Biomedical Engineering Reference
In-Depth Information
this poses a ground-loop problem for low-frequency signals, one end can be coupled to
ground through a 0.01-
F 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 iso-
lation 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 e
ยต
ective 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
ff
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 su
fi
ces to pro-
tect from 8-kV ESD events. The area protected by a nonconductive cover is more di
cult 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 dis-
plays, membrane keypads, potentiometers, and switches is su
fi
cient 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 pre-
vent or at least divert ESD currents from reaching vulnerable internal circuits.
The Real Bandwidth of Signal Lines
Protecting medical devices from EMI is especially di
cult because it often involves
sensitive electronics that can pick up and demodulate RFI. Interfering signals can be
recognized as real features of physiological signals, leading to potentially serious risks
to patients and health-care providers. Take, for example, the polling of a cellular phone,
which happens at a frequency close to that of the heart's normal rhythm. If detected and
interpreted incorrectly, a pacemaker could assume that the EMI bursts are really the heart
beating at an appropriate rhythm, causing it to inhibit the delivery of pacing therapy.
A common mistake in the design of medical instrumentation, especially of biopotential
ampli
cation and processing stages is to assume that the RF bandwidth of the circuit is
limited to the intended operational bandwidth. The limited bandwidth of an op-amp or of
a low-pass
fi
filter intended to limit the bandwidth of biopotential signals will do little to
prohibit pickup and demodulation of RF signals. If not controlled, RFI can easily induce
volt-level RF currents in biopotential ampli
fi
fi
ers designed to detect micro- or millivolt-level
signals. These RF currents will surely
find nonlinear paths (e.g., zener protection diodes
and parasitic diodes) that demodulate them, yielding high-level in-band signals that obscure,
if not completely swamp, real biopotentials.
Besides using proper shielding, one e
fi
filters on every line
connected to the outside world, especially those that convey low-level signals from patient
sensors and electrodes to high-input-impedance analog circuits. Figure 4.34 shows the
input
ff
ective solution is to place RF
fi
er
used to record intracardiac electrograms. In this circuit, individual NFM51R00P106 sin-
gle-line chip
fi
filters placed in immediate proximity of the signal-input connector of an ampli
fi
fi
filters made by muRata are used to shunt RF signals to the isolated ground plane
without a
ff
frequency of 10 MHz, yielding a minimum attenuation of 5 dB at 20 MHz, 25 dB at
ff
ecting low-frequency signals. These
fi
filters have a nominal
3-dB cuto
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