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Medical ECG Slide 12

Beginning with the analog front end, this slide shows what needs to happen before signals are processed by the front end amplifier. The analog circuitry must be able to function in the presence of defibrillation signals. Defibrillation pulses are for events where the left ventricle stops pumping blood, when there is an application of a very large voltage, 3000 to 5000 V. In the front end the neon gas lamps protect against these defibrillation voltages. When the high voltage is applied they become a low impedance path, on the order of 20 and 270 Ω. That is the first line of defense, the next is the series input impedance which does two things; it protects the patient and the electronic device from current. Manufacturers break up the series input impedance to create a two-pole low pass filter, represented on this slide as CF and CCM. It also can be seen that a differential capacitor is used in the front end with a value ten times larger than the common mode capacitor, CCM. This is done in case there is a mismatch between a protection resistance and capacitance, Rprot and CCM, which can cause common mode noise to be converted to differential noise. If the capacitor is scaled to be ten times larger, it is able to attenuate this noise to a negligible level. The protection diodes are in the circuit to protect the analog device and clamp the voltage within its safe operating range. The zener diodes are attached to provide a low impedance path to ground in the event that there is a supply transition.

PTM Published on: 2012-05-08