Biomedical Engineering Reference
In-Depth Information
This author has measured electric fields as high
as 10 kV/m induced by an 8W, 800MHz hand-
held radio at the surface of a medical device
[9]. This is 1000 times greater than the IEC
standards and results from the electric near-field
generated in the vicinity of an antenna. Conway
and Jones [10] demonstrated that, “[i]n a clinical
setting, high-power-output devices such as a two-
way radio may cause significant interference in
ventilator function. Medium-power output devices
such as mobile phones may cause minor alarm
triggers. Low-power output devices, such as Blue-
tooth appear to cause no interference to ventilator
function.”
Conducted EMI enters most commonly along
power connections to a device. Transients induced
locally from other devices (switching power
supplies and electric motors) or over the power grid
(lightning, HEMP) can gain direct entry without
proper safeguards. Common mode voltages can
develop on patient lead wires and device inter-
connects. When these lines are greater than 0.1
wavelength, they become efficient antennas [1].
Most modern equipment is well-protected
against local static discharge. Lightning, on the
other hand, can induce conducted transients as high
as 40 kA with rise time of 20 ns resulting in a
40 kV to 250 kV voltage spike, depending on the
proximity of the strike [11].
Figure 19.2
Typical Electromagnetic pulse shapes [15].
well via conduction. Frequencies above 100MHz
are efficient for radiation coupling [6,7].
Once the energy has propagated into a circuit,
there is the potential for interference. Low energy
EMI can occur in digital circuits as a propagation
delay or by exceeding the triggering threshold
of a logic device. Analog devices tend to show
increasing effects as power increases and will
usually recover after the EMI subsides. Digital
systems usually require resetting or re-locking after
an event [8]. At higher energy levels, permanent
damage to low-power integrated circuits can occur.
Figure 19.3
Electromagnetic field vectors [25]. (Permission is granted to copy, distribute and/or modify this document under the
terms of the
GNU Free Documentation License
, Version 1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts Subject to
disclaimers
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