EMI: BUILDING DEVICE COMPATIBILITY FROM THE GROUND UP
The proliferation of electronic equipment in health-care environments has led to a surge in EMI-related incidents. Regulatory agencies around the world are taking a cue from Europe and requiring that electromagnetic compatibility controls be built into electronic medical devices.Gabriel Spera
Our understanding of electromagnetic interference has grown considerably in recent years, and EMI is now recognized as a dangerous potential source of medical device failure. Performance degradation relating to EMI has resulted in inappropriate treatment, serious injury, and even patient death. The problem is growing more severe as increasing numbers of electronic devices operate in close proximity to each other, often with unforeseen consequences. The proliferation of portable and mobile communications devices contributes to the confusion of the electromagnetic environment. Add to that the growing number of electronic devices used in private homes, and the need to control EMI becomes obvious.
Harmony and Compatibility
Regulatory agencies across the world have begun to require that EMC controls be built into the design of electronic medical devices, and Europe is clearly leading the way. The EMC Directive (89/336/ EEC) and its amendments became mandatory in January 1996. The directive says, in a nutshell, that any apparatus placed on the market must neither cause excessive electromagnetic disturbances nor be unduly susceptible to them.
The EMC Directive is harmonized with the Medical Devices Directive (93/42/EEC), which means that devices that comply with the MDD do not have to be certified additionally to the EMC Directive. While the MDD is in its transitional period, manufacturers have a few options. "It is the European Commission's view that until 13 June 1998 [after which the MDD becomes mandatory], manufacturers of medical devices may comply with either the MDD or the EMC Directive, but that they must comply with one of the two," explains Nigel Harvey of SGS, a UK notified body under the MDD and a competent body under the EMC Directive. However, he adds, this situation is not the same in all member states. "The UK, for example, offers manufacturers the option of complying with neither until 13 June. This clearly results in a nonlevel playing field within the EU." According to Harvey, the MDD includes EMC within its scope, and both directives reference EN 60601-1-2:1993, a product-family standard for EMC that recommends tests for RFI, ESD, electrical fast transients, and surges. "By complying with this standard, one can comply with the EMC Directive and with the parts of the MDD [that relate to EMC] at the same time." After 13 June 1998 electromedical devices need comply only with the MDD, since the EMC Directive will no longer apply. (The Commission recently released its new guide to the EMC Directive, which spells out requirements and exceptions for medical equipment.)
Dag Bjorklof of SEMKO Medtech, a Swedish notified body for the MDD, points out that "all electronics are more or less sensitive to the electromagnetic environment. The extent of susceptibility depends on the type of IC technology used, layout, cabling, shielding, and so forth." Tim Williams of Elmac Services, an EMC consultancy in Chichester, UK, identifies two particular problem functions. The first group comprises "microprocessor control and indication circuits, which are especially sensitive to transient effects such as electrostatic discharge and electrical switching." These can occur because the microprocessor is a state machine that can be diverted to a rogue state by an incoming disturbance, Williams explains. Consequently, it can crash or behave unpredictably. The second group comprises low-level analogue amplifying circuits, especially for patient-coupled sensors that must amplify extremely weak signals in the presence of considerable environmental noise. "Designers have historically found ways to deal with in-band noise (otherwise their products simply would not work), but nowadays products are being affected by RF transmitters that are out-of-band and whose effects are ignored by the designer," Williams says.
Specific devices with a history of EMI problems include blood warmers, defibrillators, fetal monitors, and ventilators. Typical incidents have involved apnea monitors that erroneously detected respiration, wheelchairs that spontaneously moved near emergency vehicles, external defibrillators that stopped pacing in ambulances, ventilators that shut down or signaled an alarm, infusion pumps that changed infusion rate near cellular phones, fetal-heart monitors that gave incorrect readings, and oximeters that displayed faulty readings or spontaneously restarted with a loss of set parameters.
Start at the Beginning
"The only way to ensure disturbance-free function with minimal costs is to take EMC into consideration from the beginning," says Werner Hirschi, director of EMC Fribourg, an EMC consultancy in Fribourg, Switzerland. Williams agrees, adding, "Retrospective fixes are inefficient, cumbersome, and a sure way to commercial oblivion. Experience with the EMC Directive has shown that it is often more cost-effective to redesign and relaunch a volume product to meet EMC standards than it is to 'bolt on' fixes to an existing design."
Simply understanding the nature of EMI can help designers meet their EMC targets. Basically, EMI is conducted or radiated (or both) from the emitting device to the receiver device. Radiated interference manifests itself as an electromagnetic wave propagating through the air or other nonconducting media; conducted interference travels along power or signal wires. In general, lower frequencies will be conducted, and higher frequencies will be radiated. The strength of an RF field falls off in inverse proportion to the distance from the radiator.
Designers can start by identifying possible sources of interference and determining, as much as possible, whether they will be present near the device. Problems can be rectified by eliminating the contamination source, interrupting the interference path, or preventing reception. Removing or minimizing the source of interference is, of course, the most effective procedure. The threat posed by mobile telephones is so great, Bjorklof points out, that they "are often forbidden in hospitals and other sensitive locations." According to Williams, walkie-talkies and diathermy machines also emit potentially strong RF fields, as do fixed transmitters, such as hospital pager base stations. Electrostatic discharge, he says, is a serious threat in dry environments and locations where a lot of synthetic material is used, such as hospital wards with nylon bed sheets. Relays, contacts, motors, and fluorescent lighting, says Hirschi, can also give rise to transients.
Designing for Success
Hirschi stresses the importance of developing cable interfaces at the PCB level. "This has to be developed from the beginning of design and is strongly linked to the placement of the connectors on the PCB. The environment of the PCB also has to be taken into account." EMC controls, properly implemented early on, can simplify product design and reduce overall cost. For example, Hirschi says, a well-designed PCB generally allows designers to avoid using the equipment box for supplementary shielding, making plastic boxes an option. According to Bjorklof, careful design includes the use of four-layer circuit boards, protective components, and planning for the electromagnetic topology.
Proper EMC design, according to Williams, should include partitions to separate critical and noncritical sections. Grounding and PCB layout, he says, go hand in hand, and the careful placement of ground connections goes a long way toward reducing the noise voltages that are developed across ground impedance. Minimizing the value of the ground impedance itself is another approach, and a ground plane can be the easiest way to achieve a good low-impedance ground in medical electronics. Ungrounded tracks and component leads will act as antennae, radiating RF energy as a function of their length; before beginning layout, designers should identify the most critical tracks and ensure that there are no breaks in the ground-plane layer beneath them.
Dealing with common-mode interference currents at equipment interfaces is vital to achieving EMC. Unshielded I/O lines, Williams says, should be decoupled to the interface ground reference at the point at which they enter or leave the enclosure. A properly installed mains filter--preferably one with good high-frequency performance--is also essential for systems using a switched-mode power supply. Signal cables can be screened or filtered in designs that employ an interface ground; pigtails should be avoided at all costs. Designers can also try filtering at the circuit level. Good techniques include tailoring feedback networks for the minimum necessary bandwidth, installing in-line RC low-pass filters (especially at inputs), and adding low-value capacitors directly across the input pins of high-gain circuits. Lastly, systems should be encased in conductive enclosures or screened boxes with conductive gaskets or spring fingers on all seams.
Time Is Running Out
Although EMC testing is not in itself a lengthy task, the process of identifying and correcting problems certainly can be, and manufacturers are running out of time. "We can expect that there will be a significant number of devices, on the market now, that will not be in full compliance with the MDD by next June," says Harvey. In such circumstances, manufacturers will be faced with three stark choices. First, they can carry on selling, but without affixing the CE mark. "They are committing an offense," says Harvey, "but are not making any false claims about the product." Or, they can illegally affix the CE mark and continue selling. "This is a more serious offense, given that they are making false claims about the product," adds Harvey. Finally, they can stop selling until they are in compliance. "No offenses are committed," Harvey says, "but there are obvious commercial implications." Given the rather frightening alternatives, clearly the wisest course of action for manufacturers is to ensure that their devices comply fully with the relevant standards.
