A differential pair (or differential signalling) is a way of sending a signal: it consists of sending two 'copies' of a signal, one an inversion of the other, down a pair of conductors; a balanced pair, for best results. At the destination, the second is subtracted from the first. 'Differential' is also used to refer to a scheme where there is a separate signal return ground for each signal; although different in analog detail, the fundamentals and advantages are basically the same.
This approach has a number of advantages:
- It can use lower voltages to achieve a given signal-to-noise ratio
- It is less susceptible to interference
- It produces less interference
- It allows connecting two entities without having to have a common ground
Any noise that has been picked up in common on both conductors is removed when one is subtracted from the other; this reduction in noise allows use of a lower voltage to send a signal. Also, since the return path for the current of the signal in the first conductor is not ground, but the inverse current in the second conductor, this tends to cancel any electro-magnetic radiation from those currents.