【導讀】隨著信息領域各種技術的發(fā)展，在數(shù)據(jù)采集方面的技術也取得了長足的進步，采集數(shù)據(jù)的信息化是目前社會的發(fā)展主流方向。石油勘探、地震數(shù)據(jù)采集領域已經(jīng)得到應用。控領域中占到了統(tǒng)治地位。數(shù)據(jù)采集系統(tǒng)是將現(xiàn)場采集到的數(shù)據(jù)進行處理、傳輸、數(shù)據(jù)采集系統(tǒng)的主要功能是把模擬信號變成數(shù)字信號，并進。行分析、處理、存儲和顯示。國內(nèi)大大小小很多公司多開發(fā)了數(shù)據(jù)采集器和卡之。類的產(chǎn)品，這使信息的數(shù)字化提供了極大的方便。送給PC機處理，另外三個數(shù)碼管LED顯示采集的數(shù)字信息。

　　

【正文】 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。 the extraction of a sample value of the signal to be processed and the actual conversion of that sample value into a binary form. Figure 2 gives the block diagram of an analog signal acquisition module. As the analogtodigital converter (ADC) at the heart of this module may be rather expensive, it is not unusual to provide a number of different analog channels, all using the same ADC. Each analog channel in figure 2 begins with a transducer that converts an analog quantity into an electrical value. A transducer exploits some physical property of matter to perform the conversion process. For example, the thermistor is a transducer posed of a substance whose electrical resistance varies with temperature. Figure 2 An analog signal acquisition module 第 33 頁 The electrical signal from the transducer is frequently very tiny (sometimes only a few microvolts) and must be amplified before further processing in order to bring it to a level well above the noise voltages present in later circuits. After amplification es filtering, a process designed to restrict the passage of certain signals through the circuit. Filtering blocks signals with a frequency above or below a cutoff point。 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 V。 that is, the digital output corresponds to the quantized input, rather than the actual input. The difference between the actual input and the idealized input corresponds to an error of V. Figure 4 shows that the maximum error between the input and output is equal to Q/2. This error is called the quantization error. 第 36 頁 Figure 4 The error function of an ideal 3bit A/D converter The output from a real ADC can