[SI-LIST] Re: stitching capacitors for signals changing reference plane

Whether the return path will take the route of the stitching via or not
depends on whichever path is giving the lowest inductance.
a) it could be your pwr to gnd capacitance (built in path).
b) discrete caps that you may end up placing. As the Gurus pointed out,
discrete caps do not provide low path at high frequencies, rather they work
as an inductor. Esp. the pads (Footprint) starts to play more than the
capacitor itself.

From the geometry you could measure the plane to plane capacitance and place
them in such a way that built in capacitance between the planes takes care
of it.

-Qazi

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Joel Brown
Sent: Friday, October 17, 2008 8:43 AM
To: 'steve weir'
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: stitching capacitors for signals changing reference
plane

Steve,

So normally we think of the return current flowing on the plane directly
below the trace and that translates into a constant impedance typically 50
ohms. When the signal hits the transition via the return current now has to
find the closest stitching cap(s) and now it is no longer traveling directly
below the trace and will see a change in impedance which will cause
reflections. Does the formula you gave take this into account? Is it not a
problem because the distance to the cap is less than the critical distance
calculated from the risetime of the signal making the reflection not
problematic?

Joel
 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Thursday, October 16, 2008 6:10 PM
To: Joel Brown
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: stitching capacitors for signals changing reference
plane

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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