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Figure 3.10 In this analog signal isolator, the analog signal is converted into a pulse train of variable pulse width, which is then used to drive the optoisolator. At the other side of the optoisolator, the pulse train is demodulated to render the original signal. The large cylinder at the center of the board is an Optek OPI1264 optoisolator through which isolation levels of 10kV can be obtained. A similar circuit can be built using an integrated voltage-to-frequency converter. The circuit of...
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may chose to classify a failure mode that does not pose a risk to the patient or other surrounding targets as a pass. For example, the manufacturer may pass a device that fails to start operating or stops operating when exposed to EMI threats as long as these modes do not pose a threat to the patient. A common classification for device performance during testing is as follows Criteria level A normal performance within equipment specifications Criteria level B degradation or loss of function or...
Very Realistic Physiological Signal Sources
Figure 6.34 Rosell's method applied to measure electrode-skin impedances using carbon-loaded silicone electrodes 1, stimulating ring electrode 2 and 3, voltage-sensing electrodes. The electrodes are embedded at a depth of 1.5 mm within an isolating silicone rubber cast. Figure 6.34 Rosell's method applied to measure electrode-skin impedances using carbon-loaded silicone electrodes 1, stimulating ring electrode 2 and 3, voltage-sensing electrodes. The electrodes are embedded at a depth of 1.5 mm...
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Figure 4.34 RF filters are often needed on every line connected to the outside world. In this intracardiac electrogram signal input circuit, individual single-line chip filters are used to shunt RF signals to the isolated ground plane without affecting low-frequency signals. These filters have a nominal 3-dB cutoff frequency of 10MHz, yielding a minimum attenuation of 5 dB at 20 MHz, 25 dB at 100 MHz, and 30dB above 500 MHz. 4700-pF capacitors shunt RF signals that may be induced by...
Barebones Spectrum Analyzer
While an ac voltmeter can provide an indication of the field strength to which a probe is exposed, it does not provide any indication of the spectral contents of an emission. A spectrum analyzer is a tool that certainly cannot be beaten in the search for offending signals. Unfortunately, spectrum analyzers are often beyond the reach of a designer on a tight budget. For near-field sniffing, however, even the crudest spectrum analyzer will do a magnificent job. Figure 4.9 shows a simple...
Vlcd
Figure 3.25 An AAMI load converts currents into voltage signals that are amplified by an instrumentation amplifier. The RMS value of the amplified signal is converted by IC2 into a dc voltage that is measured by a 3 -digit DVM module. for frequencies under 1 kHz. The output of the AAMI load is amplified with a gain of 10 by instrumentation amplifier IC1. The RMS value of the amplified signal is then computed by IC2, a true-RMS-to-dc converter IC, and displayed by a 3 -digit DVM module. Note...
Analog Generation Of Arbitrary Waveforms
In many applications, repetitive sine, square, and triangle waves are seldom representative of the signals that the equipment under test is designed to process. For example, the heart's electrical signal is a waveform consisting of a complex mixture of these basic waveshapes intertwined with intermittent baseline segments. Since a constant live feed of such signals may be impractical or even dangerous for testing biomedical equipment, dedicated signal sources had to be developed to be capable...
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Since even a modest ESD event can develop a current of 10 A in 1 ns, the voltage induced across 1cm of PCB track will be as high as 100 V Similarly, high currents can flow through capacitances on and across circuit components. Current flowing through a capacitor is given by If an ESD event causes a change in potential of 1 kV within 1 ns, the current flowing through an unprotected input with 10-pF capacitance would be as high as 10 A. Testing for Immunity against ESD Two ESD testing techniques...
V Vvi
Figure 6.16 The linear power supply for the arb requires a 12 V ac input to produce 5 V for the logic circuitry, 5 V for the ECL logic of the high-speed DACs, and 12 V for the analog circuitry. The same circuit can be used for powering the DDS generator. Finally, a word of caution when building this arb. High-frequency clocks and signals demand proper layout techniques Prutchi, 1994 . Preferably, use a multiple-layer printed circuit board. Separate the analog ground from the digital ground and...
Sampling Rate And The Nyquist Theorem
Without doubt you have heard that according to the Nyquist theorem, a signal should be sampled at twice its frequency. Yes, and no. There is no data acquisition concept that is more quoted and less well understood than the Nyquist theorem. Let's explore what this theorem actually implies for proper data acquisition. Nyquist stated that any bandwidth-limited signal can be reconstructed from its digitized equivalent if the sample rate is at least twice the highest-frequency component. Signal...
Digital Generation Of Analog Waveforms
More complex, nonstandard, real-world stimuli waveforms can easily be created as a numerical array and played back through a digital-to-analog converter D A or DAC to yield analog waveforms of arbitrary complexity. This is the operating principle of a digital arbitrary waveform generator, or arb. Despite the simplicity of the concept, a PC program that would copy digital values stored in an array into a DAC would severely limit the maximum frequency of spectral components for the arbitrary...
D1
Figure 6.26 This bar graph display circuit monitors the intensity of bipolar current pulses delivered to the left ventricle. Figure 6.26 This bar graph display circuit monitors the intensity of bipolar current pulses delivered to the left ventricle. Figure 6.27 An external brick digital portions of the circuit. 15-V dc power supply is used to power the cardiac simulator. 5 V dc is generated onboard for the Figure 6.27 An external brick digital portions of the circuit. 15-V dc power supply is...
Standards For Protection Against Electrical Shock
It has been a long time now that medical electronic devices left the realm of experimentation and were transformed into irreplaceable tools of modern medicine. This widespread use of a very diverse variety of electronic devices compelled countries to impose regulations that ensure their efficacy and safety. In the United States, the Food and Drug Administration FDA is responsible for the regulation of medical devices. In the European Union EU , a series of directives establishes the...
Microphone Input
Figure 5.16 A 4018 Johnson counter can be made to convert a frequency-modulated pulse train into a quasi-sinusoidal tone for remote data acquisition. IC1 is wired to sequence resistors R1-R5 to produce a 10-step staircase waveform that approximates the shape of a sine wave. Since the square-to-sine converter divides the input frequency by 10, the input frequency range should be 10 times higher than the required sinusoidal tone audio range. In most instances, the powerful Fourier techniques used...
Arb Basics
As shown in Figure 6.9, an arbitrary waveform generator shares the basic building blocks of a DDS generator. Instead of a ROM sine lookup table, however, a full time-domain digital representation of the arbitrary waveform is downloaded into RAM. In addition, the counter is not thought of as a phase accumulator, and the means are provided to be able to define arbitrarily the last data point of the waveform cycle end address . In this way, the waveform can be replayed continuously by looping back...
Vout Comm
Figure 2.6 This bootstrapped design yields an ac-coupled differential amplifier that retains all of the superior performance inherent in decoupled instrumentation amplifiers. Ac voltages from the outputs of the ICIA's differential input stage are fed to the inverting inputs of their respective amplifiers via C3 and C4. This causes the ac voltage drop across R1 and R4 to be virtually zero. Ac current flow through resistors R1 and R4 is practically zero, while dc bias currents can flow freely to...
Preface
The medical devices industry is booming. Growth in the industry has not stopped despite globally fluctuating economies. The main reason for this success is probably the self-sustaining nature of health care. In essence, the same technology that makes it possible for people to live longer engenders the need for more health-care technologies to enhance the quality of an extended lifetime. It comes as no surprise, then, that the demand for trained medical-device designers has increased...
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buffers, which present an ultrahigh input impedance to an array of up to 32 electrodes. Each buffer in the array is implemented using a LinCMOS1 precision op-amp operated as a unity-gain voltage follower. An output signal has the same amplitude as that of its corresponding input. The output impedance is very low, however in the few kilohm range and can source or sink a maximum of 25 mA. As a result of this impedance transformation, the signal at the buffer's output can be transmitted over long...
Additional Protection
Regardless of how carefully you designed your instrument, absolute safety cannot be guaranteed in the real world. Despite all the safety testing and evaluation required by the FDA, medical device manufacturers still pay a premium for insurance to protect themselves from exposure against liability. For this reason, it often happens that additional or redundant hardware to ensure safety beyond the minimum requirements is cost-effective, since it will bring concomitant savings in insurance costs...
Linear Analog Isolation Using Optoisolators
A way of using an optoisolator for analog signals while maintaining good linearity is first to convert the analog signal into a pulse train of variable frequency or pulse width , which is then used to drive the optoisolator. At the other side of the optoisolator, the pulse train is demodulated to render the original signal. Another possibility is to place the optoisolator within a servo loop that makes use of the loop's error to convey a high-linearity analog signal. Yet another solution is to...
Bootstrapped Accoupled Biopotential Amplifier
Direct ac coupling of the instrumentation amplifier's inputs by way of RC high-pass filters across the inputs degrades the performance of the amplifier. This practice loads the input of the amplifier, which substantially lowers input impedance and degrades the CMRR of the differential amplifier. Although unity-gain input buffers can be used to present a high-input impedance to the biopotential source, any impedance mismatch in the ac coupling of these to an instrumentation amplifier stage...
Isolated Analog Multiplexer
The number of channels that can be acquired through the isolated A D can be expanded by using additional multiplexers. Figure 3.17 shows a PCB that implements a 64-channel Figure 3.17 The number of channels that can be acquired through the isolated A D can be expanded by using additional multiplexers. This circuit implements a 64-channel analog signal multiplexer with isolated control. Figure 3.17 The number of channels that can be acquired through the isolated A D can be expanded by using...
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safety hazard in the equipment is defective or a single external abnormal condition is present. These include interruption of the supply by opening the neutral conductor as well as interruption of the protective ground conductor. Patient leakage current between an F-type applied part and ground assumes that an external voltage equal to 110 of the maximum rated supply voltage is connected directly to the applied part. For battery-powered equipment, the external voltage that is assumed to be...
David Prutchi Michael Norris
A JOHN WILEY amp SONS, INC., PUBLICATION Copyright 2005 by John Wiley amp Sons, Inc. All rights reserved. Published by John Wiley amp Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either...
Standalone Analog Isolators
The previous design example demonstrated the use of the ISO107 isolation amplifier embedded within a circuit to provide a signal path across the isolation barrier. In many cases, having instead a self-contained general-purpose isolation module like that of Figure 3.4 can simplify the design of prototype and experimental equipment. The circuit diagram for such a module is shown in Figure 3.5. This module was designed as a stand-alone isolation board to protect subjects connected to isolated...
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TABLE 2.2 Approximate 3-dB Frequencies Required for the Reproduction of Physiological Signals with Negligible and Acceptable Levels of Distortion 3-dB Point Negligible Distortion 1 Acceptable Distortion 5 . , , , 0.0016 0.008 High-pass Hz lt -- lt - 3.1 1.4 Low-pass Hz lt -- lt - IC5 is powered via a Burr-Brown IS0107 isolation amplifier IC3, which generates isolated 12V when powered from 12V. L1, C6, C7, and C8 form a pi filter to clean the isolated 12-V power line generated by the IS0107 from...
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Corrected level recorded level antenna factor cable loss. The frequencies of interest were selected during a precompliance scan of the device in a shielded room. Figure 4.5 A prototype implantable-device programmer is being tested at an open-field test site. The device sits atop a motorized turntable. A biconical antenna is placed 10 m away from the device under test. Figure 4.5 A prototype implantable-device programmer is being tested at an open-field test site. The device sits atop a...
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Figure 1.11 Schematic diagram of a capacitive active bioelectrode. Biopotentials are coupled to buffer IC1A through resistor R1 and the capacitor formed by the biological tissues, aluminum oxide dielectric, and aluminum electrode plate. Operational amplifier IC1A is configured as a unity-gain buffer. IC1B drives a shield that protects the input from current leakage and noise. Resistors R3 and R2 reduce the gain of the shield driver to just under unity to improve the stability of the guarding...
References 1
Ackerman, B., High-Resolution Timing on a PC, Circuit Cellar INK, 24, 46-49, December 1991 January 1992. Association for the Advancement of Medical Instrumentation, Safe Current Limits for Electromedical Apparatus, ANSI AAMI Standard ES1, 1993. IEC-601-1, Medical Electrical Equipment Part 1 General Requirements for Safety, 1988 Amendments 1, 1991, and 2, 1995. IEC-601-1-1, Medical Electrical Equipment Part 1 General Requirements for Safety Section 1 Collateral Standard Safety Requirements for...
Singleended Biopotential Amplifier Arrays 15
Figure 1.13 A miniaturized version of the capacitive bioelectrode array may be assembled on a single flexible printed circuit. This assembly can be encapsulated and embedded at the appropriate position within the inner padding of a flight helmet for differential measurement of the EEG between positions Fp1 and Fp2 of the International 10-20 System. Conductive foam is used to establish non-active reference either at positions A1 and A2 or at the chin of the subject. Reprinted from Prutchi and...
Singleended Biopotential Amplifier Arrays
Single-ended op-amp amplifiers were in the past used as front-end stages for biopotential amplifiers. As we will see later, the advent of low-cost integrated instrumentation amplifiers has virtually eliminated the need to design single-ended biopotential amplifiers, and as such, the use of single-ended biopotential amplifiers is not recommended. Despite this, this section has strong educational value because it demonstrates the design principles of using single-ended amplifiers, which are...
Accoupled Instrumentation Biopotential Amplifier Front End
The circuit of Figure 2.5 embodies the classic implementation of a medium-impedance 10-MQ instrumentation biopotential amplifier based on the popular AD521 ICIA by Analog Devices. The gain of this circuit is adjustable between 10 and 1000 and maintains a CMR of at least 110 dB. This circuit offers superior dynamic performance with a minimal ac-coupled signal bandwidth -3 dB of 40kHz and low noise 1 Vp_p at G 10, 0.1-100 Hz . This circuit is an example of a biopotential amplifier front end...
Biopotential Amplifiers
In general, signals resulting from physiological activity have very small amplitudes and must therefore be amplified before their processing and display can be accomplished. The specifications and lists of characteristics of biopotential amplifiers can be as long and confusing as those for any other amplifier. However, for most typical medical applications, the most relevant amplifier characterizing parameters are the seven described below. 1. Gain. The signals resulting from...
Pasteless Biopotential Electrodes
Op-amp voltage followers are often used to buffer signals detected from biopotential sources with intrinsically high input impedance. One such application is detecting biopotential signals through capacitive bioelectrodes. One area in which these electrodes are particularly useful is in the measurement and analysis of biopotentials in humans subjected to conditions similar to those existing during flight. Knowledge regarding physiological reactions to flight maneuvers has resulted in the...
Pasteless Biopotential Electrodes 13
30 Hz. Unfortunately, standard anodization breaks down in the presence of saline e.g., from sweat , making the electrodes unreliable for long-term use. A relatively new anodization process was used by Lisa Sagi-Dolev, the former head of R amp D at the Israeli Airforce Aeromedical Center, and one of us Prutchi and Sagi-Dolev, 1993 to manufacture pasteless EEG electrodes that could be embedded in flight helmets. The hard anodization Super coating process developed by the Sanford Process...