Data Acquisition Systems
Data acquisition systems are used to acquire process operating data and store it on ， secondary storage devices for later analysis. Many or the data acquisition systems acquire this data at very high speeds and very little computer time is left to carry out any necessary, or desirable, data manipulations or reduction. All the data are stored on secondary storage devices and manipulated subsequently to derive the variables of in-terest. It is very often necessary to design special purpose data acquisition systems and interfaces to acquire the high speed process data. This special purpose design can be an expensive proposition. Powerful mini- and mainframe computers are used to combine the data acquisition with other functions such as comparisons between the actual output and the desirable output values, and to then decide on the control action which must be taken to ensure that the output variables lie within preset limits. The computing power required will depend upon the type of process control system implemented. Software requirements for carrying out proportional, ratio or three term control of process variables are relatively trivial, and microcomputers can be used to implement such process control systems. It would not be possible to use many of the currently available microcomputers for the implementation of high speed adaptive control systems which require the use of suitable process models and considerable online manipulation of data. Microcomputer based data loggers are used to carry out intermediate functions such as data acquisition at comparatively low speeds, simple mathematical manipulations of raw data and some forms of data reduction. The first generation of data loggers, without any programmable computing facilities, was used simply for slow speed data acquisition from up to one hundred channels. All the acquired data could be punched out on paper tape or printed for subsequent analysis. Such hardwired data loggers are being replaced by the new generation of data loggers which incorporate microcomputers and can be programmed by the user. They offer an extremely good method of collecting the process data, using standardized interfaces, and subsequently performing the necessary manipulations to provide the information of interest to the process operator. The data acquired can be analyzed to establish correlations, if any, between process variables and to develop mathematical models necessary for adaptive and optimal process control. The data acquisition function carried out by data loggers varies from one to 9 in system to another. Simple data logging systems acquire data from a few channels while complex systems can receive data from hundreds, or even thousands, of input channels distributed around one or more processes. The rudimentary data loggers scan the selected number of channels, connected to sensors or transducers, in a sequential manner and the data are recorded in a digital format. A data logger can be dedicated in the sense that it can only collect data from particular types of sensors and transducers. It is best to use a nondedicated data logger since any transducer or sensor can be connected to the channels via suitable interface circuitry. This facility requires the use of appropriate signal conditioning modules. Microcomputer controlled data acquisition facilitates the scanning of a large number
of sensors. The scanning rate depends upon the signal dynamics which means that some channels must be scanned at very high speeds in order to avoid aliasing errors while there is very little loss of information by scanning other channels at slower speeds. In some data logging applications the faster channels require sampling at speeds of up to 100 times per second while slow channels can be sampled once every five minutes. The conventional hardwired, non-programmable data loggers sample all the channels in a sequential manner and the sampling frequency of all the channels must be the same. This procedure results in the accumulation of very large amounts of data, some of which is unnecessary, and also slows down the overall effective sampling frequency. Microcomputer based data loggers can be used to scan some fast channels at a higher frequency than other slow speed channels. The vast majority of the user programmable data loggers can be used to scan up to 1000 analog and 1000 digital input channels. A small number of data loggers, with a higher degree of sophistication, are suitable for acquiring data from up to 15, 000 analog and digital channels. The data from digital channels can be in the form of TransistorTransistor Logic or contact closure signals. Analog data must be converted into digital format before it is recorded and requires the use of suitable analog to digital converters (ADC)． The characteristics of the ADC will define the resolution that can be achieved and the rate at which the various channels can be sampled. An in-crease in the number of bits used in the ADC improves the resolution capability. Successive approximation ADC's are faster than integrating ADC's. Many microcomputer controlled data loggers include a facility to program the channel scanning rates. Typical scanning rates vary from 2 channels per second to 10, 000 channels per second. Most data loggers have a resolution capability of ± 0.01% or better, It is also pos-sible to achieve a resolution of 1 micro-volt. The resolution capability, in absolute terms, also depends upon the range of input signals, Standard input signal ranges are 0-10 volt, 0-50 volt and 0-100 volt. The lowest measurable signal varies form 1 t, volt to 50, volt. A higher degree of recording accuracy can be achieved by using modules which accept data in small, selectable ranges. An alternative is the auto ranging facil-ity available on some data loggers. The accuracy with which the data are acquired and logged-on the appropriate storage device is extremely important. It is therefore necessary that the data acquisi-tion module should be able to reject common mode noise and common mode voltage. Typical common mode noise rejection capabilities lie in the range 110 dB to 150 dB. A decibel (dB) is a tern which defines the ratio of the power levels of two signals. Thus if the reference and actual signals have power levels of N, and Na respectively, they will have a ratio of n decibels, where n＝10 Log10（Na /Nr） Protection against maximum common mode voltages of 200 to 500 volt is available on typical microcomputer based data loggers. The voltage input to an individual data logger channel is measured, scaled and linearised before any further data manipulations or comparisons are carried out. In many situations, it becomes necessary to alter the frequency at which particu-lar channels are sampled depending upon the values of data signals received from a
particular input sensor. Thus a channel might normally be sampled once every 10 minutes. If, however, the sensor signals approach the alarm limit, then it is obviously desirable to sample that channel once every minute or even faster so that the operators can be informed, thereby avoiding any catastrophes. Microcomputer controlled intel-ligent data loggers may be programmed to alter the sampling frequencies depending upon the values of process signals. Other data loggers include self-scanning modules which can initiate sampling. The conventional hardwired data loggers, without any programming facilities, simply record the instantaneous values of transducer outputs at a regular sampling in-terval. This raw data often means very little to the typical user. To be meaningful, this data must be linearised and scaled, using a calibration curve, in order to determine the real value of the variable in appropriate engineering units. Prior to the availability of programmable data loggers, this function was usually carried out in the off-line mode on a mini- or mainframe computer. The raw data values had to be punched out on pa-per tape, in binary or octal code, to be input subsequently to the computer used for analysis purposes and converted to the engineering units. Paper tape punches are slow speed mechanical devices which reduce the speed at which channels can be scanned. An alternative was to print out the raw data values which further reduced the data scanning rate. It was not possible to carry out any limit comparisons or provide any alarm information. Every single value acquired by the data logger had to be recorded even though it might not serve any useful purpose during many data values only need recording when they lie outside the pre-set low and high limits. If the analog data must be transmitted over any distance, differences in ground potential between the signal source and final location can add noise in the interface design. In order to separate common-mode interference form the signal to be recorded or processed, devices designed for this purpose, such as instrumentation amplifiers, may be used. An instrumentation amplifier is characterized by good common-mode- rejection capability, a high input impedance, low drift, adjustable gain, and greater cost than operational amplifiers. They range from monolithic ICs to potted modules, and larger rack-mounted modules with manual scaling and null adjustments. When a very high common-mode voltage is present or the need for extremely-low com-mon-mode leakage current exists（as in many medical-electronics applications），an isolation amplifier is required. Isolation amplifiers may use optical or transformer isolation. Analog function circuits are special-purpose circuits that are used for a variety of signal conditioning operations on signals which are in analog form. When their accu-racy is adequate, they can relieve the microprocessor of time-consuming software and computations. Among the typical operations performed are multiplications, division, powers, roots, nonlinear functions such as for linearizing transducers, rims measure-ments, computing vector sums, integration and differentiation, and current-to-voltage or voltage- to-current conversion. Many of these operations can be purchased in available devices as multiplier/dividers, log/antilog amplifiers, and others. When data from a number of independent signal sources must be processed by the same microcomputer or communications channel, a multiplexer is used to channel the input signals into the A/D converter.
Multiplexers are also used in reverse, as when a converter must distribute analog information to many different channels. The multiplexer is fed by a D／A converter which continually refreshes the output channels with new information. In many systems, the analog signal varies during the time that the converter takes to digitize an input signal. The changes in this signal level during the conversion process can result in errors since the conversion period can be completed some time after the conversion command. The final value never represents the data at the instant when the conversion command is transmitted. Sample-hold circuits are used to make an acquisition of the varying analog signal and to hold this signal for the duration of the conversion process. Sample-hold circuits are common in multichannel distribution systems where they allow each channel to receive and hold the signal level. In order to get the data in digital form as rapidly and as accurately as possible, we must use an analog/digital (A/D) converter, which might be a shaft encoder, a small module with digital outputs, or a high-resolution, high-speed panel instrument. These devices, which range form IC chips to rack-mounted instruments, convert ana-log input data, usually voltage, into an equivalent digital form. The characteristics of A/D converters include absolute and relative accuracy, linearity, monotonic, resolu-tion, conversion speed, and stability. A choice of input ranges, output codes, and other features are available. The successive-approximation technique is popular for a large number of applications, with the most popular alternatives being the counter-comparator types, and dual-ramp approaches. The dual-ramp has been widely-used in digital voltmeters. D/A converters convert a digital format into an equivalent analog representation. The basic converter consists of a circuit of weighted resistance values or ratios, each controlled by a particular level or weight of digital input data, which develops the output voltage or current in accordance with the digital input code. A special class of D/A converter exists which have the capability of handling variable reference sources. These devices are the multiplying DACs. Their output value is the product of the number represented by the digital input code and the analog reference voltage, which may vary form full scale to zero, and in some cases, to negative values. Component Selection Criteria In the past decade, data-acquisition hardware has changed radically due to ad-vances in semiconductors, and price what have not changed, however, are the fundamental system problems confronting the designer. Signals may be obscured by noise, rfi，ground loops, power-line pickup, and transients coupled into signal lines from machinery. Separating the signals from these effects becomes a matter for concern. Data-acquisition systems may be separated into two basic categories: （1） those suited to favorable environments like laboratories -and（2）those required for hostile environments such as factories, vehicles, and military installations. The latter group includes industrial process control systems where temperature information may be gathered by sensors on tanks, boilers, wats, or pipelines that may be spread over miles of facilities. That data may then be sent to a central processor to provide real-time process control. The digital control of steel mills, automated chemical production, and machine tools is carried out in this kind of hostile environment. The vulnerability of the data signals leads to the requirement for isolation and other techniques.
At the other end of the spectrum-laboratory applications, such as test systems for gathering information on gas chromatographs, mass spectrometers, and other sophis-ticated instruments-the designer's problems are concerned with the performing of sen-sitive measurements under favorable conditions rather than with the problem of pro-tecting the integrity of collected data under hostile conditions. Systems in hostile environments might require components for wide tempera-tures, shielding, common-mode noise reduction, conversion at an early stage, redun-dant circuits for critical measurements, and preprocessing of the digital data to test its reliability. Laboratory systems, on the other hand, will have narrower temperature ranges and less ambient noise. But the higher accuracies require sensitive devices, and a major effort may be necessary for the required signal /noise ratios. The choice of configuration and components in data-acquisition design depends on consideration of a number of factors: 1. Resolution and accuracy required in final format. 2. Number of analog sensors to be monitored. 3. Sampling rate desired. 4. Signal-conditioning requirement due to environment and accuracy. 5. Cost trade-offs. Some of the choices for a basic data-acquisition configuration include： 1 ．Single-channel techniques. A. Direct conversion. B. Preamplification and direct conversion. C. Sample-hold and conversion. D. Preamplification, sample-hold, and conversion. E. Preamplification, signal-conditioning, and direct conversion. F. Preamplification, signal-conditioning, sample-hold, and conversion. 2. Multichannel techniques. A. Multiplexing the outputs of single-channel converters. B. Multiplexing the outputs of sample-holds. C. Multiplexing the inputs of sample-holds. D. Multiplexing low-level data. E. More than one tier of multiplexers. Signal-conditioning may include: 1. Radiometric conversion techniques. B. Range biasing. D. Logarithmic compression. A. Analog filtering. B. Integrating converters. C. Digital data processing. We shall consider these techniques later, but first we will examine some of the components used in these data-acquisition system configurations. Multiplexers When more than one channel requires analog-to-digital conversion, it is neces-sary to use time-division multiplexing in order to connect the analog inputs to a single
converter, or to provide a converter for each input and then combine the converter outputs by digital multiplexing. Analog Multiplexers Analog multiplexer circuits allow the timesharing of analog-to-digital converters between a numbers of analog information channels. An analog multiplexer consists of a group of switches arranged with inputs connected to the individual analog channels and outputs connected in common（as shown in Fig. 1）．The switches may be ad-dressed by a digital input code. Many alternative analog switches are available in electromechanical and solid-state forms. Electromechanical switch types include relays, stepper switches, cross-bar switches, mercury-wetted switches, and dry-reed relay switches. The best switching speed is provided by reed relays（about 1 ms）．The mechanical switches provide high do isolation resistance, low contact resistance, and the capacity to handle voltages up to 1 KV, and they are usually inexpensive. Multiplexers using mechanical switches are suited to low-speed applications as well as those having high resolution requirements. They interface well with the slower A/D converters, like the integrating dual-slope types. Mechanical switches have a finite life, however, usually expressed in number of operations. A reed relay might have a life of 10 operations, which would allow a 3-year life at 10 operations/second. Solid-state switch devices are capable of operation at 30 ns, and they have a life which exceeds most equipment requirements. Field-effect transistors （FETs） are used in most multiplexers. They have superseded bipolar transistors which can introduce large voltage offsets when used as switches. FET devices have a leakage from drain to source in the off state and a leakage from gate or substrate to drain and source in both the on and off states. Gate leakage in MOS devices is small compared to other sources of leakage. When the device has a Zener-diode-protected gate, an additional leakage path exists between the gate and source. Enhancement-mode MOS-FETs have the advantage that the switch turns off when power is removed from the MUX. Junction-FET multiplexers always turn on with the power off. A more recent development, the CMOS-complementary MOS-switch has the advantage of being able to multiplex voltages up to and including the supply voltages. A± 10-V signal can be handled with a ± 10-V supply. Trade-off Considerations for the Designer Analog multiplexing has been the favored technique for achieving lowest system cost. The decreasing cost of A/D converters and the availability of low-cost, digital integrated circuits specifically designed for multiplexing provide an alternative with advantages for some applications. A decision on the technique to use for a given sys-tem will hinge on trade-offs between the following factors: 1. Resolution. The cost of A/D converters rises steeply as the resolution increases due to the cost of precision elements. At the 8-bit level, the per-channel cost of an analog multiplexer may be a considerable proportion of the cost of a converter. At resolutions
above 12 bits, the reverse is true, and analog multiplexing tends to be more economical. 2. Number of channels. This controls the size of the multiplexer required and the amount of wiring and interconnections. Digital multiplexing onto a common data bus reduces wiring to a minimum in many cases. Analog multiplexing is suited for 8 to 256 beyond this number, the technique is unwieldy and analog errors be-come difficult to minimize. Analog and digital multiplexing is often combined in very large systems. 3. Speed of measurement, or throughput. High-speed A/D converters can add a considerable cost to the system. If analog multiplexing demands a high-speed con-verter to achieve the desired sample rate, a slower converter for each channel with digital multiplexing can be less costly． 4. Signal level and conditioning. Wide dynamic ranges between channels can be difficult with analog multiplexing. Signals less than 1V generally require differential low-level analog multiplexing which is expensive, with programmable-gain amplifiers after the MUX operation. The alternative of fixed-gain converters on each channel, with signal-conditioning designed for the channel requirement, with digital multi-plexing may be more efficient. 5. Physical location of measurement points. Analog multiplexing is suited for making measurements at distances up to a few hundred feet from the converter, since analog lines may suffer from losses, transmission-line reflections, and interference. Lines may range from twisted wire pairs to multiconductor shielded cable, depending on signal levels, distance, and noise environments. Digital multiplexing is operable to thousands of miles, with the proper transmission equipment, for digital transmission systems can offer the powerful noise-rejection characteristics that are required for 29 Data Acquisition Systems long-distance transmission. Digital Multiplexing For systems with small numbers of channels, medium-scale integrated digital multiplexers are available in TTL and MOS logic families. The 74151 is a typical example. Eight of these integrated circuits can be used to multiplex eight A/D con-verters of 8-bit resolution onto a common data bus. This digital multiplexing example offers little advantages in wiring economy, but it is lowest in cost, and the high switching speed allows operation at sampling rates much faster than analog multiplexers. The A/D converters are required only to keep up with the channel sample rate, and not with the commutating rate. When large numbers of A/D converters are multiplexed, the data-bus technique reduces system interconnections. This alone may in many cases justify multiple A/D converters. Data can be bussed onto the lines in bit-parallel or bit-serial format, as many converters have both serial and parallel outputs. A variety of devices can be used to drive the bus, from open collector and tristate TTL gates to line drivers and optoelectronic isolators. Channel-selection decoders can be built from 1-of-16 decoders to the required size. This technique also allows additional reliability in that a failure of one A/D does not affect the other channels. An important requirement is that the multiplexer operate without introducing unacceptable errors at the sample-rate speed. For a digital MUX system, one can determine the speed from propagation delays and the time required to charge the bus capacitance.
Analog multiplexers can be more difficult to characterize. Their speed is a func-tion not only of internal parameters but also external parameters such as channel, source impedance, stray capacitance and the number of channels, and the circuit lay-out. The user must be aware of the limiting parameters in the system to judge their ef-fect on performance. The nonideal transmission and open-circuit characteristics of analog multiplexers can introduce static and dynamic errors into the signal path. These errors include leakage through switches, coupling of control signals into the analog path, and inter-actions with sources and following amplifiers. Moreover, the circuit layout can com-pound these effects. Since analog multiplexers may be connected directly to sources which may have little overload capacity or poor settling after overloads, the switches should have a break-before-make action to prevent the possibility of shorting channels together. It may be necessary to avoid shorted channels when power is removed and a chan-nels-off with power-down characteristic is desirable. In addition to the chan-nel-addressing lines, which are normally binary-coded, it is useful to have inhibited or enable lines to turn all switches off regardless of the channel being addressed. This simplifies the external logic necessary to cascade multiplexers and can also be useful in certain modes of channel addressing. Another requirement for both analog and digital multiplexers is the tolerance of line transients and overload conditions, and the ability to absorb the transient energy and recover without damage.
数据采集系统
数据采集系统是用来获取数据处理和存储在二级存储设备， 为后来的分析。 许多或数据 采集系统获得数据以很高的速度和非常小的计算机时间去进行任何必要的， 或需要， 数据操 作或减少。 所有的数据都存储在二级存储设备和操纵随后得出的变量的利益。 这是很经常需 要设计特殊用途的数据采集系统和接口获得高速过程数据。 这一特殊目的的设计是一个昂贵 的命题。 强大的小型和大型计算机结合使用的数据采集与其他职能， 如比较实际输出与期望输出 值，并决定对控制作用，必须采取确保输出变量躺在预设的限制。计算所需功率将取决于类 型的过程控制系统的实施。软件需求进行比例，比率或三项控制的过程变量都比较琐碎，和 微型计算机可以用来执行过程控制系统。 它不可能使用许多现有微机实施高速度自适应控制 系统，需要使用合适的过程模型和大量的在线数据处理。 微机数据记录仪是用来进行中间等功能的数据采集在较低的速度， 简单的数学运算的数 据和某些形式的数据约简。第一代的数据记录器，没有任何可编程计算机设施，使用简单的 低速数据采集到一百频道。 所有获得的数据可以被穿孔纸带或印刷为以后的分析。 这种硬数 据记录器， 取而代之的是新一代数据记录器纳入微机和可编程的用户。 他们提供了一个非常 好的方法收集数据的过程中，使用标准接口，并随后执行必要的操作提供感兴趣的信息，经 营者的过程。获得的数据可以分析，建立相关，如果有的话，变量之间的过程和发展的数学 模型的必要的适应性和过程优化控制。 数据采集功能进行数据记录器不同从 9 到另一个系统。 简单的数据记录系统获取的数据 从几个渠道而复杂的系统，可以接收数据从数百，甚至数千，输入通道分布在一个或多个进 程。初步数据记录器扫描选定的通道，连接到传感器或传感器，在一个连续的方式，数据被 记录在数字格式。 数据记录仪能专注在这个意义上， 它可以只收集数据从特定类型的传感器 和传感器。 最好的方法是使用一个专用数据记录仪， 因为任何传感器或传感器可以连接到通 道通过适当的接口电路。这个设备需要使用适当的信号调理模块。 微机控制的数据采集方便扫描大量传感器。 扫描速度取决于信号的动态是指一些渠道必 须扫描在非常高的速度， 以避免混淆的错误而很少有损失的信息的扫描其它渠道以较慢的速 度。 在一些数据应用程序更快的渠道需要采样速度高达每秒 100 次而缓慢的渠道可以每五分 钟采样一次。 传统的硬， 不可编程数据记录器样本的所有频道顺序和采样频率对所有通道必 须是相同的。这个程序的结果积累的大量数据，其中一些是不必要的，而且也减慢了全面有 效的取样频率。微机数据记录器可以用来扫描一些快速通道在更高的频率比其他速度慢通 道。 绝大部分的用户可编程数据记录器可以用来扫描多达 1000 个模拟和 1000 个数字输入通 道。少量数据记录器，具有更高的复杂程度，适用于从数据获取高达 15，000 模拟和数字通 道。数据从数字频道的形式可以是晶体管-晶体管逻辑或接点闭合信号。模拟数据必须被转 换成数字格式之前，它是记录和需要使用适当的模拟到数字转换器（模数转换器） 。特色的 模数转换器将确定的分辨率可以达到的速度在不同的渠道进行采样。 一个能在使用的位数的 提高分辨能力。 逐次逼近模数转换器的速度比积分的。 许多微机控制数据记录器包括一个设 备编程通道扫描率。典型的扫描率从 2 通道每秒 10，000 通道每秒。
大多数数据记录器具有分辨能力± 0.01%或更好，也有可能实现一个分辨率 为 1 微伏。该决议的能力，绝对值，还取决于输入信号范围，标准输入信号范围 为 0 - 50- 100 伏伏伏，和。最低可测信号的变化从 1 伏到 50 伏，。更高程度的 记录精度可以达到使用模块接受数据量小，可选择的范围。另一种是自动测距设 备可在某些数据记录器。
准确性与数据采集和登录的适当的存储设备是非常重要的。 因此有必要的数据采集模块 应该能够拒绝共模噪声和共模电压。典型的共模噪声抑制能力范围在 110 分贝至 150 分贝。 分贝（分贝）是燕鸥定义比功率水平的信号。因此，如果参考和实际信号功率水平，和 n 分别，他们将有一个比的分贝，当
n＝10 Log10（Na /Nr）
防止最大共模电压为 200 到 500 伏特是典型的微机数据记录器。 电压输入个人数据记录仪测量信道， 规模和线性化之前， 任何进一步的数据操作或比较 进行。 在许多情况下， 有必要改变的频率在这项通道采样取决于价值的数据信号从收到一个特 定的输入传感器。因此，渠道可能通常是每 10 分钟采样一次。如果，然而，传感器信号的 警报限制，那么它显然是可取的样品通道每分钟或甚至更快，使运营商可以得知，从而避免 任何灾难。 微机控制智能数据记录器可以通过编程改变采样频率取决于价值的过程信号。 其 他数据记录器包括扫描模块，可以启动采样。 传统的硬数据记录器，无需任何编程，只记录瞬时值传感器的输出在采样间隔。这种原 始数据通常是很小的典型用户。 是有意义的， 这个数据必须被线性化和规模， 使用校准曲线， 以确定真正价值的变量在适当的工程单位。 之前的可用性可编程数据记录器， 此功能通常在 离线模式在小型或大型计算机。原始数据值是打了对纸胶带，在二进制或八进制代码，输入 到随后的计算机用于分析的目的和转换为工程单位。纸带速度缓慢，机械设备，降低速度， 通道可以扫描。另一个是打印出来的原始数据值，进一步减少了数据的扫描速度。这是不可 能进行任何限制比较或提供任何报警信息。 每一个单值获得的数据记录器已被记录， 即使它 可能没有任何用处在随后的分析； 许多数据值只需要记录时， 他们不在预先设定的最低和最 高限额。 如果模拟数据必须在任何传输距离， 差别在地电位之间的信号源和最后的位置， 可以添 加噪声中的界面设计。为单独的共模干扰的信号记录或加工，设备设计用于这一目的，如仪 表放大器，可用于。仪表放大器具有共模排斥能力，高输入阻抗，低漂移，可调增益，和更 大的成本比运算放大器。 它们的范围从单片集成电路模块和较大的盆栽， 机架式模块手动缩 放和调整。 当一个非常高的共模电压存在或需要极低共模漏电流的存在 （如在许多医学电子 学的应用） ，隔离放大器是必需的。隔离放大器可以使用光学或隔离变压器。 模拟电路的专用电路，用于各种信号调理操作信号，在模拟形式。当他们的精度是足够 的，他们可以减轻微处理器软件和计算耗时。在典型的操作是乘法，除法，权力，根，非线 性函数为线性传感器，边测量，计算矢量和，整合和分化，以及电压或电压-电流转换。许 多这些行动可在现有设备作为乘数/分频器，日志/反对数放大器，及其他。 当数据从一个独立的信号源必须处理由同一计算机或通信信道， 一个多路复用器是用于 信道输入信号的模数转换器。 多路复用器还用于反向， 如当一个转换器必须分配模拟信息到许多不同的渠道。 多路复 用器是由一个转换器，不断刷新输出通道与新的信息。 在许多系统，模拟信号在不同的时间，以数字化输入信号转换器。在这个信号的变化的 转化过程中，会产生错误，因为转换期间可以完成后一段时间的转换命令。最后的值不代表 数据的瞬间转换命令传输。 采样保持电路进行收购的不同模拟信号和把握这个信号的持续时 间的转换过程。采样保持电路的多路分配系统，使每个通道接收和持有信号电平。 为了获得数字形式的数据尽快和尽可能准确，我们必须使用一个模拟/数字转换器（模 数） ，这可能是一个轴编码器，一个小模块的数字输出，或高解析度，高速面板仪表。这些 设备，其中形成集成电路芯片安装在机架上的仪器，ana-log 转换输入数据，通常电压，等 效成数字形式。 特征一个模数转换器包括绝对和相对精度，线性度， 单调， 拆分，转换速度，
稳定性。一个选择的输入，输出代码，和其他功能可用。流行的逐次逼近技术的大量应用， 与最流行的替代品正在计数器类型，和双斜方法。该双斜已被广泛应用于数字电压表。 模数转换器转换成数字格式为等效模拟表示。 基本的转换器包括一个电路的加权电阻值 或比例，每个由一个特定水平或重量的数字输入数据，形成输出电压或电流，根据数字输入 码。一类特殊的数模转换器，有能力处理变量的参考来源。这些设备是乘法数模转换器。其 输出值数量的产品所代表的数字输入码和模拟参考电压， 这可能形成全为零， 以及在某些情 况下，消极价值观。
组件的选择标准
在过去十年中， 数据采集硬件发生了根本的变化， 由于半导体中提出， 并且价格也下来； 什么都没有改变，但是，是基本的系统设计师面临的问题。信号可能掩盖噪声，射频干扰， 接地回路，电源线，和瞬态耦合到信号线机械。分离从这些信号的影响成为一个值得关注的 问题。 数据采集系统可分为 2 类： （1）那些适合有利环境的实验室和（2）所需的恶劣环境如 工厂，车辆，和军事设施。后者包括工业过程控制系统在温度信息可能是由传感器收集到的 坦克，锅炉，管道，或可能传播英里以上的设施。数据可以被发送到一个中央处理器提供实 时过程控制。数字控制米尔斯，自动化生产，机械工具，是进行这种敌对的环境。脆弱性的 数据信号导致隔离要求和其他技术。 在光谱的另一端实验室应用，如测试系统收集资料，气相色谱仪，质谱仪，等精密仪器 设计问题的关注与表演的敏感测量的有利条件下， 而不是与问题阐释收集数据的完整敌对的 条件下。 系统在恶劣的环境中可能需要广泛的温度元件， 屏蔽， 共模噪声降低， 转换在早期阶段， 冗余电路的关键测量，和预处理的数字数据测试其可靠性。实验室系统，另一方面，会有较 窄的温度范围和减少环境噪声。 但高精度需要敏感的设备， 和一个主要的努力可能是所需的 信号/噪声比。 选择配置和部件数据采集的设计取决于考虑的一些因素： 1．分辨率和精度要求在最后格式。 2．一些模拟传感器监测。 3．采样率需要。 4．由于环境和精度信号调理要求。 5．成本的权衡。 一些选择一个基本数据采集配置包括： 1．单通道技术。 2．多通道技术。 我们应考虑这些技术后，但首先我们将检视一些常用的组件在这些数据采集系统配置。
多路复用器
当有一个以上的通道需要模拟到数字的转换， 有必要使用时分复用为连接的模拟输入到 一个单一的转换器，或提供一个转换为每个输入，然后结合变频器输出的数字多路复用。
模拟多路复用器
模拟多路复用器电路允许的分时的模拟数字转换器之间的一个数字模拟信息渠道。 一个 模拟多路复用器由一组开关的输入连接到模拟输出通道和共同连接。 该开关可以处理的数字 输入码。
许多其他的模拟开关可在机电和固态形式。机电开关类型包括继电器，步进开关，交叉 开关，开关，干簧继电器开关汞润。最好的开关速度是由舌簧继电器（约 1 毫秒） 。机械开 关提供高隔离电阻，接触电阻低，并有能力处理电压 1k V，他们通常是廉价。多路复用器 使用机械开关适合低速应用， 以及那些具有高分辨率的要求。 他们很好地与较慢的模数转换 器，如积分斜率类型。机械开关有一个有限的生命，然而，通常表示操作数。簧片继电器， 可能一生中有 109 个操作，这将使一个 3 年的生活在 10 个业务/秒。 固态开关设备能够运行在 30 纳秒，和他们有一个超过大多数设备要求。场效应晶体管 （FET）大部分都是采用多路复用器。他们已经取代了双极晶体管可以引进大的电压偏移时 用作开关。
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