Файл: Digital design with CPLD applications and VHDL (R. Dueck, 2000).pdf
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C H A P T E R 1 2 • Interfacing Analog and Digital Circuits |
Electronic circuits and signals can be divided into two main categories: analog and digital. Analog signals can vary continuously throughout a defined range. Digital signals
take on specific values only, each usually described by a binary number.
Many phenomena in the world around us are analog in nature. Sound, light, heat, position, velocity, acceleration, time, weight, and volume are all analog quantities. Each of these can be represented by a voltage or current in an electronic circuit. This voltage or current is a copy, or analog, of the sound, velocity, or whatever.
We can also represent these physical properties digitally, that is, as a series of numbers, each describing an aspect of the property, such as its magnitude at a particular time. To translate between the physical world and a digital circuit, we must be able to convert analog signals to digital and vice versa.
We will begin by examining some of the factors involved in the conversion between analog and digital signals, including sampling rate, resolution, range, and quantization.
We will then examine circuits for converting digital signals to analog, since these have a fairly standard form. Analog-to-digital conversion has no standard method. We will study several of the most popular: simultaneous (flash) conversion, successive approximation, and dual slope (integrating) conversion.
12.1 Analog and Digital Signals
K E Y T E R M S
Continuous Smoothly connected. An unbroken series of consecutive values with no instantaneous changes.
Discrete Separated into distinct segments or pieces. A series of discontinuous values.
Analog A way of representing some physical quantity, such as temperature or velocity, by a proportional continuous voltage or current. An analog voltage or current can have any value within a defined range.
Digital A way of representing a physical quantity by a series of binary numbers.
A digital representation can have only specific discrete values.
Analog-to-digital converter A circuit that converts an analog signal at its input to a digital code. (Also called an A-to-D converter, A/D converter, or ADC.)
Digital-to-analog converter A circuit that converts a digital code at its input to an analog voltage or current. (Also called a D-to-A converter, D/A converter, or DAC.)
Electronic circuits are tools to measure and change our environment. Measurement instruments tell us about the physical properties of objects around us. They answer questions such as “How hot is this water?”, “How fast is this car going?”, and “How many electrons are flowing past this point per second?” These data can correspond to voltages and currents in electronic instruments.
If the internal voltage of an instrument is directly proportional to the quantity being measured, with no breaks in the proportional function, we say that it is an analog voltage. Like the property being measured, the voltage can vary continuously throughout a defined range.
For example, sound waves are continuous movements in the air. We can plot these movements mathematically as a sum of sine waves of various frequencies. The patterns of magnetic domains on an audio tape are analogous to the sound waves that produce them and electromagnetically represent the same mathematical functions. When the tape is played, the playback head produces a voltage that is also proportional to the original sound waves. This analog audio voltage can be any value between the maximum and minimum voltages of the audio system amplifier.