A novel auto-balancing capacitance-to-pulsewidth converter (CPC) that uses sinusoidal excitation, and operates in a closed-loop configuration, is presented in this paper. Unlike most of the existing CPCs, the proposed interface circuit is compatible with both single-element and differential capacitive sensors. In addition, it provides a pulse-width modulated (PWM) signal which can easily be digitized using a counter. From this PWM signal, a ratio output is derived when a single-element sensor is interfaced, and a ratiometric output is obtained for a differential sensor. The final digital output is independent of the nominal capacitance of the sensor and has a linear characteristic irrespective of the sensor characteristic being linear or inverse. The CPC is designed such that the PWM output depends on the change in the sensor capacitance alone. It is insensitive to parasitic capacitance and has very low sensitivity to the non-idealities of the components and ICs used. The effects due to some of the non-idealities are automatically corrected by the negative feedback based autobalancing employed. The effect of component mismatch is significantly reduced by a one-time correction mechanism. These benefits are achieved without the use of any complex or expensive analog building blocks. The prototype exhibits a maximum non-linearity error of less than 0.7%, a resolution of 13.02 effective number of bits (ENOB), a signal-to-noise ratio (SNR) of 80.12 dB, and a rise time of 5 ms. Thus, the proposed simple, yet effective, low-power, low cost, auto-balancing CPC can be used to interface a wide range of existing and new capacitive sensors to digital systems.
A novel approach to directly interface a capacitivelycoupled resistive sensor to a microcontroller is presented in this paper. The existing measurement schemes for such sensors are complex. In addition, the coupling capacitance often also holds important data. The proposed simple measurement system, for such series RC sensors, is capable of measuring both the resistance and the coupling capacitance. A detailed analysis on the effect of the non-idealities on the resistance measurement showed that it is independent of the accuracy of the charging capacitor, supply voltage and preset threshold voltage. The performance of the proposed scheme has been evaluated by building suitable prototypes. Initially, a setup was designed such that the measurement was not limited by the non-idealities of the microcontroller. The test results from this showed a maximum error of 0.28% and 0.96% for the resistance and capacitance measurement, respectively. The subsequent study with the microcontroller interface exhibited a maximum error of 0.91% (resistance) and 2.94% (capacitance). Noise and resolution studies have also been conducted and the results presented. The accuracy of the prototype is promising, with a measurement time of 5 ms per parameter. This is a practical, low-power, low-cost measurement system as it provides digital data on the resistance and capacitance, in series, using only a microcontroller, and a couple of passive components.
A novel approach to interface capacitively-coupled resistive sensors directly to a microcontroller is presented in this paper. The existing schemes for the measurement of the resistance of a sensing element that is inaccessible, unless through capacitive coupling, are complex. This paper presents a direct microcontroller interface solution for this category of sensors. When a resistive sensor is coupled to the measurement system through capacitances, one of the main challenges is to make the resistance measurement insensitive to variations in the coupling capacitances. This is achieved in the proposed direct microcontroller approach, without using the synchronous demodulation technique adopted by conventional approaches. Additionally, the output is independent of the value of the charging capacitor, the supply voltage and the preset threshold voltage. The feasibility study of the proposed scheme has been conducted in two steps by building suitable prototypes. The test results from the initial study, in which the experimental setup was designed in such a way that its performance was not limited by the non-idealities of the microcontroller, showed a maximum error of 0.28%. The subsequent study with the microcontroller interface exhibited a maximum error of 1.48%. Thus, the accuracy of the proposed system is quite promising, with a measuring time of a few milliseconds. Higher accuracy can be achieved by using a microcontroller with higher time resolution, and a comparator with lesser delay and noise. This scheme does not use any expensive parts and can hence realize a highly practical, low-power and low-cost measurement system.
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