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 >> >> >> >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >> or to administer your membership from a web page, go to: >> //www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> >> >> > > > -- > 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 > (401) 284-1840 Fax > > Oregon office > (503) 430-1065 Business > (503) 430-1285 Fax > > http://www.teraspeed.com > This e-mail contains proprietary and confidential intellectual property of > Teraspeed Consulting Group LLC > ---------------------------------------------------------------------------- > -------------------------- > Teraspeed(R) is the registered service mark of Teraspeed Consulting Group > LLC > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > -- 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 (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------------------------------------ Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu