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Concept
Differential signalling
Summary
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires in a twisted-pair or ribbon cable or traces on a printed circuit board.
Electrically, the two conductors carry voltage signals which are equal in magnitude, but of opposite polarity. The receiving circuit responds to the difference between the two signals, which results in a signal with a magnitude twice as large.
The symmetrical signals of differential signalling may be referred to as balanced, but this term is more appropriately applied to balanced circuits and balanced lines which reject common-mode interference when fed into a differential receiver. Differential signalling does not make a line balanced, nor does noise rejection in balanced circuits require differential signalling.
Differential signalling is to be contrasted to single-ended signalling which drives only one conductor with signal, while the other is connected to a fixed reference voltage.
Contrary to popular belief, differential signalling does not affect noise cancellation. Balanced lines with differential receivers will reject noise regardless of whether the signal is differential or single-ended, but since balanced line noise rejection requires a differential receiver anyway, differential signalling is often used on balanced lines. Some of the benefits of differential signalling include:
Doubled signal voltage between the differential pair (compared to a single-ended signal of the same nominal level), giving 6 dB extra headroom.
Common-mode noise between the two amps (e.g. from imperfect power supply rejection) is easily rejected by a differential receiver.
Longer cable runs are possible due to this increased noise immunity and 6 dB extra headroom.
At higher frequencies, the output impedance of the output amplifier can change, resulting in a small imbalance.
Official source
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