Introduction

The physical world is a place that is characterized by processes that are continuous in time and amplitude. Parameters such as temperature, speed, pressure and length in the physical world are all continuous functions of time and value; that is, for the temperature of an object to change from one value at one time to a different higher or lower value at a later time, it will have to pass through every arbitrarily chosen intervening temperature. No parameter can change value instantly.

When we measure these parameters it is normal to use a sensor to produce a voltage or current output that varies in sympathy with the parameter that we are measuring, so we might, for example, represent temperature between 0 C and 100 C as a voltage between 0 V and 10 V. The voltage then is varying in ratio 100 mV/1 C with the parameter that we are measuring. The word analogue is derived from the Greek analogia ( ana, according to; and logos, ratio) meaning equality of ratios and is commonly used to describe electrical signals that represent measured parameters and the systems that process these signals directly.

The advent of digital electronic systems at the end of the second world war, followed by the invention of the transistor and then the integrated circuit changed the way we process, display and store data. Digital systems are now dominant, replacing devices such as moving coil meters and chart recorders in almost all applications. In order for digital...