[SI-LIST] Re: Does differential signalling really need complimentary signal ??
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 01 Aug 2013 05:48:39 -0700
Digital differential signals are two complementary CODED signals that
may have little line to line coupling: loosely coupled, or perhaps as
much as 15% line to line coupling: tightly coupled. Differential
signals have several advantages, one of which is better noise rejection
relative to the power common at the receiver and a much smaller common
mode current component throughout the channel than either of the
constituent signals by themselves. Differential signals do not have to
swing the voltage rails, and most do not. Neither do they have to swing
above and below Vss. Differentially coded signals need to swing in
opposite directions on each transition. USB uses differential signaling.
Steve.
On 8/1/2013 5:32 AM, Rajan Hansa wrote:
> Hello Experts,
> I have confusion over differential signalling so please help to clear my
> clouded mind.
>
> I was reading a book by Howard Johnson where "differential signaling is
> defined as transmission of *two complementary signals* over two-wire
> signaling and if both wires/traces are identical & matched *so that both
> wires/traces have equal coupling to the reference system*, any AC currents
> induced in the reference system by one wire are counteracted by equal and
> opposite signals induced by the complementary wire"
>
> That means complimentary signal in differential signalling helps to
> mitigate the effect of parasitic coupling between differential trace and
> reference system and I think that's the biggest motivation of using
> differential signal at high frequency.
>
> Complementary signals if I understood correctly is sending opposite signal *
> simultaneously *to both traces that means +V to one trace and -V to another
> trace but in many designs I don't see complementary signals are used for
> differential signalling like in USB 2.0 I see + 200 mv (dp) and 0 V (dn)
> for active high and 0 V (dp) and + 200 mv (dn) for active low ( If it was
> complimentary signal it would have been +100 volt (dp) and -100 volt (dn)
> for active high and -100 volt (dp) and +100 volt (dn) for active low )
>
> My question is how can we mitigate the effect of parasitic coupling between
> differential trace and reference system when signal is not complimentary or
> is there something am I missing ??
>
> Rajan
>
>
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