Chapter 4 - Electromagnetic Compatibility And Medical Devices: Paying Attention to PCB Layout

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, capable
of achieving a speed of 200+ mph!—its magnificent turbo-charged V-12 engine purring
while cruising down the road at a speed at the limit of human reflexes. Waking up to reality,
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 considers
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 appropriate
bandwidth—no quasi-dc signals anymore! Interestingly, the same PCB layout practices
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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Chapter 4 - Electromagnetic Compatibility And Medical Devices: Paying Attention to PCB Layout

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