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