【正文】 模塊設(shè)計實例導(dǎo)航 [M]. 北京： 人民郵電出版社， [10] 沈紅衛(wèi) .單片機應(yīng)用系統(tǒng)設(shè)計實例與分析 [M]. 北京： 北京航空航天大學(xué)出版社， [11] 賴麒文 .8051單片機 C語言徹底應(yīng)用 [M]. 北京： 北京科學(xué)出版社 [12] 李光飛 、李良兒、樓然苗 .單片機 C程序設(shè)計實例指 [M]. 北京： 北京航空航天大學(xué)出版社 [13] 譚浩強 .C語言設(shè)計 [M]. 北京： 清華大學(xué)出版社 20xx [14] 羅炎林 .數(shù)字電路 [M]. 北京： 機械工業(yè)出版社， 1997. [15] 梁明理、 鄧仁清 .電子線路 [M]. 北京： 高等教育出版社， 20xx [16] 老虎工作室 .電路設(shè)計與制版 Protel 99SE典型實例 [M]. 北京： 人民郵電出版社 P1P115 [17] 精英科技 .電路板設(shè)計完全手冊 [M]. 北京： 中國電力出版社 [18] 何立民 .MCS51系列單片機應(yīng)用系統(tǒng)設(shè)計系統(tǒng)配置與接口技術(shù) [M]. 北京： 北京航空航天大學(xué)出版社 1990 [19] 康華光 .電子技術(shù)基礎(chǔ) [M]. 北京： 北京高等教育出版社 [20] 李華嵩 、 王偉 .Protel電路原理圖與 PCB設(shè)計 108例 [M]. 北京： 中國青年電子出版社 P2P126 [21] John of Power on Wesley publishing pany,1991 第 31 頁 附錄 一 外 文翻譯 Analog Signals A signal is said to be analog if it falls between two arbitrary levels, Vx and Vy, and can assume any one of an infinite number of values between Vx and Vy. If the analog signal, V(t), is timedependent, it is a continuous function of time, so that its slope, dV/dt, is never infinite, which would imply an instantaneous change of value. Figure 1 illustrates how both an analog voltage and a digital voltage vary with time. Figure 1 Analog and digital signals Analog signals are processed by analog circuits. The principal feature of an analog circuit is its ability to process an analog signal faithfully, without distorting it— hence the expression hifidelity. A typical analog signal is produced at the output terminals of a microphone as someone speaks into it. The voltage varies continuously over some finite range, depending only on the loudness of the speech and on the physical characteristics of the microphone. An amplifier may be used to increase the amplitude of this timevarying signal to a level suitable for driving a loudspeaker. If the voltage gain of the amplifier is A, and the voltage from the microphone V(t), the output of the amplifier is equal to A?V(t). The output signal 第 32 頁 from the amplifier, like the input, has an infinite range of values, but within a range A times that of the signal from the microphone. Signal Acquisition The conversion of an analog quantity into a digital value requires two separate operations。 for example, if the signal from the transducer contains useful frequency ponents only in the range 0 to 20 Hz (as one might expect from, say, an electrocardiogram), it is beneficial to filter out all signals of a higher frequency. These out of band signals represent unwanted noise and have no useful effect on the interpretation of the electrocardiogram. Moreover, it is necessary for the filter to cut out all frequencies above one half the rate at which the analog signal is sampled. The reasons for this are explained later. The outputs of the filters are fed to an electronic switch called a multiplexer that selects one of the analog input channels for processing. The multiplexer is controlled by the digital system to which the signal acquisition module is connected. The only purpose of the multiplexer is to allow one analogtodigital converter to be connected to several inputs. The analog output of the multiplexer is applied to the input of the last analog circuit in the acquisition module, the sample and hold (S/H) circuit. The sample and hold circuit takes an almost instantaneous sample of the ining analog signal and holds it constant while the analogtodigital converter, ADC, is busy determining the digital value of the signal. If the input signal is changing rapidly, the output of an ADC (which takes an appreciable time to perform its conversion) would be meaningless without a S/H circuit to staticize the input. 第 34 頁 The analogtodigital converter (DAC) transforms the voltage at its input into an mbit digital value, where m varies from typically 4 to 16 or more. Several types of analogtodigital converter are discussed at the end of this section. Signal Quantization Two fundamental questions have to be asked when considering any analogtodigital converter. Into how many levels or values should the input signal be divided and how often should the conversion process be carried out? Let’s look at an ideal threebit analogtodigital converter that converts a voltage into a binary code. As the analog input to this ADC varies in the range 0 V to V, its digital output varies from 000 to 111. Figure 3 provides a transfer function for this ADC. Figure 3 The transfer function of an ideal 3bit A/D converter 第 35 頁 Consider the application of a linear voltage ramp input from V to V to this ADC. Initially the analog input is V and the digital output 000. As the input voltage rises, the output remains at 000 until the input passes V, at which point the output code jumps from 000 to 001. The output code remains at 001 until the input rises above V. Clearly, for each V change in the input, the output code changes by one unit. Figure 3 shows that the input can change in value by up to 1 V without any change taking place in the output code. The resolution of an ADC, Q, is the largest change in its input required to guarantee a change in the output code and is V in this example. The resolution of an ADC is expressed indirectly by the number of bits in its output code, where resolution = Vmaximum/2n1. For example, an 8bit ADC with an input in the range 0 V to + V has a resolution of V/255 = V = mV. Table 1 gives the basic characteristics of ADCs with digital outputs ranging from 4 to 16 bits. The column labeled value of Q for 10 V FS in table indicates the size of the step (., Q) if the maximum output of the ADC is 10 V. The abbreviation FS means fullscale. Figure 4 provides a graph of the difference or error between the analog input of a 3bit ADC and its digital output. Suppose that the analog input is V. The corresponding digital output is 110 which represents