Biomedical Engineering Reference
In-Depth Information
Although designers usually protect power supplies from the high-voltage transients that
may be encountered under the conditions simulated by EFT and high-energy surges, the
EMI component of these events is often forgotten. High-voltage surges may bypass the
power supply and associated
fi
filters completely and attack the device's circuitry directly.
The same
filtering recommendations apply then to protecting sensitive analog and digital
circuits against the EMI produced by power line surges.
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Paying Attention to PCB Layout
Although in earlier sections we advocated maintaining clock speeds low, as well as rise and
fall times as slow as possible, some medical device applications really demand lightning-
fast processing. Let's digress and imagine what it would be like to own a Ferrari F40, capa-
ble of achieving a speed of 200
cent turbo-charged V-12 engine purring
while cruising down the road at a speed at the limit of human re
mph!—its magni
fi
fl
exes. Waking up to real-
ity, though, you would seldom (if ever) be able to
floor the gas pedal of this marvel. Even
if disregarding the legal limit, our roads are just not designed to support much more than
half the maximum speed of a loaded sports car. The awesome power of sports engines can
be let loose only in special race tracks, constructed with the right materials and slants.
Although you may not consider adding a Ferrari to your estate at this moment, its power
does relate to the topic of this chapter, as you are probably using increasingly fast logic
and microprocessors in your projects. However, in close resemblance to the sports car
analogy, very high bus speeds result in interconnection delays within the same order of
magnitude as on-chip gate delays, and for this reason typical PCB design, which consid-
ers traces as low-frequency conductors rather than as high-frequency transmission lines,
will ensure that such a project turns into a very impressive and expensive paperweight.
Some 20 years ago, while some of us were building microcomputers with 2-MHz Z80s,
8080s, and CDP1802s, engineers designing with ECL technology already faced problems
related to the implementation of printed circuit boards, backplanes, and wiring for high-
speed logic circuits. Today, however, multihundred megahertz and even gigahertz buses are
commonplace, and we face strict regulations on the RF emissions escaping from such
wideband sources. For this reason, we should all acknowledge that the utopian idea that
digital signals behave as ones and zeros must be replaced by a more realistic approach that
involves RF transmission line theory. Through this new approach, printed circuits are
designed to convey pulse transmissions with minimal distortion through channels of appro-
priate bandwidth—no quasi-dc signals anymore! Interestingly, the same PCB layout prac-
tices that are useful in the design of high-speed circuitry apply to the design of circuitry
with increased immunity against EMI.
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Transmission Line Model of PCB Track
PCB design for high-speed logic and RFI immunity demands the use of power and ground
planes, and plain double-sided PCBs are not recommended. In the former, a surface
stripline track such as that depicted in Figure 4.35 will have an impedance
Z
t
given by
87
1
5
8
.
w
9
8
h
Z
t
ε
ln
0.
.4
1
t
t
t
is the dielectric constant of the PCB dielectric,
h
the height of the track above the
ground or power plane, and
w
t
and
t
t
the width and thickness of the track, respectively. A
PCB track buried within the
where
ε
fiberglass-epoxy laminate will have its impedance reduced by
about 20% compared with that of a surface track.
This PCB track can be modeled as a transmission line [Magid, 1972], and a short pulse
applied to one end of this transmission line will appear on the other side, supplying the
fi
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