[SI-LIST] Re: What is meant by a differential signal

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
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