Multisensor thiết bị đo đạc thiết kế 6o (P4)

LINEAR SIGNAL CONDITIONING TO SIX-SIGMA CONFIDENCEINTRODUCTION Economic considerations are imposing increased accountability on the design of analog I/O systems to provide performance at the required accuracy for computerintegrated measurement and control instrumentation without the costs of overdesign. Within that context, this chapter provides the development of signal acquisition and conditioning circuits, and derives a unified method for representing and upgrading the quality of instrumentation signals between sensors and data-conversion systems....
Nội dung trích xuất từ tài liệu:
Multisensor thiết bị đo đạc thiết kế 6o (P4) 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)4LINEAR SIGNAL CONDITIONING TOSIX-SIGMA CONFIDENCE4-0 INTRODUCTIONEconomic considerations are imposing increased accountability on the design ofanalog I/O systems to provide performance at the required accuracy for computer-integrated measurement and control instrumentation without the costs of overde-sign. Within that context, this chapter provides the development of signal acquisi-tion and conditioning circuits, and derives a unified method for representing andupgrading the quality of instrumentation signals between sensors and data-conver-sion systems. Low-level signal conditioning is comprehensively developed for bothcoherent and random interference conditions employing sensor–amplifier–filterstructures for signal quality improvement presented in terms of detailed device andsystem error budgets. Examples for dc, sinusoidal, and harmonic signals are provid-ed, including grounding, shielding, and noise circuit considerations. A final sectionexplores the additional signal quality improvement available by averaging redun-dant signal conditioning channels, including reliability enhancement. A distinctionis made between signal conditioning, which is primarily concerned with operationsfor improving signal quality, and signal processing operations that assume signalquality already at the level of interest. An overall theme is the optimization of per-formance through the provision of methods for effective analog design.4-1 SIGNAL CONDITIONING INPUT CONSIDERATIONSThe designer of high-performance instrumentation systems has the responsibility ofdefining criteria for determining preferred options from among available alterna-tives. Figure 4-1 illustrates a cause-and-effect outline of comprehensive methodsthat are developed in this chapter, whose application aids the realization of effectivesignal conditioning circuits. In this fishbone chart, grouped system and device op- 7576 LINEAR SIGNAL CONDITIONING TO SIX-SIGMA CONFIDENCE FIGURE 4-1. Signal conditioning design influences.tions are outlined for contributing to the goal of minimum total instrumentation er-ror. Sensor choices appropriate for measurands of interest were introduced in Chap-ter 1, including linearization and calibration issues. Application-specific amplifierand filter choices for signal conditioning are defined, respectively, in Chapters 2and 3. In this section, input circuit noise, impedance, and grounding effects are de-scribed for signal conditioning optimization. The following section derives modelsthat combine device and system quantities in the evaluation and improvement ofsignal quality, expressed as total error, including the influence of random and co-herent interference. The remaining sections provide detailed examples of these sig-nal conditioning design methods. External interference entering low-level instrumentation circuits frequently issubstantial and techniques for its attenuation are essential. Noise coupled to signalcables and power buses has as its cause electric and magnetic field sources. For ex-ample, signal cables will couple 1 mV of interference per kilowatt of 60 Hz load foreach lineal foot of cable run of 1 ft spacing from adjacent power cables. Most inter-ference results from near-field sources, primarily electric fields, whereby an effec-tive attenuation mechanism is reflection by nonmagnetic materials such as copperor aluminum shielding. Both foil and braided shielded twinax signal cable offer at-tenuation on the order of –90 voltage dB to 60 Hz interference, which degrades byapproximately +20 dB per decade of increasing frequency. 4-1 SIGNAL CONDITIONING INPUT CONSIDERATIONS 77 For magnetic fields absorption is the effective attenuation mechanism requiringsteel or mu metal shielding. Magnetic fields are more difficult to shield than electricfields, where shielding effectiveness for a specific thickness diminishes with de-creasing frequency. For example, steel at 60 Hz provides interference attenuationon the order of –30 voltage dB per 100 mils of thickness. Applications requiringmagnetic shielding are usually implemented by the installation of signal cables insteel conduit of the necessary wall thickness. Additional magnetic field attenuationis furnished by periodic transposition of twisted-pair signal cable, provided no sig-nal returns are on the shield, where low-capacitance cabling is preferable. Mutualcoupling between computer ...