Larry, What you said about the current distribution at high frequency(>100Mhz) is true, but my questions is whether the current distribution is important or the voltage variation on the power plane is more important than current distribution? Let's say, there are 2 points on the power plane and the most (>93%) current flows on the surface near digital circuit. The current flow near analog circuit is very small (<7%), but it has big loop (large inductance) and it produces same voltage drop between those 2 points as large current on the surface of digital circuit. IF the anolg circuit is sensitive to the voltage noise, then it is a problem. Jim, another thing you need to be aware is that low frequency may be a killer for your anolog circuit. In your current stack up, it is diffcult for to control the lower frequency current return path on layer 4. Thanks. Zhiping -- Zhiping Yang, Ph. D. Hardware Engineer Cisco Systems 270 West Tasman Drive Mail Stop:SJCG/2/2 San Jose, CA 95134 | | email: zhiping@xxxxxxxxx :|: :|: Tel : 408 525 5690 :|||: :|||: Fax : 408 526 5504 .:|||||||:..:|||||||:. ***************************************************** Larry Smith wrote: > Jim - I don't believe that the high frequency return currents on your > digital traces will have much effect on your analog traces even though > they share the same ground plane (layer 4). > > The skin depth at 100 MHz is about 0.26 mil compared with the 0.7 mil > thickness of half oz copper. The skin depth is essentially the depth > that the magnetic field penetrates into the copper. At 100 MHz, very > little magnetic field (approximately 1/[e^(.7/.26)] = 7% ) will > penetrate through the copper plane. Even less of it will reach an > analog trace. At higher frequencies, the penetration will be even less. > > How sensitive are your analog signals? For digital signals, I would > not worry about 7% magnetic field penetration. > > regards, > Larry Smith > Sun Microsystems. > > > Delivered-To: si-list@xxxxxxxxxxxxxx > > From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx> > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] return currents > > Date: Wed, 26 Sep 2001 07:32:10 -0400 > > MIME-Version: 1.0 > > Content-Transfer-Encoding: 8bit > > X-archive-position: 946 > > X-listar-version: Listar v1.0.0 > > X-original-sender: james.f.peterson@xxxxxxxxxxxxx > > X-list: si-list > > > > > > hello, > > > > Stackup : > > > > 1 - gnd > > 2 - digital sig > > 3 - digital sig > > 4 - gnd > > 5 - analog sig > > 6 - analog sig > > 7 - gnd > > 8,9,10,11 ..... > > > > notice that the digital signals from layer 3 and the analog signals from > > layer 5 will probably have return currents on layer 4. > > > > question : > > will the digital return currents cause noise in the analog section, since > > they both share layer 4 for return currents? (My first guess is yes, but > > someone mentioned that the skin depth for the return currents is small so > > they can share layer 4 without effecting each other.) > > > > thanks for your input. > > Jim > > ------------------------------------------------------------------ > > 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 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 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 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