Biomedical Engineering Reference
In-Depth Information
Figure 3.3
er of Figure 3.2. Note that
the gap in components and conductors which forms the insulation barrier is traversed only by the
ISO107 isolation ampli
Layout of an instrument that incorporates the ECG ampli
fi
er. An external ELPAC model MED113TT medical-grade power supply is
used to power the circuit from the 120 V ac power line.
fi
the de
brillation pulse may appear at the ECG recording electrodes as well as between the
isolated patient ground and the power line ground. The front-end protection circuit places
330-k
fi
resistors (R4 in series with R6, R25, and R5 in series with R7) in series with the
patient leads to limit the peak de
brillator input current to under 10 mA. For this applica-
tion, 2-W carbon-composition high-voltage-rating resistors are chosen, to withstand the
several dozen watts of instantaneous power that may be dissipated during each de
fi
fi
brillation
pulse.
Since voltages close to the full 5000-V de
brillator capacitor initial voltage could
appear across these resistors, care must be taken to ensure that current does not
fi
nd an
alternative path by producing a spark or by creeping across the printed circuit. The insula-
tion required to withstand the peak voltage of the de
fi
brillator pulse should be chosen to
be a minimum air clearance of 7 mm and a minimum creepage distance of 1 2 mm. This
separation would also apply to the isolation barrier between the applied part and all other
parts of the medical instrument.
A second consideration must be made for equipment that may be used in the operating
room. Here the applied part of the instrument may be exposed to very strong RF currents
coming from an electrosurgery (ESU) unit used for either cauterizing wounds or cutting
tissue. Usually, continuous-wave or gated damped sinusoids are applied between a large-
area electrode on the patient's back and the scalpel electrode. Through RF heating, tissues
are cut and blood is coagulated, causing small ruptured vessels to close. The RF compo-
nent of the ESU waveform typically is within the range 200 kHz to 3 MHz, and power
levels into a 500-
fi
load range from 80 to 750 W. Open-circuit voltages range from approx-
imately 300 V and can be as high as 9 kV.
If the circuit of Figure 3.2 were used in the presence of ESU, the path of RF leakage
current would probably be from the ESU electrodes into one or more of the device's patient
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