Ludovic, Coupling between supply rails is not a problem as long as noise is properly kept under control . Vertical coupling between the Vcc1 and Vcc2 layers will be strong in the frequency range where connected capacitors loose their effectiveness. Assuming that you treat each supply rail properly within its own domain, a potential complication arises when the noise signatures are significantly different among the coupled rails. Suppose you have a 3.3V rail, feeding generic glue logic, vertically coupled to a supply puddle of an analog supply rail. Probably you could tolerate 100s of mV noise on the 3V rail, whereas a few mV may be the limit on a sensitive analog rail. Spectral signature of the noise and noise sensitivities have to be factored in, but this scenario would be risky unless you quiet the 3V rail to an unrealistic (and otherwise unnecessary) low noise level. The solution: avoid coupling supply rails with very different noise requirements and noise characteristics. Regards, Istvan novak SUN Microsystems Ludovic Levieil wrote: >Hello All, >In my current board design I have the following stack up: > > ....... >---------------- GND (solid plane) >------ ----- --- VCC1 (splitted plane) >--- ----- ------ VCC2 (splitted plane) >---------------- GND (solid plane) > ....... > >4 mils separate GND and VCC planes >5 mils separate VCC1 and VCC2 planes > >Both VCC planes are splitted in different power domains and I am wondering >: > - if having two coupled VCC planes is good/acceptable when >thinking about noise ?? > - if there is a problem in having one power domain on on plane >overlapping at least two power domains on the other plane ?? > >Thanks > >Ludovic Levieil > >------------------------------------------------------------------ > > ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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