Analysis for the Implementation of Capacitive Couple Readout Circuit for Contact-less ECG and EEG

Abstract

In modern medicine, the two most common and actively used techniques for measuring electrophysiological signal for human are Electrocardiography (ECG) and Electroencephalography (EEG) systems. These physiological signals are very weak in amplitude in terms of few millivolts down to 10’s of millivolts. The major amount of power is located at very low frequencies from below 1Hz up to 40 Hz. The measurement of the above parameters is very tough job in electrical circuit designs. Generally, the measurement of these parameters is done by electrodes with direct skin contact by applying of some gels. For this, time consuming clinical set up is required. This paper introduces a method for the measuring the above type of signals using a capacitive coupled sensor. This sensor is completely non-contact type, but this method has to face some difficulties, such as: high input impedance and interfaces and some circuit nose problems. A capacitive coupled sensor is realized by the printed circuit board (PCB) bottom layer as a one capacitor plate and the signal source is considered as another plate. In this, PCB layer mask acting as an insulator. The high input impedance amplifier gives the gain for the sensed signals is 60 dB and filtering also taken place. Two measurements were made off using the same circuit for two different input impedances. The results of these measurements show that high input impedance is a crucial parameter for the functionality of the sensor circuit and the high input impedance circuit provides always good signal to noise ratio (SNR).