A/D and D/A Converters

Introduction

A device which converts signals in analog form (voltages) into digital form is termed an analog-to-digital (A/D) converter or encoder and the device which performs the reverse process is referred to as a digital-to-analog (D/A) converter. A variety of techniques is available for expressing a voltage in its corresponding digital form or deriving a voltage which is proportional to a number. These techniques are surveyed in the next sections of the textbook.

Inputs of A/D converters are continuously variable (analog) quantities. Outputs of A/D converter are discrete (digital) quantities or codes representing, usually, binary or decimal numbers. Many A/D converters are capable of acting as D/A converters. Some converters are capable of acting in one direction only.

This section contains a discussion of digital codes used in converters. Knowledge of these codes is helpful in understanding the principles of operation of the main types of converters.

Digital Codes

Digital codes are ways of expressing numbers by means of discrete electrical states. In most general use are decimal and binary codes. For many purposes it is convenient to use the decimal number system. It is possible to build devices with ten discrete states that can be used directly to instrument the numbers 0 to 9. But it is general practice, especially in the converter and computer fields, to use one of a number of binary-coded decimal schemes rather than a "pure" decimal code. These schemes commonly use a combination of four devices (digits) that are basically binary to produce ten discrete states.

Binary-coded-decimal schemes are widely used where human beings are to read data, whereas binary codes are popular where data are to be processed by a large-scale scientific computer. Most of these use straight binary notation internally, translating to and from decimal form for outputs and inputs, respectively. Straight binary code is sometimes displayed in octal form (digits 0 to 7) by combining binary digits in groups of three starting from the binary point. The great advantages of straight binary (or octal) code are the ease of instrumenting it and the efficiency with which it uses components. For example, the number 1,023 can be represented by ten binary digits (all 1's), whereas 999 requires three sets of four binary digits in a binary-coded-decimal code.