Multisensor thiết bị đo đạc thiết kế 6o (P5)

DATA CONVERSION DEVICES AND ERRORS Data conversion devices provide the interfacing components between continuoustime signals representing the parameters of physical processes and their discrete-time digital equivalent. Recent emphasis on computer systems for automated manufacturing and the growing interest in using personal computers for data acquisition and control have increased the need for improved understanding of the design requirements of real-time computer I/O systems.
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Multisensor thiết bị đo đạc thiết kế 6o (P5) Multisensor Instrumentation 6 Design. By Patrick H. Garrett Copyright © 2002 by John Wiley & Sons, Inc. ISBNs: 0-471-20506-0 (Print); 0-471-22155-4 (Electronic)5DATA CONVERSION DEVICESAND ERRORS5-0 INTRODUCTIONData conversion devices provide the interfacing components between continuous-time signals representing the parameters of physical processes and their discrete-timedigital equivalent. Recent emphasis on computer systems for automated manufactur-ing and the growing interest in using personal computers for data acquisition andcontrol have increased the need for improved understanding of the design require-ments of real-time computer I/O systems. However, before describing the theory andpractice involved in these systems it is advantageous to understand the characteriza-tion and operation of the various devices from which these systems are fabricated.This chapter provides detailed information concerning A/D and D/A data conversiondevices, and supporting components including analog multiplexers and sample-holddevices. The development of the individual error budgets representing these devicesis also provided to continue the quantitative methodology of this text.5-1 ANALOG MULTIPLEXERSField-effect transistors, both CMOS and JFET, are universally used as electronicmultiplexer switches today, displacing earlier bipolar devices that had voltage off-set problems. Junction FET switches have greater device electrical ruggedness andapproximately the same switching speeds as CMOS devices. However, CMOSswitches are dominant in multiplexer applications because of their unfailing turnoff,especially when the power is removed, unlike JFET devices, and their ability tomultiplex signal levels up to the power supply voltages. Figure 5-1 shows a CMOSanalog switch circuit where a stable ON resistance is achieved of about 100 se-ries resistance by the parallel p- and n-channel devices. Terminating a CMOS mul-tiplexer with a high-input-impedance voltage follower eliminates any voltage di-vider errors possible as a consequence of the ON resistance. Figure 5-2 presents 9596 DATA CONVERSION DEVICES AND ERRORS FIGURE 5-1. CMOS analog switch. FIGURE 5-2. Multiplexer interconnections and tiered array. 5-2 SAMPLE-HOLDS 97TABLE 5-1. Multiplexer Switch CharacteristicsType ON Resistance OFF Isolation Sample RateCMOS 100 70 dB 10 MHzJFET 50 70 dB 1 MHzReed 0.1 90 dB 1 KHzinterconnection configurations for a multiplexer, and Table 5-1 lists multiplexerswitch characteristics. Errors associated with analog multiplexers are tabulated in Table 5-2, and aredominated by the average transfer error defined by equation (5-1). This error is es-sentially determined by the input voltage divider effect, and is minimized to a typi-cal value of 0.01%FS when the AMUX is followed by an output buffer amplifier.The input amplifier associated with a sample-hold device often provides this high-impedance termination. Another error that can be significant is OFF-channel leak-age current that creates an offset voltage across the input source resistance. Vi – V0 Transfer error = × 100% (5-1) Vi5-2 SAMPLE-HOLDSSample-hold devices provide an analog signal memory function for use in sampled-data systems for temporary storage of changing signals for data conversion purpos-es. Sample-holds are available in several circuit variations, each suited to specificspeed and accuracy requirements. Figure 5-3 shows a contemporary circuit thatmay be optimized either for speed or accuracy. The noninverting input amplifierprovides a high-impedance buffer stage, and the overall unity feedback minimizessignal transfer error when the device is in the tracking mode. The clamping diodesensure that the circuit remains stable during the hold mode when the switch is open.The inclusion of S/H devices in sampled-data systems must be carefully considered.The following examples represent the three essential applications for sample-holds.Table 5-3 lists representative sample-hold errors. TABLE 5-2. Representative Multiplexer Errors REED CMOS Transfer error 0 .0 1 % ...