In this paper, a comprehensive model for the electronic noise properties and frequency-dependent responses of printed circuit board (PCB)-based as well as textile noncontact capacitive electrodes is presented. For bioelectric diagnostics, noncontact capacitive electrodes provide an interesting alternative to classical galvanically coupled electrodes, since such a low-cost diagnostic system can be applied without preparation time and in mobile wireless environments. For even higher user comfort, textile capacitive electrodes are preferable. This paper provides a thorough study of the influence of the electrical components of capacitive electrodes and textile capacitive electrodes, as well as their surface area and circumferences on the resulting noise properties of the electrode by independently measuring the corresponding noise spectra. Consequently, the equivalent noise model is developed. The most important noise source is the high input bias resistance, which, in combination with the involved capacitance, results in an apparent $1/f^2$-power noise spectrum. By comparing the noise measurements with the noise model of the electrode, we conclude that the surface of the electrode contributes to an additional $1/f$ -power noise in the noise spectrum. We also found that the highest possible coupling capacitance is most favorable for low-noise behavior. Therefore, electrodes with electrically conducting fabric surfaces are investigated. Due to this, it is possible to enlarge the surface of the electrode and maintain a small distance between the body and the surface of the electrode. We show that with the use of textile capacitive electrodes, it is possible to reduce the noise characteristics considerably. Our findings in this paper provide a necessary source for further optimization of capacitive electrodes for bioelectric measurement applications.
Abstract:The rigid surface of the conventional PCB-based capacitive electrode produces an unde ned distance between the skin and the electrode surface. Therefore, the capacitance introduced by them is uncertain and can vary from electrode to electrode due to their di erent positions on the scalp. However, textile electrodes which use conductive fabric as electrode surfaces, are bendable over the scalp. Therefore, it provides a certain value of the capacitance which is predictable and calculable accurately if the e ective distance to the scalp surface can be determined. In this paper noise characteristics of textile electrodes with di erent fabric sizes as electrode's surface and capacity calculations related to each size are presented to determine the e ective distances for each electrode size.
Analog dividers are widely used in analog systems. Analog realization of such circuits suffer from limited dynamic range and non-linearity issues, therefore, extra circuitry should be required to compensate these types of shortcomings. In this paper a gain controllable, analog divider is proposed based on data converters. Our circuit can be implemented both in current and voltage mode by selecting proper architectures. The resolution, power consumption and operation speed can be controlled by proper selecting of components. Another advantage of our circuit is its gain programmability. Moreover, the gain can be adjusted independently based on the relationship between input signals. Our proposed method offers two different gain control abilities, one for situation that the numerator signal is bigger than the denominator, and another gain is applied when the denominator is larger than the numerator. As a result, no extra amplifier is required for signal amplification. Moreover, the input and output signal nature can be chosen arbitrarily in this circuit, i.e. input signal may be a voltage signal while the output signal is current. Simulation results from SPICE confirm the proper operation of the circuit.
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