Biomedical Engineering Reference
In-Depth Information
In fact, ignoring capacitor and inductor resonance is the most common mistake in
designing circuits with good EMI/EMC characteristics. Parasitic inductance and capaci-
tance cause components and PCB tracks to resonate, often at frequencies as low as 1 to
30 MHz. For example, many 10-nF decoupling capacitors with
2
-in. leads resonate at
around 20 MHz. Above the resonant point, capacitors act as inductors and inductors act as
capacitors, which typically render
1
filters useless against high frequencies.
Regardless of the usefulness of these RF test tools, their price puts them out of reach
for most electronics hobbyists and small engineering
fi
firms. But don't be discouraged:
Building successful high-immunity and high-speed circuits on a budget is possible by
adopting conservative design policies. As you may realize by now, an analog circuit sim-
ulator could be as helpful as a digital circuit simulator in the design of your next circuit.
For critical circuits, do not assume ideal capacitors and inductors, but rather, consider the
RF characteristics of these components in your analysis.
If possible, use ceramic surface-mounted capacitors and short, fat PCB tracks to keep
inductance low. As far as inductors are concerned, ferrites perform better than wire-wound
types. This is because ferrites have high resonant frequencies and absorb large amounts
of energy at resonance. If wire-wound inductors are nevertheless required, place small
ferrites in series with the inductor (e.g., mount small ferrite beads on the inductor's leads)
to protect the circuit over a wider bandwidth. Of course, make sure that the magnetic
fi
fl
ux
created by currents
flowing through wire-wound inductors does not couple to adjacent
components and PCB traces.
fl
Duct Tape and Test-Time Bandages
Despite thorough engineering, surprises do happen during compliance testing. Inevitably,
while you are still giving the last touches to your design, the marketing department has
already sold a few dozen units to their most prestigious customers and eagerly awaits get-
ting their hands on the very prototype you are testing for the next trade show. “This is not
good,” you think. Next comes the calculation of how long you can live on your credit cards
before
finding another job in a distant corner of the world where no one would have heard
about your mistake.
In reality, vulnerabilities in well-designed devices can often be patched up at the
test site. Understand, however, that any modi
fi
cations you make to pass the tests will
have to be implemented in the actual product. Don't go overboard implementing every
possible solution at once. Although adding a capacitor here, a ferrite there, and some
shielding to a cable may sound trivial, each of these modi
fi
cations can turn into a logistic
nightmare during production. Moreover, unbudgeted additions multiplied by the number
of units to be produced over time may end up reducing the pro
fi
fi
t margin to intolerable
levels.
In addition, and before applying any corrective measures to a medical device prototype,
however, make sure that by
c line, component, or assembly,
you do not violate insulation or leakage requirements. With that said, let's look at the
essential elements of a
fi
filtering or shielding a speci
fi
fi
first-aid kit.
Ferrites
first thing most people use to try to clean the signals from a sus-
pect line. A huge variety of ferrites are available; they come in all sizes and provide atten-
uation at diverse frequency ranges. Ferrites also come with various installation options:
Some require you to pass the cable or lead component through them, while some have
clamps that make it easy to retro
Ferrites are the
fi
fi
it equipment without disconnecting wires. Ferrites are
also available to speci
erential or common-mode problems. Ferrite diag-
nostic kits are available from various vendors (e.g., Fair-Rite), and test houses usually have
plenty of ferrite beads and chokes available for their customers.
fi
cally treat di
ff
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