CH3 - CHAPTER 3 ADC and DAC Most of the signals directly...

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35 CHAPTER 3 ADC and DAC Most of the signals directly encountered in science and engineering are continuous : light intensity that changes with distance; voltage that varies over time; a chemical reaction rate that depends on temperature, etc. Analog-to-Digital Conversion (ADC) and Digital-to-Analog Conversion (DAC) are the processes that allow digital computers to interact with these everyday signals. Digital information is different from its continuous counterpart in two important respects: it is sampled , and it is quantized . Both of these restrict how much information a digital signal can contain. This chapter is about information management : understanding what information you need to retain, and what information you can afford to lose. In turn, this dictates the selection of the sampling frequency, number of bits, and type of analog filtering needed for converting between the analog and digital realms. Quantization First, a bit of trivia. As you know, it is a digital computer, not a digit computer. The information processed is called data, not data. Why then, is analog-to-digital conversion generally called: ize and ization, rather than ize and digital ization ? The answer is nothing you would expect. When electronics got around to inventing digital techniques, the preferred names had already been snatched up by the medical community nearly a century before. Digitalize and digitalization mean to administer the heart stimulant digitalis . Figure 3-1 shows the electronic waveforms of a typical analog-to-digital conversion. Figure (a) is the analog signal to be digitized. As shown by the labels on the graph, this signal is a voltage that varies over time . To make the numbers easier, we will assume that the voltage can vary from 0 to 4.095 volts, corresponding to the digital numbers between 0 and 4095 that will be produced by a 12 bit digitizer. Notice that the block diagram is broken into two sections, the sample-and-hold (S/H), and the analog-to-digital converter (ADC). As you probably learned in electronics classes, the sample-and-hold is required to keep the voltage entering the ADC constant while the
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The Scientist and Engineer's Guide to Digital Signal Processing 36 conversion is taking place. However, this is not the reason it is shown here; breaking the digitization into these two stages is an important theoretical model for understanding digitization. The fact that it happens to look like common electronics is just a fortunate bonus. As shown by the difference between (a) and (b), the output of the sample-and- hold is allowed to change only at periodic intervals, at which time it is made identical to the instantaneous value of the input signal. Changes in the input signal that occur between these sampling times are completely ignored. That is, sampling converts the independent variable (time in this example) from continuous to discrete.
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CH3 - CHAPTER 3 ADC and DAC Most of the signals directly...

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