  Low ESR filter capacitors are a major 
contributor in reducing ripple and "Noise" in modern switching 
regulator designs. 
Jeff

Jeffrey L. Adams  eengineer@erols.com  Web  
http://www.erols.com/eengineer
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