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Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation - Northrop.pdf
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Preface

Reader Background

This text is intended for use in a classroom course on analysis and application of analog electronic circuits in biomedical engineering taken by junior or senior undergraduate students specializing in biomedical engineering. It will also serve as a reference book for biophysics and medical students interested in the topics. Readers are assumed to have had introductory core courses up to the junior level in engineering mathematics, including complex algebra, calculus, and introductory differential equations. They also should have taken an introductory course in electronic circuits and devices. As a result of taking these courses, readers should be familiar with systems block diagrams and the concepts of frequency response and transfer functions; they should be able to solve simple linear ordinary differential equations and perform basic manipulations in linear algebra. It is also important to have an understanding of the working principles of the various basic solid-state devices (diodes, bipolar junction transistors, and field-effect transistors) used in electronic circuits in biomedical applications.

Rationale

The interdisciplinary field of biomedical engineering is demanding in that it requires its followers to know and master not only certain engineering skills (electronics, materials, mechanical, photonic), but also a diversity of material in the biological sciences (anatomy, biochemistry, molecular biology, genomics, physiology, etc.). This text was written to aid undergraduate biomedical engineering students by helping them to understand the basic analog electronic circuits used in signal conditioning in biomedical instrumentation. Because many bioelectric signals are in the microvolt range, noise from electrodes, amplifiers, and the environment is often significant compared to the signal level. This text introduces the basic mathematical tools used to describe noise and how it propagates through linear systems. It also describes at a basic level how signal-to-noise ratio can be improved by signal averaging and linear filtering.

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© 2004 by CRC Press LLC

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Analysis and Application of Analog Electronic Circuits

Bandwidths associated with endogenous (natural) biomedical signals range from dc (e.g., hormone concentrations or dc potentials on the body surface) to hundreds of kilohertz (bat ultrasound). Exogenous signals associated with certain noninvasive imaging modalities (e.g., ultrasound, MRI) can reach into the tens of megahertz. Throughout the text, op amps are shown to be the keystone of modern analog signal conditioning system design. This text illustrates how op amps can be used to build instrumentation amplifiers, isolation amplifiers, active filters, and many other systems and subsystems used in biomedical instrumentation.

The text was written based on the author’s experience in teaching courses in electronic devices and circuits, electronic circuits and applications, and biomedical instrumentation for over 35 years in the electrical and computer engineering department at the University of Connecticut, as well as on his personal research in biomedical instrumentation.

Description of the Chapters

Analysis and Application of Analog Electronic Circuits in Biomedical Engineering is organized into 12 chapters, an index, and a reference section. Extensive examples in the chapters are based on electronic circuit problems in biomedical engineering.

In Chapter 1, Sources and Properties of Biomedical Signals, the sources of bioelectric phenomena in nerves and muscles are described. The general characteristics of biomedical signals are set forth and we examine the general properties of physiological systems, including nonlinearity and nonstationarity.

In Chapter 2, Models for Semiconductor Devices Used in Analog Electronic Systems, we describe the midand high-frequency models used for analysis of pn junction diodes, BJTs, and FETs in electronic circuits. The high-frequency behavior of basic oneand two-transistor amplifiers is treated and the Miller effect is introduced. This chapter also describes the properties of photodiodes, photoconductors, LEDs, and laser diodes.

In Chapter 3, The Differential Amplifier, this important analog electronic circuit architecture is analyzed for BJT and FET DAs. Midand highfrequency behavior is treated, as well as the factors that lead to a desirable high common-mode rejection ratio. DAs are shown to be essential subcircuits in all op amps, comparators, and instrumentation amplifiers.

© 2004 by CRC Press LLC

Preface

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In Chapter 4, General Properties of Electronic Single-Loop Feedback Systems, we introduce the four basic kinds of electronic feedback (positive/ negative voltage feedback and positive/negative current feedback) and describe how they affect linear amplifier performance.

Chapter 5, Feedback, Frequency Response, and Amplifier Stability, presents Bode plots and the root-locus technique as design tools and means of predicting closed-loop system stability. The effects of negative voltage and current feedback, as well as positive voltage feedback, on an amplifier’s gain and bandwidth, and input and output impedance are described. The design of certain “linear” oscillators is treated.

In Chapter 6, Operational Amplifiers, we examine the properties of the ideal op amp and how its model can be used in quick pencil-and- paper circuit analysis of various op amp circuits. Circuit models for various types of practical op amps are described, including current feedback op amps. Gain-bandwidth products are shown to differ for different op amp types and circuits. Analog voltage comparators are introduced and practical circuit examples are given. The final subsection illustrates some applications of op amps in biomedical instrumentation.

In Chapter 7, Analog Active Filters, we illustrate three major architectures easily used to design for op amp-based active filters. These include the Sallen and Key quadratic AF, the oneand two-loop biquad AF, and the GIC-based AF. Voltage and digitally tunable AF designs are described and examples are given; AF applications are discussed.

In Chapter 8, Instrumentation and Medical Isolation Amplifiers, we describe the general properties of instrumentation amplifiers (IAs) and some of the circuit architectures used in their design. Medical isolation amplifiers (MIAs) are shown to be necessary to protect patients from electrical shock hazard during bioelectric measurements. All MIAs provide extreme galvanic isolation between the patient and the monitoring station. We illustrate several MIA architectures, including a novel direct sensing system that uses the giant magnetoresistive effect. Also described are the current safety standards for MIAs.

In Chapter 9, Noise and the Design of Low-Noise Amplifiers for Biomedical Applications, descriptors of random noise, such as the probability density function; the autoand cross-correlation functions; and the autoand cross-power density spectra, are introduced and their properties discussed. Sources of random noise in active and passive components are presented and we show how noise propagates statistically through LTI filters. Noise factor, noise figure, and signal- to-noise ratio are shown to be useful measures of a signal conditioning system’s noisiness. Noise in cascaded amplifier stages, DAs, and feedback amplifiers is treated. Examples of noise-limited signal

© 2004 by CRC Press LLC

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Analysis and Application of Analog Electronic Circuits

resolution calculations are given. Factors affecting the design of lownoise amplifiers and a list of low-noise amplifiers are presented.

Digital Interfaces, Chapter 10, details these particular interfaces, as well as derivation of aliasing and the sampling theorem. Analog-to-dig- ital and digital-to-analog converters are described. Hold circuits and quantization noise are also treated.

In Chapter 11, Modulation and Demodulation of Bioelectric Signals, we illustrate the basics of modulation schemes used in instrumentation and biotelemetry systems. Analysis is conducted on AM; singlesideband AM (SSBAM); double-sideband suppressed carrier (DSBSC) AM; angle modulation including phase and frequency modulation (FM); narrow-band FM; delta modulation; and integral pulse frequency modulation (IPFM) systems, as well as on means for their demodulation.

In Chapter 12, Examples of Special Analog Circuits and Systems in Biomedical Instrumentation, we describe and analyze circuits and systems important in biomedical and other branches of instrumentation. These include the phase-sensitive rectifier; phase detector circuits; voltageand current-controlled oscillators, including VFCs and VPCs, phase-locked loops, and applications; true RMS converters; IC thermometers; and four examples of complex measurement systems developed by the author.

In addition, the comprehensive references at the end of the book contain entries from periodicals, the World Wide Web, and additional texts.

Features

Some of the unique contents of this text are:

Section 2.6 in Chapter 2 describes the properties of photonic sensors and emitters, including PIN and avalanche photodiodes, and photoconductors. Signal conditioning circuits for these sensors are given and analyzed. This section also describes the properties of LEDs and laser diodes, as well as the circuits required to power them.

Chapter 8 gives a thorough treatment of the design of instrumentation amplifiers and medical isolation amplifiers. Also described in detail are current safety standards for MIAs.

A comprehensive treatment of noise in analog signal conditioning systems is given in Chapter 9.

©2004 by CRC Press LLC

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Chapter 10 on digital interfaces examines the designs of many types of ADCs and DACs and introduces aliasing and quantization noise as possible costs for going to or from analog or digital domains.

Chapter 11 illustrates the use of phase-locked loops to generate or demodulate angle-modulated signals, including phase and frequency modulation as well as AM and DSBSCM signals.

Chapter 12 describes an applications-oriented collection of analog circuit “building blocks,” including: phase-sensitive rectifiers; phase detectors; phase-locked loops; VCOs and ICOs, including VFCs and VPCs; true RMS converters; IC thermometers; and examples of complex biomedical instrument systems designed by the author that use op amps extensively.

Many illustrative examples from medical electronics are given in the chapters.

Home problems that accompany each chapter (except Chapter 1, Chapter 8, and Chapter 12) stress biomedical electronic applications.

Robert B. Northrop

Chaplin, Connecticut

© 2004 by CRC Press LLC