DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

104 DESIGN OF SAFE MEDICAL DEVICE PROTOTYPESor capacitively coupled sensors, a 10-M resistor to the isolated ground plane from eachinput should be used. When ac coupling is desired, IC4A, R8, and C5 are used to offsetIC1’s reference to suppress a baseline composed of components in the range dc to 0.48 Hz.Also for ac coupling, any remaining baseline at IC1’s output may be eliminated by a highpassfilter (1.59 Hz at 3 dB) formed by C6 and R9.IC1’s output signal is amplified by IC4b. Notice that the gain of this stage is fixed at101. Galvanic isolation is provided by IC3, a Burr-Brown ISO107 isolation amplifier IC.This type of IC resembles an operational amplifier but is designed with an internal isolationbarrier between its input and output pins. The ISO107’s signal channel has a smallsignalbandwidth of 20 kHz and provides an isolation barrier rated at a continuous 2500 V.In addition to providing a signal channel across the isolation barrier, the ISO107 has aninternal dc/dc converter which powers the isolated side of the ISO107 circuitry and providesisolated power (15 V at 15 mA typical) for the rest of the circuitry of the appliedpart (i.e., IC1 and IC2). The isolation rating of the barrier for the dc/dc converter is thesame as that for the signal channel. In total, the 60-Hz leakage current through IC3 doesnot exceed 2 A with 240 V ac applied across its isolation barrier. The output gain of IC3is selected through jumpers JP7–JP9 to provide gains of 1, 10, or 100. IC3’s output is thenlow-pass filtered by IC4C. With the component values shown, the filter has a cutoff frequencyof 300 Hz. You may recalculate these values to match the bandwidth required byyour application.In one position of SW1, the filter’s output is buffered directly by IC4D and presented tothe output of the biopotential amplifier. In the other position, SW1 redirects the output ofIC4C to a tunable-frequency notch filter before being buffered by IC4D. This makes it easyto eliminate 50/60-Hz power line hum that may have been picked up through commonmodeimbalances between the differential patient connections.As shown up to this point, patient leakage and auxiliary currents have been kept withinallowed limits by virtue of appropriate selection of the components for the circuit. However,appropriate layout and interconnection are as important in ensuring a safe design. To do so,every conductive point belonging to the isolated portion of the circuit must be separated fromevery conductive point in the nonisolated side of the circuit by the required air clearance andcreepage distances corresponding to reinforced insulation at the rated working voltage. Thelayout of a prototype instrument that incorporates this ECG amplifier is shown in Figure 3.3.Since there is a 30-mm separation between the closest pins across the ISO107 isolationbarrier, and considering that the internal isolation barrier is rated at a continuous 2500 Vat 60 Hz, the standards would consider this barrier to be equivalent to 1000 V ac–rated reinforcedinsulation. This separation would also be needed between all other isolated andnonisolated points of the circuit. Most commonly, a biopotential amplifier is operated inenvironments where the power line voltage is the highest potential of concern and has amaximum rated value of 240 V RMS . According to Table 3.1, this would require an air clearanceof 5 mm and an 8-mm creepage distance. Remember that these distances also applyto the separation of any point on the isolated side and any conductive fastening means inconnection with any nonisolated part of a medical instrument.Amplifying the electrical activity produced by the heart introduces a number of additionalrequirements addressed by the front-end protection circuit shown in Figure 3.2.Physicians conducting electrophysiological diagnosis and therapy of conditions involvingthe heart assume the possibility of ventricular fibrillation during a procedure. Revertingfibrillation back into a normal rhythm driven by the sinus node of the heart involves brieflyforcing high current through the heart. To overcome tissue resistivity, this implies thedelivery of a high-energy, high-voltage pulse.Typical external defibrillators deliver this pulse by discharging a 32-F capacitor chargedup to 5000 V dc through a 500-H inductor directly into paddle electrodes placed on thechest of the patient, who may be assumed to act as a 100- resistor. A sizable fraction of