Your point is very well taken Robert. The operative term is "signal reference plane", regardless of DC voltage. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Robert Sefton Sent: Monday, March 11, 2002 10:30 PM To: ahilton@xxxxxxxxxxxxx Cc: [SI-LIST] Subject: [SI-LIST] Re: (no subject) > Goutham.S@xxxxxxxxxx wrote: > > 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? > > The problem with running the 2.5 V signals beside a 5V palne is this - > > Controlled impedance implies a relationship between the conductor that carries > the signal and the conductor that is used for the return path. In order for a > plane to be a return path for a signal, the plane *must be connected* to the > circuitry for the signal. The 2.5V drivers and receivers are not *connected* to > the 5V plane, except through what is likely to be a very high-inductance path. > > If you really must do this, you could put a decoupling capacitor from 2.5V to 5V > at both ends of the signal to provide a lower-inductance path to the plane. For > very high speed signals with tight timing parameters, this inductance may still > be too high. If the 2.5V and 5V supplies have the same impedance to ground, what difference does the DC voltage on the plane make? Robert Sefton ------------------------------------------------------------------ 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