What is differential signal?

A differential signal is a signal that represents the difference between two physical quantities using a single value. Strictly speaking, all voltage signals are differential because one voltage can only be measured relative to another voltage. In some systems, the system ground is used as a voltage reference. When ground is used as a reference for voltage measurements, the signal arrangement is called single-ended. We use this term because the signal is represented by the voltage on a single conductor.
A differential signal, on the other hand, is applied to two conductors. The signal value is the difference in voltage between the two conductors. Although not strictly necessary, the average of the two voltages will often be the same. A way to compare a differential signal is to think of two people on a seesaw. When one person is moved up, the other person is moved down - but their average position remains the same. Continuing with the seesaw analogy, a positive value would mean that the person on the left is higher than the person on the right, while a negative value would mean that the person on the right is higher than the person on the left. Zero would mean that both people are at the same level.

Figure 1 Differential signal represented by a seesaw

In electrical applications, the two seesaws are represented by a pair of wires labeled V+ and V-. When V+>V-, the signal is defined as a positive signal, and when V+

Figure 2 Differential signal waveforms and single-ended equivalents

Figure 2 The average voltage around which the differential pair swings is set to 2.5V. When each signal in the pair is limited to 0-5V amplitude, offsetting the differential pair provides a maximum range of signal swing. This is often the case when operating from a single 5V supply. When
using differential signaling instead of single-ended signaling, we replace a single wire with a pair of wires, increasing the complexity of any associated interface circuits. So what tangible benefits does differential signaling provide to justify the increased complexity and cost? The
first benefit of differential signaling is that small signals can be easily identified because you are controlling the 'reference' voltage. In a ground-referenced, single-ended signaling system, the exact value of the measured signal depends on the consistency of the 'ground' within the system. The farther the signal source and signal receiver are apart, the greater the possibility that there will be differences in the voltage values ​​of their local grounds. The signal value recovered from differential signaling is largely independent of the exact value of the 'ground', but within a certain range. The second major benefit of differential signaling is that it is highly immune to external electromagnetic interference (EMI). An interference source affects each end of the differential signal pair to almost the same degree. Since the voltage difference determines the signal value, any interference that occurs on both conductors will be ignored. In addition to being less sensitive to interference, differential signaling generates less EMI than single-ended signaling. The third benefit provided by differential signaling is the ability to accurately handle "bipolar" signals in a single-supply system. In order to handle bipolar signals in a single-ended, single-supply system, we must establish a virtual ground at some arbitrary voltage between ground and the power rail (usually the midpoint). Voltages above the virtual ground represent positive signals, and voltages below the virtual ground represent negative signals. Next, the virtual ground must be correctly distributed throughout the system. With differential signaling, there is no need for such a virtual ground, which allows us to handle and propagate bipolar signals with high fidelity without relying on the stability of the virtual ground.