Differential signalling

Differential signaling 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 (typically twisted together) or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The opposite technique is called single-ended signaling. Differential pairs are usually found on printed circuit boards, in twisted-pair and ribbon cables, and in connectors.

Provided that the source and receiver impedances in the differential signaling circuit are equal, external electromagnetic interference tends to affect both conductors identically. Since the receiving circuit only detects the difference between the wires, the technique resists electromagnetic noise compared to one conductor with an un-paired reference (ground). The technique works for both analog signaling, as in balanced audio—and digital signaling, as in RS-422, RS-485, Ethernet over twisted pair, PCI Express, DisplayPort, HDMI, and USB.

The electronics industry, particularly in portable and mobile devices, continually strives to lower supply voltage to save power and reduce emitted electromagnetic radiation. A low supply voltage, however, reduces noise immunity. Differential signaling 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 ${\displaystyle V_{S}}$. The high logic level is ${\displaystyle V_{S}\,}$ and the low logic level is 0 V. The difference between the two levels is therefore ${\displaystyle V_{S}-0\,\mathrm {V} =V_{S}}$. Now consider a differential system with the same supply voltage. The voltage difference in the high state, where one wire is at ${\displaystyle V_{S}\,}$ and the other at 0 V, is ${\displaystyle V_{S}-0\,\mathrm {V} =V_{S}}$. The voltage difference in the low state, where the voltages on the wires are exchanged, is ${\displaystyle 0\,\mathrm {V} -V_{S}=-V_{S}}$. The difference between high and low logic levels is therefore ${\displaystyle V_{S}-(-V_{S})=2V_{S}\,}$. 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.

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