Capacitance measuring method and apparatus

Abstract

An apparatus for performing ratiometric capacitance measurements by comparing the output of a digital-to-analog converter with the voltage at a tap between an unknown capacitance and a first inherent capacitance of the apparatus. A common input voltage is supplied to both the series circuit, including the first inherent and unknown capacitances, and to the digital-to-analog converter which accomplishes attenuation of the input voltage in response to a digital value. The digital value is generated by a successive approximation register supplying a digital output relating the unknown capacitance to the known capacitance. The apparatus is adapted to ratiometrically measure the first inherent capacitance by supplying the common input voltage to a series circuit, including a known reference capacitance and the first inherent capacitance, such that the digital output of the successive approximation register relates the first inherent capacitance to the reference capacitance. The apparatus is further adapted to ratiometrically measure a second inherent capacitance by supplying the common input voltage to a series circuit, including the first and second inherent capacitances, such that the digital output of the successive approximation register relates the second inherent capacitance to the first inherent capacitance. The measured value of the unknown capacitance is then corrected by the amount of the second inherent capacitance appearing in parallel with the unknown capacitance during measurement thereof with reference to the first inherent capacitance.