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

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.=20
        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. =20
       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?=20
Bharath


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