DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

148 ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICESby which equipment must possess sufficient immunity to operate as intended in the presenceof interference [Gubisch, 1995].Regulatory bodies around the world have developed standards and regulations coveringboth emissions and immunity that designers must take very seriously. Failure to complywith EMI and EMC regulations can have a serious impact on everyone associated with aproduct, starting with the designer, through the manufacturer, the marketing and distributionnetwork, and extending even to customers. The consequences of noncomplianceinclude halting manufacturing and distribution, levying fines, and the publication of publicnotices of noncompliance to warn potential customers and other agencies. These considerationsbecome especially important in the case of medical equipment, since it ofteninvolves 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 devicesthrough the EN-60601-1-2 standard (Medical Electrical Equipment—Part 1: GeneralRequirements for Safety; Section 2: Collateral Standard: Electromagnetic Compatibility—Requirements and Tests) as well as the EN-55011 standard (Limits and Methods of Measurementof Radio Disturbance Characteristics of Industrial, Scientific and Medical RadioFrequency 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 posea hazard to the patient as a “pass.” In the United States, the FDA is adopting many of theIEC-60601-1-2 requirements but is imposing restrictions on a manufacturer’s ability toadopt pass/fail criteria. The FDA prescribes that a passing result corresponds to maintainingclinical utility. In addition, there are discrepancies between the immunity levels recommendedby 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 adviseengineers to keep updated on the latest versions.Assuring compliance with the rules involves an extensive series of tests. The EMI andEMC standards enforced by the various regulatory agencies clearly define the constructionof test sites as well as the test procedures to be followed. Even a fairly spartan facilitycapable of conducting these tests ends up costing over $100,000 just to set up, and for thisreason, most companies hire an outside test lab at the rate of $1500 to $3000 per day toconduct testing. Considering how fast charges can accumulate during testing, it is obviouslynot a smart move simply to hire a test lab and wait for the results. Rather, designersshould familiarize themselves with the relevant EMI and EMC standards and make surethat compliance requirements are considered at every stage in the design process.In this chapter we present the major EMI/EMC requirements for medical devices, lookat the theory of how circuits produce EMI, and describe some low-cost tools and methodsthat will allow you to identify and isolate the sources of EMI that inevitably make it intoa circuit.EMISSIONS FROM MEDICAL DEVICESThe FCC’s main concern with RF emissions from electronic devices is possible interferencewith communications devices such as commercial radio and TV receivers. From thepoint of view of agencies regulating medical devices (in the United States the FDA), theconcern about unintentional electromagnetic emissions extends to the way in which theycould interfere with diagnostic or therapeutic medical devices. Note the word unintentional,since these standards do not apply directly to medical devices that intentionallygenerate electromagnetic signals (e.g., telemetry ECG transmitters, electrosurgery equipment,magnetic resonance imagers) which require special emissions that limit exemptionsat specifically allocated frequency bands.