An improved capacitance-to-digital converter for leaky capacitive sensors

This paper presents a capacitance-to-digital converter (CDC) that employs a novel approach to perform an accurate measurement of capacitance of a leaky capacitive sensor. This CDC employs a sinusoidal source for excitation, which is advantageous for various sensing applications, including ice detection, liquid level measurement, humidity measurement, proximity sensing, and so on. Recently, an impedance-to-digital converter (IDC) based on the dual-slope conversion technique has been reported. It can measure the value of capacitance even when a parallel resistance is present, but requires a large number of sinusoidal excitation cycles to complete a conversion, leading to poor update rate. The CDC proposed in this paper gives much higher (about 125 times) update rate compared with the IDC as it requires only a few excitation cycles for the conversion, but still gives an accurate output due to the use of a specially designed clock, which helps to count the number of charge packets received by the integrator capacitor during the deintegration. Other than the operation of the CDC, this paper also describes an outcome of a thorough analysis conducted to quantify the effect of various circuit parameters on the output of the new CDC. A prototype of the improved CDC has been developed, and its performance parameters, such as accuracy (±0.27%), conversion time (24 ms), effect of parallel resistance, and so on, have been tested, and the results are reported in this paper.