Chapter 4 - Electromagnetic Compatibility And Medical Devices: Shields Up!

Shields Up!

Shielding and grounding (reflection and conduction) are the primary methods of guarding
against EMI entry and exit to and from a circuit. Chances are that you will not build your
own enclosure. Rather, you will probably use an off-the-shelf case or hire an enclosure
manufacturer to supply you with custom-made enclosures. In either case, look at the enclosure’s
data sheets for EMC specifications. The authors’ preference is to use enclosures
which have a conductive cage that is contained completely inside a plastic enclosure without
any exposed metallic parts.

If a conductive enclosure is chosen, ensure that the conductive surface is as electrically
continuous as possible. For a split enclosure, ensure as good an electrical contact as possible
between the parts. Openings in the case that are required for display windows, cooling
slots, and so on, must be kept as small as possible. If the size of the opening is larger
than 1/20 of a potential offending EMI component, use transparent grilles to close the RF gap.
Finally, ensure that unshielded lines that carry offending signals do not pass directly
through a shielded enclosure. Use shielded cables for high-sensitivity inputs.

EMI grounding requires different, sometimes conflicting considerations from those
used to protect low-frequency low-level signal lines. The first difference is the issue of single-
point versus distributed grounding. Single-point grounding of circuits is a common
practice in the design of low-noise electronic circuits because it eliminates ground loops.
This assumption is valid only up to a few megahertz. At higher frequencies in the radio
spectrum, line inductances and parasitic capacitances become significant elements, voiding
the effectiveness of single-point grounding. For example, for the 300-MHz components of
an ESD event, a 0.25-cm length of wire or PCB track acts as a one-fourth wavelength
antenna, providing maximum voltage at the ungrounded end. As such, any cable that is
longer than 1/10 to 1/20 of offending spectral components should be grounded at both ends. If
this poses a ground-loop problem for low-frequency signals, one end can be coupled to
ground through a 0.01-μF capacitor.

Whenever possible, the shield of external cables should be properly terminated to the
equipment enclosure. Poor termination, which may be imposed by leakage current and isolation
requirements, may result in capacitive coupling of EMI to signal lines. So, by all
means, and as long as isolation and leakage requirements permit, bond the cable shield
directly to the device’s conductive enclosure. Contrary to the suggestions above, when the
potential problem is ESD, the effective solution is not to shield with a conductive layer
but rather, to insulate. By not allowing an ESD spark to occur at all, there are no bursts of
electric and magnetic fields to radiate EMI.

For this purpose, plastic enclosures, plastic knobs and switch caps, membrane keyboards,
plastic display windows, and molded lampholders help eliminate ESD discharge points. As
a rule of thumb, a 1-mm thickness of PVC, ABS, polyester, or polycarbonate suffices to protect
from 8-kV ESD events. The area protected by a nonconductive cover is more difficult to
assess because surface contamination by fingerprints and dust attract moisture from the air
to form a somewhat conductivity paths through which ESD can creep. During 8-kV testing,
an ESD gun can produce sparks that follow random paths over a supposedly nonconductive
surface all the way to a metallic part 5 cm away. The same happens on metallic surfaces
painted with nonconductive paint, where surface sparks seek pinhole defects on the paint.

A very common design mistake is to assume that 15-kV-rated insulation on LCD displays,
membrane keypads, potentiometers, and switches is sufficient to protect circuitry
connected to these components. The problem is that although ESD won’t go through the
insulation, it will creep to the edges of the insulation and hit wiring on the edges of these
components. As such, extend the dielectric protection of panel-mounted controls to prevent
or at least divert ESD currents from reaching vulnerable internal circuits.

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Chapter 4 - Electromagnetic Compatibility And Medical Devices: Shields Up!

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