Biomedical Engineering Reference
In-Depth Information
To use the test system, increase the variac's output until you read 5.55 V RMS across R1.
At this point, 4.5 A should be circulating through the two loops of L1. Correcting for the
di
field produced by a square loop versus that of a circular loop, and
applying the Biot-Savart law, the magnetic
ff
erence in magnetic
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field at the center of the loop should be
(4.5 A
2)/0.9 m
10 A/m.
GOOD DESIGN PRACTICES, REMEDIES, AND DUCT TAPE
No cure for EMC problems is better than prevention. Trouble avoidance in EMC is accom-
plished by considering the emissions and susceptibility aspects of EMC at every stage of
the design process. The following important questions must be part of the circuit design,
selection of components, and packaging:
• Will this part of the design generate or be susceptible to interference?
• What are the characteristics of the interference?
• At what frequency or frequencies does it occur?
• From where is it most likely to originate?
• Which radiated and/or conducted path(s) can the interference take from source to
victim?
ed, you must decide what to do to
reduce their impact. There are four broad solutions to an individual EMC problem:
Once potential sources of interference are identi
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1. Prevention . Eliminate the sources of potential interference.
2. Re
ection . Keep internally generated signals inside the device and keep external
interference outside the device's enclosure.
3. Absorption . Use
fl
filtering materials to absorb interfering signals.
4. Conduction . Divert interfering signals to the device's RF ground.
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filter networks and
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Fortunately, most of the rules and perils are known in the war against EMI. Designing
an instrument to pass EMC testing is, in all likelihood, all that will be needed to ensure
proper performance under real-world situations. Avoid overdesign. The authors are not
aware of a single medical device malfunction attributed to interference by unknown UFO
radiation. All you need to do is
figure out the potential level of interference that you may
encounter, and design within these limits.
Kendall [1998] proposed a simple way of estimating the amount of protection that may
be needed in a medical device to counteract an EMI threat. His step-by-step procedure
demonstrates how to estimate the protection that needs to be incorporated in the design of
an analog comparator with 5-mV sensitivity.
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1. Start by identifying the RF threat level. For example, if the applicable standard for
your device establishes immunity against radiated interference at 3 V/m, use this
level for your calculations.
2. Multiply the threat level by the
field uniformity of the test chamber in which the
device will be exposed to EMI. A factor of 2 is appropriate for ferrite-lined chambers,
while a factor of 4 is typically used for semianechoic chambers. Assuming a ferrite-
lined chamber, the uniform
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V/m).
3. Account for losses between the source and the victim. A minimum theoretical loss
of
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field will be 2
3(V/m)
6(V/m)
136(dB
µ
14 dB would happen in the case in which the source and the victim are both
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