Biomedical Engineering Reference
In-Depth Information
the user that the probe is potentially charged. To comply with the standard, the probe must
have speci
8
-in. smooth rounded-
head bolt as the probe tip for air discharge. For contact-discharge tests, use the pointed
edge of a
fi
c dimensions. However, good results are obtained using a
3
4
-in. steel nail.
During operation, the discharge return ground cable of the generator must be connected
to earth ground. The ground cable should be at least 2 m long and have insulation rated at
12 kV or more. Dc power for the ESD gun can be obtained from a 12-V battery pack or a
12-V dc adapter with a current rating of at least 400 mA. Finally, make a calibration dial
to be placed around the shaft of potentiometer R2 by measuring the voltage across C5
using a high-impedance high-voltage probe and a digital voltmeter.
The standards call for a ground reference plane to be placed in the
1
floor of the labora-
tory. The plane should be an aluminum or copper sheet no thinner than 0.25 mm, covering
an area no smaller than 1 m
2
, and projecting at least 0.5 m beyond all sides of the device
under test. This is the ground reference to which the ESD simulator should connect. The
plane must also be connected to the protective earth ground. The device under test should
be placed on a nonconductive test table 0.8 m high. All nonconductive construction (e.g.,
all wood) is necessary because metal objects in the table construction would distort the RF
fi
fl
field. The table should be placed no closer than 1 m to the
walls of the laboratory or any other metallic object. A 1.6 m
fields radiated by the ESD event
fi
0.8 m metallic sheet hori-
zontal coupling plane covered with a 0.5-mm insulating support is placed between the
tabletop and the device under test. This coupling plane must be connected to the reference
ground plane via two 470-k
Ω
resistors in series.
Indirect Injection of ESD Fields
Since ESD events generate large amounts of RFI, it is
not always necessary for the ESD event to happen between a charged body and the med-
ical device itself. A discharge between two bodies in the vicinity of the medical device may
su
ce to cause a failure. For this reason, IEC-61000-4-2 speci
fi
es that testing shall also be
done by generating EMI
fields through ESD between the ESD simulator and the isolated
horizontal coupling plane, as well as between the ESD simulator and an isolated vertical
coupling plane. The vertical coupling plane is e
fi
ff
ectively an antenna of dimensions
0.5 m
0.5 m that is placed on the horizontal coupling plane but is isolated from it. An
ESD generator is then placed in the center of the vertical edge, and at least 10 impulses of
either polarity are applied. The vertical coupling plane must also be connected to the ref-
erence ground plane via two 470-k
Ω
resistors in series.
Susceptibility to Radiated Electromagnetic Interference
IEC-61000-4-3 speci
es a modulated RFI test of 3 V/m as representative of the radiated
electromagnetic interference that may be caused on a medical device by wireless commu-
nication equipment. For critical equipment such as life-support devices, 10 V/m is used for
testing. Currently, tests should be performed at frequencies of 26 MHz through 1 GHz with
1 kHz at 80% amplitude modulation, but there is serious talk about extending the upper limit
to 3 GHz. The frequency band is covered in steps of 1% of the fundamental frequency. For
frequencies of 26 to 200 MHz, a biconical transmit antenna is commonly used. For fre-
quencies above 200 MHz, a double-ridged horn transmit antenna is the popular choice.
As shown in Figure 4.21, the testing is usually performed in a shielded enclosure with
anechoic material placed throughout the enclosure to minimize re
fi
ections. The transmit
antenna is typically located 3 m from the device under test. An isotropic
fl
field strength
meter is placed inside the room at a location physically close to the device under test and
used as a secondary indication of the
fi
field strength. Testing is performed utilizing linearly
polarized antennas, with the device under test exposed to both vertically and horizontally
polarized
fi
fi
fields on each of four sides. In addition to the frequency sweep, the device under
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