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. | Multisensor Instrumentation 6a Design. By Patrick H. Garrett Copyright 2002 by John Wiley Sons Inc. ISBNs 0-471-20506-0 Print 0-471-22155-4 Electronic 5 DATA CONVERSION DEVICES AND ERRORS 5-0 INTRODUCTION 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. However before describing the theory and practice involved in these systems it is advantageous to understand the characterization and operation of the various devices from which these systems are fabricated. This chapter provides detailed information concerning A D and D A data conversion devices and supporting components including analog multiplexers and sample-hold devices. The development of the individual error budgets representing these devices is also provided to continue the quantitative methodology of this text. 5-1 ANALOG MULTIPLEXERS Field-effect transistors both CMOS and JFET are universally used as electronic multiplexer switches today displacing earlier bipolar devices that had voltage offset problems. Junction FET switches have greater device electrical ruggedness and approximately the same switching speeds as CMOS devices. However CMOS switches are dominant in multiplexer applications because of their unfailing turnoff especially when the power is removed unlike JFET devices and their ability to multiplex signal levels up to the power supply voltages. Figure 5-1 shows a CMOS analog switch circuit where a stable ON resistance is achieved of about 100 it series resistance by the parallel p- and n-channel devices. Terminating a CMOS multiplexer with a high-input-impedance voltage follower eliminates any voltage .
