[SI-LIST] Re: Decoupling capacitors

I have some experience with problems in RFI.  To get some systems to pass FCC 
class B with enough margin it is necessary to add low value caps.  The best is 
setting them at two orders of magnitude in sequence from 10pf to whatever.  I 
like 10pf, 1nf, 0.1uf, 10uf placed in groups and a couple of extras at the 
corners of the board or plane.  

They really do make a difference in radiated emissions, whether they help SI or 
not

dav0
dlieby@xxxxxxxxxx
dav0@xxxxxxxxxxxxxxxx

"mittalr@xxxxxxxxxx" wrote:
> 
> 
> On a slightly tangential topic. Have you ever wondered the need for 100pf 
> caps. Everyone just seems to sprinkle them as a safety measure but I see no 
> need for them because
> - the IC by itself will have about 50-100pF. I checked with many IC guys and 
> the ICs where you really need decoupling are generally big enough to have 
> that much decoupling on-chip.
> - On top of this you have inter-plane caps.
> 
> I have seen many of my designs get away with no 100pF caps. I would use 1nF 
> and 0.1uF, rather than 100pf, 10nF and 0.1uF. Anyone have any other 
> experience...
> 
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