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

Joel, closer is better.  But anything within several inches is "close" 
compared to a 5ns Tr signal.  If your PDN has a capacitor density for 
power delivery similar to the guidelines I recommended your signals will 
be very happy.

Best Regards,


Steve.
Joel Brown wrote:
> 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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>
>
> --
> Steve Weir
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-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(866) 675-4630 Business
(707) 780-1951 Fax

Main office
(401) 284-1827 Business 
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http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
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