[SI-LIST] Re: stitching capacitors for signals changing reference plane
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Joel Brown <joel@xxxxxxxxxx>
- Date: Thu, 16 Oct 2008 18:09:59 -0700
Joel,
Q. If the signal rise time is slow (say greater than 5 ns) can the stitching
capacitor be located further from the signal via?
Is there a relationship between rise time and distance from via to cap that
is effective?
A. A stitch capacitor will be effective even far away from such a slow
rising signal. A first cut rule of thumb is the cap can be as far away
as 2"/ns risetime*eR^0.5, or more simply 1"/ns in typical material. The
round trip is then about 1/3 Tr or less.
Q. Is there a case where the stitching capacitors are not needed if the signal
is rise time is slow enough?
A. Not really in practice. It is an issue of how many caps are needed /
can be effective. If your signals were audio sine waves they would go
through the bulk capacitors. If they were low audio they would go
through the voltage regulator.
If you are looking for a rule of thumb for a 40 mil cavity board using
0402 caps, I suggest:
0402 caps side mount vias: 5/4 caps / signal * ns rise time. For 5ns
rise time: one cap for every four signals.
X2Y caps: 5/16 caps / signal * ns rise time. For 5ns rise time: one
cap for every sixteen signals.
Best Regards,
Steve
Joel Brown wrote:
> Consider the following stackup:
>
> L1 - Signal
> L2 - GND
> L3 - PWR
> L4 - Signal
>
> For signals transitioning from L1 to L4 a stitching capacitor may be needed
> to provide a return path.
> If the signal rise time is slow (say greater than 5 ns) can the stitching
> capacitor be located further from the signal via?
> Is there a relationship between rise time and distance from via to cap that
> is effective?
> Is there a case where the stitching capacitors are not needed if the signal
> is rise time is slow enough?
> If so, where does the current return go and why is it not a problem?
>
> Thanks - Joel
>
>
>
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