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.
Advantages 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 (20log2 ≈ 6.02) 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. When driven in differential mode by two identical amplifiers, this impedance change will be the same for both lines, and thus cancelled out. Differential signalling works for both analog signalling, as in balanced audio, and in digital signalling, as in RS-422, RS-485, Ethernet over twisted pair, PCI Express, DisplayPort, HDMI and USB.
Suitability for use with low-voltage electronics
The electronics industry, particularly in portable and mobile devices, continually strives to lower supply voltage to save power. A low supply voltage, however, reduces noise immunity. Differential signalling helps to reduce these problems because, for a given supply voltage, it provides twice the noise immunity of a single-ended system. To see why, consider a single-ended digital system with supply voltage V S {\displaystyle V_{S}} . The high logic level is V S {\displaystyle V_{S}\,} and the low logic level is 0 V. The difference between the two levels is therefore V S − 0 V = V S {\displaystyle V_{S}-0\,\mathrm {V} =V_{S}} . Now consider a differential system with the same supply voltage. The voltage difference in the high state, where one wire is at V S {\displaystyle V_{S}\,} and the other at 0 V, is V S − 0 V = V S {\displaystyle V_{S}-0\,\mathrm {V} =V_{S}} . The voltage difference in the low state, where the voltages on the wires are exchanged, is 0 V − V S = − V S {\displaystyle 0\,\mathrm {V} -V_{S}=-V_{S}} . The difference between high and low logic levels is therefore V S − ( − V S ) = 2 V S {\displaystyle V_{S}-(-V_{S})=2V_{S}\,} . This is twice the difference of the single-ended system. If the voltage noise on one wire is uncorrelated to the noise on the other one, it takes twice as much noise to cause an error with the differential system as with the single-ended system. In other words, differential signalling doubles the noise immunity.
Comparison with single-ended signalling In single-ended signalling, the transmitter generates a single voltage that the receiver compares with a fixed reference voltage, both relative to a common ground connection shared by both ends. In many instances, single-ended designs are not feasible. Another difficulty is the electromagnetic interference that can be generated by a single-ended signalling system that attempts to operate at high speed.
Relation to balanced interfaces
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