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Differential signaling is a method of transmitting information electrically by means of
two complementary signals sent on two separate wires.
Tolerance of ground offsets
In a system with a differential receiver, desired signals add and noise is subtracted away.
At the end of the connection, the receiving device reads the difference between the two
signals. Since the receiver ignores the wires' voltages with respect to ground, small
changes in ground potential between transmitter and receiver do not affect the receiver's
ability to detect the signal.
Suitability for use with low-voltage electronics
In the electronics industry, and particularly in portable and mobile devices, there is a
continuing tendency to lower the supply voltage in order to save power and reduce
unwanted emitted radiation. A low supply voltage, however, causes problems with
signaling because it reduces the noise immunity. Differential signaling helps to reduce
these problems because, for a given supply voltage, it gives twice the noise immunity of a
single-ended system.
To see why, consider a single-ended digital system with supply voltage VS. The high
logic level is VS and the low logic level is 0 V. The difference between the two levels is
therefore VS − 0V = VS. Now consider a differential system with the same supply
voltage. The voltage difference in the high state, where one wire is at VS and the other at
0 V, is VS − 0V = VS. The voltage difference in the low state, where the voltages on the
wires are exchanged, is 0V − VS = − VS. The difference between high and low logic
levels is therefore VS − ( − VS) = 2VS. This is twice the difference of the single-ended
system. The result is that it takes twice as much noise to cause an error with the
differential system as with the single-ended system. In other words, the noise immunity is
doubled.
[edit] Resistance to electromagnetic interference
This advantage is not actually due to differential signaling itself, but to the common
practice of transmitting differential signals on balanced lines.[1][2] Single-ended signals
are still resistant to interference if the lines are balanced and terminated by a differential
amplifier.
Comparison with single-ended signaling
In single-ended signaling, 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.
The widely used RS-232 system is an example of single-ended signaling, which uses ±12
V to represent a signal, and anything less than ±3 V to represent the lack of a signal. The
high voltage levels give the signals some immunity from noise, since few naturally
occurring signals can create that sort of voltage. They also have the advantage of
requiring only one wire per signal. However, they also have a serious disadvantage: they
cannot run at high speeds. The effects of capacitance and inductance, which filter out
high-frequency signals, limit the speed. Large voltage swings driving long cables also
require significant power from the transmitting end. This problem can be reduced by
using smaller voltages, but then the chance of mistaking random environmental noise for
a signal becomes much more of a problem. In many instances single-ended designs are
not feasible. Another difficulty is the electromagnetic interference that can be generated
by a single-ended signaling system which attempts to operate at high speed.
Differential signaling is used with a balanced pair of conductors. For short cables and low
frequencies, the two methods are equivalent, so cheap single-ended circuits with a
common ground can be used with cheap cables. As signalling speeds become faster,
wires begin to behave as transmission lines.