[SI-LIST] Re: Decoupling capacitors
- From: Chris Padilla <cpad@xxxxxxxxx>
- To: DLieby@xxxxxxxxxx
- Date: Fri, 17 May 2002 11:24:59 -0700
This is my experience as well. In several cost reduced boards in which
they immediately strip out "superfluous" layers, i.e. ground planes, I've
had to make it up with many, appropriately placed/routed (sometimes
appropriately mixed) sub-100 pF capacitors. Thank goodness my 10/100 and
10/100/1000 48-port ethernet boards with miles of Cat5 UTP loading them do
not need to pass Class B...Class A can be challenging enough!
Chris Padilla
EMC Engineer
Cisco Systems
At 10:20 AM 5/17/2002 -0700, Lieby David wrote:
>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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