Biomedical Engineering Reference
In-Depth Information
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 of
curbs and piers. Similar reports of
improper operation of apnea monitors, anesthetic gas monitors, and ECG and EEG moni-
tors due to electromagnetic interference prompted government agencies to look carefully
at these occurrences and establish regulations by which equipment must possess su
ff
cient
immunity to operate as intended in the presence of interference.
Complying with these regulations is not easy. The technologies involved in modern cir-
cuit design have considerably blurred the boundaries between the digital and analog
worlds. Suddenly, multihundred megahertz and even gigahertz clocks became common-
place in high-performance digital circuits, making it necessary to consider every connec-
tion 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, ana-
log e
ects 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 reg-
ulations concerning electromagnetic compatibility are now being enforced around the
world in an e
ff
ort to ensure that devices do not interfere with each other. In the United
States the FCC regulates the testing and certi
ff
cation of all electronic devices that generate
or use clock rates above 9 kHz [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 e
fi
ect, which not only establishes requirements against causing undue
interference to radio and telecommunications equipment, but also institutes requirements
ff
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