Biomedical Engineering Reference
In-Depth Information
D1
BZD227C10
FILT1
NFM51R00P106
3
1
+
+
From
intracardiac
electrodes
C1
4700pF
-
1
1
J
1
J2
2
Ref
ISO
ISO
2
3
2
3
D2
BZD227C10
FILT2
NFM51R00P106
1
3
-
C2
4700pF
2
ISO
ISO
D3
BZD227C10
FILT3
NFM51R00P106
1
3
Ref
C3
4700pF
2
ISO
ISO
Figure 4.34
RF filters are often needed on every line connected to the outside world. In this intracardiac electrogram signal input circuit,
individual single-line chip filters are used to shunt RF signals to the isolated ground plane without affecting low-frequency signals. These
filters have a nominal
3-dB cutoff frequency of 10 MHz, yielding a minimum attenuation of 5 dB at 20 MHz, 25 dB at 100 MHz, and 30 dB
above 500 MHz. 4700-pF capacitors shunt RF signals that may be induced by electrosurgery. Zener diodes with their cathodes connected to
a common point to limit surge voltages that may be caused by defibrillation.
filters is used to shunt
RF signals that may be induced by electrosurgery tools. The value of 4700 pF was selected
for this design to reduce the possibility that electrosurgery currents circulating through the
electrodes in contact with the heart would reach a level capable of ablating myocardial tis-
sue. Finally, zener diodes with their cathodes connected to a common point are used to
limit the voltage imposed on the inputs of the ampli
100 MHz, and 30 dB above 500 MHz. The 4700 pF following these
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er by high-voltage transients such as
de
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brillation.
It is a good idea to
filter every line connected to the outside world, even those that carry
high-level signals over low-impedance paths (e.g., high-level analog or digital outputs)
because if left un
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ltered, that line can act as an antenna picking up EMI from outside the
device and reradiating it within the device's enclosure. One of the most sensitive and often
disregarded lines in a medical device is the reset line. This is because EMI or ESD cou-
pling into a reset line can cause a reset, often leading to temporary interruption of service,
which may leave a patient without the support of a life-sustaining therapy until function is
restored. For this reason, reset lines and components that can evoke reset events (e.g., reset
switches, microprocessor watchdogs, and power supply supervisors) must be thoroughly
shielded and decoupled.
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