The origin of differential is from the differential amplifier first =20 designed in the forties, which amplified the difference between two =20 input signals and rejected any common value between the two. It =20 consisted of two common emitter amplifiers with shorted emitters =20 connected to an "ideal" current source to gnd. In pcb context, differential pair means, more practically, that the e =20 field lines from one track terminate on the other, and thus =20 effectively the transmission line is formed between the two tracks and =20 not between any one track and GND/VCC plane. All consequences of this like characteristic impedance etc then =20 depends on the geometries of and between the two tracks. The main advantage of this from signal integrity and noise point of =20 view is that the system of the two tracks is relatively immune of GND/ =20 VCC plane noises and other things such as GND bounce Aditya Reliant PCB Design Services Quoting "Mirmak, Michael" <michael.mirmak@xxxxxxxxx>: > Bharath, > > "Differential" sometimes carries several meanings, not just "current > flowing in one signal has to return back through the counterpart." In > many systems, differential simply means that the receiver evaluates > logic levels by looking at the *difference* between two incoming > signals. Such a system can benefit from immunity to common-mode noise > while still using a return path other than one of the incoming signals. > > - Michael Mirmak > Intel Corp. > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of Bharathkumar Raju > Sent: Tuesday, January 22, 2008 4:06 AM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] What is meant by a differential signal > > Dear Members, > In my understanding unlike ethernet signals (Tx+ & Tx- and Rx+ & > Rx-) which are true differntial the USB signals are not true > differential.=3D20 > Differential means current flowing in one signal has to return > back through the counterpart. In the ethernet protocol Tx+ current will > return through Tx- and vice versa. The same holds good for Rx pair also. > In an ideal differential system current flowing through the midpoint > (ground) will be zero. =3D20 > This is not the case for USB protocol. Here ground is the datum > through which the return current flows and under no circumstances the > ground current will be zero. Then why USB is identifed as a differential > system?=3D20 > Bharath > > > Unlimited freedom, unlimited storage. Get it now, on > http://help.yahoo.com/l/in/yahoo/mail/yahoomail/tools/tools-08.html/ > ------------------------------------------------------------------ > 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: =3D20 > =09=09//www.freelists.org/archives/si-list > or at our remote archives: > =09=09http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > =09=09http://www.qsl.net/wb6tpu > =3D20 > > ------------------------------------------------------------------ > 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: > =09=09//www.freelists.org/archives/si-list > or at our remote archives: > =09=09http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > =09=09http://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