- •Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation
- •Dedication
- •Preface
- •Reader Background
- •Rationale
- •Description of the Chapters
- •Features
- •The Author
- •Table of Contents
- •1.1 Introduction
- •1.2 Sources of Endogenous Bioelectric Signals
- •1.3 Nerve Action Potentials
- •1.4 Muscle Action Potentials
- •1.4.1 Introduction
- •1.4.2 The Origin of EMGs
- •1.5 The Electrocardiogram
- •1.5.1 Introduction
- •1.6 Other Biopotentials
- •1.6.1 Introduction
- •1.6.2 EEGs
- •1.6.3 Other Body Surface Potentials
- •1.7 Discussion
- •1.8 Electrical Properties of Bioelectrodes
- •1.9 Exogenous Bioelectric Signals
- •1.10 Chapter Summary
- •2.1 Introduction
- •2.2.1 Introduction
- •2.2.4 Schottky Diodes
- •2.3.1 Introduction
- •2.4.1 Introduction
- •2.5.1 Introduction
- •2.5.5 Broadbanding Strategies
- •2.6 Photons, Photodiodes, Photoconductors, LEDs, and Laser Diodes
- •2.6.1 Introduction
- •2.6.2 PIN Photodiodes
- •2.6.3 Avalanche Photodiodes
- •2.6.4 Signal Conditioning Circuits for Photodiodes
- •2.6.5 Photoconductors
- •2.6.6 LEDs
- •2.6.7 Laser Diodes
- •2.7 Chapter Summary
- •Home Problems
- •3.1 Introduction
- •3.2 DA Circuit Architecture
- •3.4 CM and DM Gain of Simple DA Stages at High Frequencies
- •3.4.1 Introduction
- •3.5 Input Resistance of Simple Transistor DAs
- •3.7 How Op Amps Can Be Used To Make DAs for Medical Applications
- •3.7.1 Introduction
- •3.8 Chapter Summary
- •Home Problems
- •4.1 Introduction
- •4.3 Some Effects of Negative Voltage Feedback
- •4.3.1 Reduction of Output Resistance
- •4.3.2 Reduction of Total Harmonic Distortion
- •4.3.4 Decrease in Gain Sensitivity
- •4.4 Effects of Negative Current Feedback
- •4.5 Positive Voltage Feedback
- •4.5.1 Introduction
- •4.6 Chapter Summary
- •Home Problems
- •5.1 Introduction
- •5.2.1 Introduction
- •5.2.2 Bode Plots
- •5.5.1 Introduction
- •5.5.3 The Wien Bridge Oscillator
- •5.6 Chapter Summary
- •Home Problems
- •6.1 Ideal Op Amps
- •6.1.1 Introduction
- •6.1.2 Properties of Ideal OP Amps
- •6.1.3 Some Examples of OP Amp Circuits Analyzed Using IOAs
- •6.2 Practical Op Amps
- •6.2.1 Introduction
- •6.2.2 Functional Categories of Real Op Amps
- •6.3.1 The GBWP of an Inverting Summer
- •6.4.3 Limitations of CFOAs
- •6.5 Voltage Comparators
- •6.5.1 Introduction
- •6.5.2. Applications of Voltage Comparators
- •6.5.3 Discussion
- •6.6 Some Applications of Op Amps in Biomedicine
- •6.6.1 Introduction
- •6.6.2 Analog Integrators and Differentiators
- •6.7 Chapter Summary
- •Home Problems
- •7.1 Introduction
- •7.2 Types of Analog Active Filters
- •7.2.1 Introduction
- •7.2.3 Biquad Active Filters
- •7.2.4 Generalized Impedance Converter AFs
- •7.3 Electronically Tunable AFs
- •7.3.1 Introduction
- •7.3.3 Use of Digitally Controlled Potentiometers To Tune a Sallen and Key LPF
- •7.5 Chapter Summary
- •7.5.1 Active Filters
- •7.5.2 Choice of AF Components
- •Home Problems
- •8.1 Introduction
- •8.2 Instrumentation Amps
- •8.3 Medical Isolation Amps
- •8.3.1 Introduction
- •8.3.3 A Prototype Magnetic IsoA
- •8.4.1 Introduction
- •8.6 Chapter Summary
- •9.1 Introduction
- •9.2 Descriptors of Random Noise in Biomedical Measurement Systems
- •9.2.1 Introduction
- •9.2.2 The Probability Density Function
- •9.2.3 The Power Density Spectrum
- •9.2.4 Sources of Random Noise in Signal Conditioning Systems
- •9.2.4.1 Noise from Resistors
- •9.2.4.3 Noise in JFETs
- •9.2.4.4 Noise in BJTs
- •9.3 Propagation of Noise through LTI Filters
- •9.4.2 Spot Noise Factor and Figure
- •9.5.1 Introduction
- •9.6.1 Introduction
- •9.7 Effect of Feedback on Noise
- •9.7.1 Introduction
- •9.8.1 Introduction
- •9.8.2 Calculation of the Minimum Resolvable AC Input Voltage to a Noisy Op Amp
- •9.8.5.1 Introduction
- •9.8.5.2 Bridge Sensitivity Calculations
- •9.8.7.1 Introduction
- •9.8.7.2 Analysis of SNR Improvement by Averaging
- •9.8.7.3 Discussion
- •9.10.1 Introduction
- •9.11 Chapter Summary
- •Home Problems
- •10.1 Introduction
- •10.2 Aliasing and the Sampling Theorem
- •10.2.1 Introduction
- •10.2.2 The Sampling Theorem
- •10.3 Digital-to-Analog Converters (DACs)
- •10.3.1 Introduction
- •10.3.2 DAC Designs
- •10.3.3 Static and Dynamic Characteristics of DACs
- •10.4 Hold Circuits
- •10.5 Analog-to-Digital Converters (ADCs)
- •10.5.1 Introduction
- •10.5.2 The Tracking (Servo) ADC
- •10.5.3 The Successive Approximation ADC
- •10.5.4 Integrating Converters
- •10.5.5 Flash Converters
- •10.6 Quantization Noise
- •10.7 Chapter Summary
- •Home Problems
- •11.1 Introduction
- •11.2 Modulation of a Sinusoidal Carrier Viewed in the Frequency Domain
- •11.3 Implementation of AM
- •11.3.1 Introduction
- •11.3.2 Some Amplitude Modulation Circuits
- •11.4 Generation of Phase and Frequency Modulation
- •11.4.1 Introduction
- •11.4.3 Integral Pulse Frequency Modulation as a Means of Frequency Modulation
- •11.5 Demodulation of Modulated Sinusoidal Carriers
- •11.5.1 Introduction
- •11.5.2 Detection of AM
- •11.5.3 Detection of FM Signals
- •11.5.4 Demodulation of DSBSCM Signals
- •11.6 Modulation and Demodulation of Digital Carriers
- •11.6.1 Introduction
- •11.6.2 Delta Modulation
- •11.7 Chapter Summary
- •Home Problems
- •12.1 Introduction
- •12.2.1 Introduction
- •12.2.2 The Analog Multiplier/LPF PSR
- •12.2.3 The Switched Op Amp PSR
- •12.2.4 The Chopper PSR
- •12.2.5 The Balanced Diode Bridge PSR
- •12.3 Phase Detectors
- •12.3.1 Introduction
- •12.3.2 The Analog Multiplier Phase Detector
- •12.3.3 Digital Phase Detectors
- •12.4 Voltage and Current-Controlled Oscillators
- •12.4.1 Introduction
- •12.4.2 An Analog VCO
- •12.4.3 Switched Integrating Capacitor VCOs
- •12.4.6 Summary
- •12.5 Phase-Locked Loops
- •12.5.1 Introduction
- •12.5.2 PLL Components
- •12.5.3 PLL Applications in Biomedicine
- •12.5.4 Discussion
- •12.6 True RMS Converters
- •12.6.1 Introduction
- •12.6.2 True RMS Circuits
- •12.7 IC Thermometers
- •12.7.1 Introduction
- •12.7.2 IC Temperature Transducers
- •12.8 Instrumentation Systems
- •12.8.1 Introduction
- •12.8.5 Respiratory Acoustic Impedance Measurement System
- •12.9 Chapter Summary
- •References
Instrumentation and Medical Isolation Amplifiers |
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8.5Medical-Grade Power Supplies
The only practical way to achieve extreme galvanic isolation and still couple significant power from the mains to a low-voltage regulated dc supply that powers a medical amplifier is by toroidal transformer. The transformer can operate at line frequency or, for lighter weight and improved efficiency, can operate at frequencies in the 10to 100-kHz range. A power oscillator must be used to drive the transformer’s primary in the latter case. If a highfrequency oscillator is used, the transformer can be made smaller and lighter (a smaller core and fewer turns on the windings are required) and the output filter capacitor can be smaller and lighter.
A toroidal transformer is used for efficiency in magnetic coupling the primary to secondary windings; the toroidal core allows the primary and secondary to be separate physically on the core to minimize capacitive coupling between them. By physically separating the windings and insulating them from the core with special insulation, breakdown voltages between primary and secondary can be made 8 kV or greater and primary-to-secondary dc resistance can be on the order of 1012 Ω and greater. The secondary ac voltage is rectified by the usual means; the raw dc is low-pass filtered by a capacitor, and then regulated by a feedback regulator. The net result is a dc supply with guaranteed extreme galvanic isolation between the input power lines (high, low, and ground) and the output dc terminals (ground and VCC).
Many power supply manufacturers make medical grade units; for example, GlobTek Inc. (www.globtek.com) offers a broad line of medical-grade, standalone power supplies including wall plug-in “bricks” and open-frame units. Their class II double insulated units withstand 4 kV ac or 5.65 kV dc input–output potential and have less than 100-μA leakage.
8.6Chapter Summary
This chapter described the properties and uses of instrumentation amplifier ICs, including those that can be made from low-noise op amps. Requirements for and properties of medical isolation amplifiers were also examined. Galvanic isolation is required for patient safety in all settings (surgical, clinical, outpatient). Power supply isolation is achieved by batteries or special trans- former-coupled power supplies. Isolation amplifier analog output must also be isolated. One way this can be done is by DSBSC modulation of a highfrequency ac carrier by the physiological signal, and coupling the modulated carrier to the output with a transformer, where the signal is recovered with a demodulator.
© 2004 by CRC Press LLC
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Analysis and Application of Analog Electronic Circuits |
Another way of coupling the physiological signal to the output with isolation is to use a photo-optic coupler and transmit the signal directly in analog form (see Section 6.6.4 of Chapter 6). If the isolated physiological signal is sampled and converted by a serial output analog-to-digital converter, the digital output can also be transmitted through a digital photo-optic coupler to the output. Still another way of coupling an isolated signal to the output was by converting the signal to a pulse-width modulated (PWM) TTL square wave and coupling this wave to the output with two 1-pF capacitors. At the output, this wave is highly differentiated into a spike train. Flip-flops recover the PWM wave at the output and the signal is demodulated by averaging.
The physiological effects of electroshock were described, including induced cardiac fibrillation, apnea, and burns. Certification agencies for medical electronic devices worldwide were listed and some of their standards compared. The U.S. was seen to have five major certifying agencies; Europe had two. A trend in the U.S. is to converge on the IEC standards. To meet European marketing standards for medical electronic devices, U.S. manufacturers must meet IEC and CENELEC standards that, paradoxically in some areas, are less stringent than NFPA and other U.S. standards.
Finally, medical-grade power supplies that run off the mains were discussed.
© 2004 by CRC Press LLC
9
Noise and the Design of Low-Noise Amplifiers for Biomedical Applications
9.1Introduction
The noise considered in this chapter provides one limitation to the precision of biomedical measurements. Other factors that limit resolution are the distortion caused by signal conditioning system nonlinearity and quantization in the analog-to-digital conversion (ADC) of input signals. The degree to which a biosignal is resolvable can be determined by the signal-to-noise ratio at the output of the signal conditioning system. The noise in this chapter is considered to arise in a circuit, measurement system, or electrode from completely random processes. Although any physical quantity can be "noisy,” this chapter will generally consider only completely random noise voltages and currents; these will be defined as being stationary (the physical process giving rise to the noise does not change with time) and having zero mean (zero additive dc components). An unwanted dc component accompanying a noise source in practice can be from an electrochemical EMF arising in a recording electrode or from amplifier dc offset voltage or bias current. The reduction of unwanted dc components will not be treated here.
Coherent interference (CI) can also be present at the output of biomedical signal conditioning systems. As its name suggests, CI generally has its origins in periodic, manmade phenomena, such as power line-frequency electric and magnetic fields; radio-frequency sources such as radio and television broadcast antennas; certain poorly shielded computer equipment; spark discharge phenomena such as automotive ignitions and electric motor brushes and commutators; and inductive switching transients generated by SCR motor speed controls, etc. It is well known that minimization of CI is often “arty” and may involve changing the grounding circuits for a system; shielding with magnetic and/or electric conducting materials; filtering; using isolation transformers, etc. (Northrop, 1997, Section 3.9). In this chapter, however, attention will be focused on purely random, stationary, incoherent noise.
Minimizing the impact of incoherent noise in a measurement system often involves a prudent choice of low-noise amplifiers and components, certain
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© 2004 by CRC Press LLC