Analog and digital

Analog and digital are the two ways in which electronics can carry information. An analog signal is a voltage, the electrical level on a wire, that rises and falls smoothly, in step with something in the real world, such as temperature, sound or light. A digital signal carries information as numbers, written in bits, which are ones and zeros. The real world is analog. Computers work only in digital numbers. So wherever a computer meets the real world, some chip must translate between the two.

Here is how it works in general. A sensor turns something physical into an analog voltage. A microphone, for example, produces a voltage that rises and falls with the sound waves that reach it. This voltage is often tiny, a few thousandths of a volt, so an amplifier makes it larger. Then an analog-to-digital converter, or ADC, measures the voltage at fixed intervals. This is called sampling. For music, the converter measures 48,000 times a second. Each measurement is rounded to the nearest of a fixed number of levels and written down as a number. With 16 bits, there are 65,536 levels. Now a processor or a microcontroller can store the numbers, send them or calculate with them. To go back, a digital-to-analog converter, or DAC, turns numbers into a smoothly changing voltage again, which can drive a speaker or a motor.

Each translation loses a little detail. Measuring more often, and with more levels, keeps more detail, but uses more power and produces more numbers to store. Chips that contain both analog and digital circuits are called mixed-signal chips. Analog and mixed-signal ICs are the converters and amplifiers that do this translation, and they are sold by the billion.

Take, for example, a phone's touch screen. A touch controller measures tiny changes in electric charge across a grid of fine wires behind the glass, converts them into numbers, and works out where the finger is. The display driver does the opposite: it turns numbers into the voltages that set the brightness of each pixel.

Consider an electric scooter's battery pack. Sensor chips measure each cell's temperature as an analog signal. The readings are converted into numbers, so that a controller can cut the power if one cell becomes too hot.

How is analog and digital different from AC and DC? The words are often confused. AC and DC describe electric power. AC changes direction many times a second, while DC always moves in one direction. Analog and digital describe information: whether it is carried as a smoothly changing level or as numbers. A battery gives DC power, and a circuit running on that battery can still carry analog signals, digital signals or both. Power semiconductors handle the power. Analog and digital chips handle the information.