What’s the difference between a digital and an analog signal?

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Always wondered what the difference is between a digital and an analog signal?

Digital signal

A digital signal is very simple – it’s either on or off.  For example, when a DOL motor starter trips due to a thermal overload, a set of contacts will close in the overload relay, which will communicate to the control system that the motor has failed.

Digital signals can also be received as pulses.  For example, one pulse per litre of fluid passing through a flow meter, to totalise (add up) a regular flow.  In general a digital signal is simpler, cheaper to implement and more reliable than an analog signal.  Digital instruments include level switches, pressure switches, flow switches and running/fault signals.

Analog signal

An analog signal is a little more complex.  Rather than being off or on, an analog signal is continuously variable.  For example, an analog level sensor in a tank of fluid will return a variable signal based on the level in the tank.  A control system will interpret this signal to provide the user with the exact % level.

In addition, an analog output signal is used to dictate the required motor speed to a variable speed drive.  The most common type of analog signal is 4-20mA, but also 0-10V and mV types may be used.  Analog instruments are generally costlier than digital instruments to implement and require calibration, but their versatility and accuracy makes them essential for any more complex process control or where a particular setpoint must be maintained.  Analog instruments include level sensors, pressure sensors, flow meters, water quality meters and motor speed signals.