Re: gEDA-user: bypass caps

[Stuart Brorson <sdb@xxxxxxxxxx>]

> 0.1 uF or 1 uF ?  Same footprint (0603), the 1uFs are a few cents more
> each (er, ~ 40% more cost for 10x the capacitance).
>
> Don't know if the 0.1uFs have some benefits inductance-wise or
> ESR-wise, in general.

This question (and those like it) is the topic of constant debate on
SI-LIST.  My take-away is that if you are doing an agressive high
speed design, then you need to worry about the inductance of your
caps.  Caps turn inductive at high frequencies (lead inductance, stray
inductance, etc.)  The 1.0uF will turn inductave at a lower frequency
than the 0.1uF cap.  Therefore, the SI gurus suggest that you use a
pair of caps in parallel -- 1uF and 0.01uF say -- to maximize the
frequency region over which your bypass circuit looks capacitave.

That being said, I have never actually used this advice.  I haven't
been doing RF boards that sensitive to SI issues.  I tend to use
either an 0.01uF or 0.1uF cap on each power pin, and sprinkle a couple
of 10uF Tantalum caps around the board for bulk decoupling.  For small
boards, just one 10uF tantalum cap at each power supply is
sufficient.  That approach works just fine in my experience.

For your furnace controller, you probably won't see any difference
between 1.0uF and 0.1 uF; either one will work for you.  It's not
necessarily in the  same class as the GHz server motherboards or 10
GHz router boards that the SI gurus argue about.

You *are* going to use a GND plane on the board's back, aren't you?
Putting a GND plane on the back of your board is 10000 times more
important than the difference between 1.0uF and 0.1uF bypass caps.

Stuart


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