Electromagnetic Compatibility And Medical Devices

Have you heard about the wheelchair that moved on its own every time a police car passed by? No, it's not part of a joke. This actually happened, and several people were seriously injured when radio signals from the two-way communications equipment on emergency vehicles and boats, CB, and amateur radios interfered with proper operation of the control circuitry of powered wheelchairs, sending some off curbs and piers. Similar reports of improper operation of apnea monitors, anesthetic gas monitors, and ECG and EEG monitors due to electromagnetic interference prompted government agencies to look carefully at these occurrences and establish regulations by which equipment must possess sufficient immunity to operate as intended in the presence of interference.

Complying with these regulations is not easy. The technologies involved in modern circuit design have considerably blurred the boundaries between the digital and analog worlds. Suddenly, multihundred megahertz and even gigahertz clocks became commonplace in high-performance digital circuits, making it necessary to consider every connection between components as an RF transmission line. At the same time that the need for higher performance pushes designers toward high-speed technology, the marketplace is demanding more compact, lighter, and less power-hungry devices. With smaller size, analog effects again enter into consideration, because as components and conductors come into close proximity, coupling between circuit sections becomes a real problem.

Obviously, self-interference within a circuit must be eliminated to make the product workable, but this still does not make the product marketworthy. This is because strict regulations concerning electromagnetic compatibility are now being enforced around the world in an effort to ensure that devices do not interfere with each other. In the United States the FCC regulates the testing and certification of all electronic devices that generate or use clock rates above 9kHz [Dash and Strauss, 1995]. In principle, the FCC's charter is to protect communications from unwanted electromagnetic interference (EMI). In the European Common Market, on the other hand, an electromagnetic compatibility (EMC) directive is now in effect, which not only establishes requirements against causing undue interference to radio and telecommunications equipment, but also institutes requirements

Design and Development of Medical Electronic Instrumentation By David Prutchi and Michael Norris ISBN 0-471-67623-3 Copyright © 2005 John Wiley & Sons, Inc. '

by which equipment must possess sufficient immunity to operate as intended in the presence of interference [Gubisch, 1995].

Regulatory bodies around the world have developed standards and regulations covering both emissions and immunity that designers must take very seriously. Failure to comply with EMI and EMC regulations can have a serious impact on everyone associated with a product, starting with the designer, through the manufacturer, the marketing and distribution network, and extending even to customers. The consequences of noncompliance include halting manufacturing and distribution, levying fines, and the publication of public notices of noncompliance to warn potential customers and other agencies. These considerations become especially important in the case of medical equipment, since it often involves sensitive electronics that can be affected adversely by electromagnetic interference, leading to potentially serious hazards to patients and health-care providers.

The European Community regulates emissions and immunity of medical devices through the EN-60601-1-2 standard (Medical Electrical Equipment—Part 1: General Requirements for Safety; Section 2: Collateral Standard: Electromagnetic Compatibility— Requirements and Tests) as well as the EN-55011 standard (Limits and Methods of Measurement of Radio Disturbance Characteristics of Industrial, Scientific and Medical Radio Frequency Equipment). In EN-60601-1-2, pass/fail criteria are defined by the manufacturer. As a result, the manufacturer may chose to classify a failure mode that does not pose a hazard to the patient as a "pass." In the United States, the FDA is adopting many of the IEC-60601-1-2 requirements but is imposing restrictions on a manufacturer's ability to adopt pass/fail criteria. The FDA prescribes that a passing result corresponds to maintaining clinical utility. In addition, there are discrepancies between the immunity levels recommended by European authorities and the FDA. Because of these differences in opinion, as well as because the standards are relatively new, changes occur often, and we advise engineers to keep updated on the latest versions.

Assuring compliance with the rules involves an extensive series of tests. The EMI and EMC standards enforced by the various regulatory agencies clearly define the construction of test sites as well as the test procedures to be followed. Even a fairly spartan facility capable of conducting these tests ends up costing over $100,000 just to set up, and for this reason, most companies hire an outside test lab at the rate of $1500 to $3000 per day to conduct testing. Considering how fast charges can accumulate during testing, it is obviously not a smart move simply to hire a test lab and wait for the results. Rather, designers should familiarize themselves with the relevant EMI and EMC standards and make sure that compliance requirements are considered at every stage in the design process.

In this chapter we present the major EMI/EMC requirements for medical devices, look at the theory of how circuits produce EMI, and describe some low-cost tools and methods that will allow you to identify and isolate the sources of EMI that inevitably make it into a circuit.

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