Biomedical Engineering Reference
In-Depth Information
Ferrite is even available in tape form (actually, it is a ferrite-coated tape) and can be
applied to PCBs and as a shield on the chassis, enclosure, or other surfaces. Before buy-
ing tape speci
cally for this application, try the magnetic backing sold in arts and craft
stores to make refrigerator magnets out of photographs. It works wonders!
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Connector Filters
If a signal line on a connector is a suspect for noncompliance, the
easiest and cleanest way to apply in-line
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filtering is either to exchange the original connector
for a
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filtered one or to insert a
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filtered adapter between the original plug and its target socket.
D-type
filtering adaptors are widely available and come with a
feedthrough capacitor in the range 50 to 2000 pF with or without ferrite inductors. You should
also consider using ferrite plates with holes to match your connectors' pinout con
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filtered connectors and
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guration.
Connector Shields
Very often, shielded wires are used to protect sensitive or noisy lines,
only to terminate at an unshielded connector. Connector backshields can easily be
retro
tted onto many cables. Make sure, however, that by connecting the cable's shield to
an exposed metallic connector you do not violate insulation requirements.
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Capacitors and In-Line Filters
Small capacitors can be tacked on to suspect lines. These
are especially useful when dealing with problems that may be occurring on a PCB. It is
always a good idea to take an assortment of pF- and nF-range chip capacitors to the test
house. If additional
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filtering is needed, some surgery can be done to a PCB to retro
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t in-
line
filter modules. Beware, however, that the addition of capacitors to the circuit of a med-
ical device may change its leakage characteristics, causing the device to fail leakage and/or
hipot safety tests.
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Cable Shielding
fixing EMC
problems. Mesh with and without zip-on sheaths, as well as conductive foils with or with-
out adhesive backing, are available and can be applied to cables with ease. You will need
to decide how thick a shield you use and how you connect the shield to ground.
Cable shielding is the next step in troubleshooting and
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Enclosure Gaskets
Metallic enclosures are often assumed to be bulletproof barriers
against EMI, both incoming as well as outgoing. However, no instrument is perfectly
sealed, since cables, displays, and controls couple the inside of the instrument to the out-
side environment. In fact, a metallic case can sometimes act as a resonator, guiding EMI
to (or from) the vulnerable (or o
ff
ensive) circuit.
ced to control
an EMI problem, the next step is looking for gaps in the enclosure which may require
shielding. Here, you can use metallic foils and tapes to improve contact along enclosure
seams and determine if a permanent solution could be achieved through the use of con-
ductive EMC gaskets. In addition, temporary application of conductive foils to display
windows and ventilation apertures is a very useful diagnostic to determine if conductive
transparent screens (e.g., very
Assuming that
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filtering and shielding of cables and controls has not su
fine wire mesh) need to be applied to these openings. Many
of these shielding materials are available from Chomerics.
If the problem is ESD, consider even tiny gaps or joints in enclosure shields which are
weak spots, because they divert very large fast currents as they
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flow around the enclosure,
causing current density hot spots that emit strong EMI through the shield and into the
enclosure. In fact, for the very high frequency components of ESD pulses, gaps and joints
may act as slot antennas that help get EMI into the enclosure.
On a di
fl
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erent front, consider that metallic shields can sometimes be completely trans-
parent to o
fields that have a dominant magnetic components. In these cases, vul-
nerable parts of the metallic enclosure may require further shielding with a material that
ff
ensive
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