Biomedical Engineering Reference
In-Depth Information
provides a low-reluctance magnetic path for the interference
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field. The idea is to make the
shield attract
field away from the sensitive com-
ponent. The magnetic sheets mentioned above are a good start for solving these problems.
If nonmagnetic shields are needed (e.g., to form a shield close to a CRT or a magnetic sen-
sor), you may try one of the shielding alloys produced by Magnetic Shield Corporation.
Magnetic Shield sells a $150 engineering kit that includes various 10 in.
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flux lines to itself and divert the magnetic
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15 in. sheets as
well as some braided sleeving made of their CO-NETIC and NETIC alloys. The kit even
includes an ac magnetic
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field probe that can be used with a DVM or oscilloscope to meas-
ure magnetic
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fields from 10 Hz to 3 kHz.
Conductive Spray Paint
Many medical devices are not built with metallic enclosures. If
extensive shielding of the case becomes necessary, an alternative to changing the design to
use a conductive enclosure is to spray-paint the enclosure using conductive paint. EMI
spray paints are available to provide varying degrees of EMI shielding, all the way from
light, graphite-based paints to provide mild shielding against ESD through nickel/chrome-
loaded sprays that can divert strong magnetic
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fields away from sensitive components.
Shielding Components
If you got this far down the list of quick patches, chances are that
you may need to go back to the drawing board (or more likely, the PCB layout station).
Before going back home, though, you may try to shield individual components. You could
apply one of various available conductive foils and tapes directly to PCBs (to shield tracks)
or to components. There are even precut conductive cardboard boxes that can be used to
shield entire sections of a circuit. However, if you need to build a complete village of pro-
tective housings on your PCB, it may be worthwhile biting the bullet and going back to the
lab to reengineer the product.
CONCLUDING REMARKS
Designing medical equipment that can pass EMI/EMC compliance testing without
xes or
delays never happens by mistake. Rather, it involves considering compliance with EMI and
EMC regulations from the very beginning of product formulation and design. A head start
in the battle against EMI can be obtained by developing a
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first prototype free of foresee-
able trouble. This is possible by carefully selecting the technologies that ful
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ll the product
requirements while minimizing EMI, observing good design and construction practices,
and making extensive use of circuit simulation tools. Near-
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eld probing of the
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rst proto-
type should reveal real-world EMI e
ects that escaped from the limited view of initial
modeling. In addition, RF techniques prove to be essential in the design of circuits that can
exploit the power of modern high-speed processors. Correcting any problems through
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filtering, shielding, or redesign is still inexpensive at this early stage in the design, and the
second prototype will already have a good chance of passing compliance testing with min-
imal rework.
In this chapter we have presented only a few of the ways in which technology selection,
circuit design, and layout techniques in
uence the generation of and immunity against EMI.
A discussion of detailed techniques to control EMI by virtue of good design is beyond the
scope of this topic. However, many topics and articles have been published that disclose the
secrets of the EMI/EMC world, all the way from Maxwell's equations, through the legali-
ties of regulation, into the tricks of the trade for taming EMI [Mardiguian, 1992; Marshman,
1992; Williams, 1999]. Considering the stiff
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economical, technical, and legal penalties
brought by manufacturing a medical product that does not comply with EMI and EMC reg-
ulations, you should be motivated to keep EMC in sight at every turn of the design process.
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