Goutham, The 2.5V signal's return current needs to find the shortest path back to it's source to reduce EMI and noise issues. That is possible if only either a common ground plane or the 2.5V plane is adjacent to this signal. If you any have other plane like the 5V then the return current has to find a path thru' the 5V plane and will have to take a circuitous path. Effectively increasing the inductance of the path traced which is not desirable. One way to reduce this "long path" is to couple the two power planes thru' small capacitors so as to provide a easier path for these currents. Which will enable you to have a 5V underneath the 2.5V signals only if you have no other choice should you resort to this. I do not know the answer to the second question. Girish ----- Original Message ----- From: <Goutham.S@xxxxxxxxxx> To: <si-list@xxxxxxxxxxxxx> Sent: Sunday, March 10, 2002 10:36 PM Subject: (no subject) > > > > Hi, > > Can any one explain why the following things normally system designers > follow/prefer: > > > 1.0 2.5V signals should refer only to 2.5V or GROUND PCB plane and not 3V/5V > plane. > > >-----If 2.5V signals on signal layer are having 5V reference plane > adjacent to the signal layer then what is the problem? > > 2.0 Ethernet PHY signals should refer to only transformer secondary supply > voltage PLANE. > > >-----If System designer ignores the above point what will happen? > > I feel the first point is meaning less for single ended traces (because for Z > control just plane is required), what you say? > > > > > Regards > Goutham Sabavat > > > > ------------------------------------------------------------------ > 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