Biomedical Engineering Reference
In-Depth Information
safety hazard in the equipment is defective or a single external abnormal condition is
present. These include interruption of the supply by opening the neutral conductor as well
as interruption of the protective ground conductor. Patient leakage current between an
F-type applied part and ground assumes that an external voltage equal to 110% of the max-
imum rated supply voltage is connected directly to the applied part. For battery-powered
equipment, the external voltage that is assumed to be connected to the F-type applied part
is 250 V. In addition, it must be noted that grounding of the patient is considered to be a
normal condition.
The allowable leakage current levels have been set as a compromise between achievable
performance and overall risk. Although a 60-Hz current as low as 10
A
fl
flowing through
the heart may cause ventricular
fibrillation (a disorganized quivering of the lower chambers
of the heart muscle that quickly leads to death) under highly speci
fi
c conditions, the prob-
ability of such an event is only 0.2%. Under more realistic clinical conditions, however, a
50-
fi
flowing from a CF-type applied part through an intracardiac catheter has an
overall probability of just 0.1% of causing ventricular
A current
fl
fi
fibrillation. This probability is very
similar to that of causing
fibrillation due to the irritation caused by mere mechanical con-
tact of the catheter with the heart wall. Obviously, for equipment that does not come in
direct contact with the heart, allowable leakage currents have been increased up to the point
where even under single-fault condition, the probability of causing ventricular
fi
fibrillation is
no higher than 0.1%, even though the actual current may be perceptible to the patient.
fi
DESIGN EXAMPLE: ISOLATED DIFFERENTIAL ECG AMPLIFIER
Let's use a simple circuit as an example to illustrate the various considerations regarding
the safe design of a medical instrument. Figure 3.2 presents the schematic diagram of a
simple biopotential ampli
erential ECG signal through surface
ECG electrodes. In the circuit, signals picked up by electrodes attached to the patient's
skin are ampli
fi
er intended to detect a di
ff
er IC. The gain
of the front-end stage is programmable between unity and 500 by jumpers JP2-JP5. Since
IC1 is dc-coupled, care must be exercised in the selection of gain so that the ampli
fi
ed by IC1, a Burr-Brown INA110 instrumentation ampli
fi
fi
er is
not saturated by dc o
set voltages accompanying the biopotential signal. For example, to
use this circuit as part of a surface ECG ampli
ff
fi
er, the gain must be calculated to cope with
o
300 mV. In general, IC1's gain should be kept low so that dc
coupling does not result in its saturation. Potentiometer R1 is used to trim the input o
ff
set potentials of up to
set
to IC1. R1-R3 can be omitted from the circuit for most applications that do not require
extreme dc precision.
Direct connection of IC1's inputs to patient electrodes is possible since the ampli
ff
er uses
a maximum bias current of 50 pA, and the FDH300 low-leakage diodes used to protect the
inputs of IC1 contribute no more than an additional 1 nA each to the patient auxiliary cur-
rent. The total 54 nA maximum is well under the allowed 0.01 mA auxiliary current for CF-
type applied parts. If the application permits it, however (e.g., if the skin-electrode interface
has a su
fi
ciently low impedance), it is a good idea to add resistors larger than 300 k
in
series with the patient connections. These resistors would e
ff
ectively limit the auxiliary cur-
rent
flowing through the patient to less than 0.05 mA in case a fault in IC1 or in D1-D4
short-circuit the patient's connection with one of the isolated power rails.
Depending on the biopotential signal being ampli
fl
ed, either dc or ac coupling are
required. For dc coupling, IC1 is referenced to the isolated ground plane I
G1
, which also
serves as the patient common input. Since the INA110 has FET inputs, bias currents drawn
through input source resistances have a negligible e
fi
ect on dc accuracy. However, a return
path must always be provided to prevent charging of stray capacitances which may satu-
rate the INA110. If this ampli
ff
fi
er would be needed to amplify completely
fl
floating sources
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