An interference-insensitive switched-capacitor CDC

Abstract

A new interference-insensitive Switched Capacitor (SC) Capacitance-to-Digital Converter (CDC) is presented in this paper. The insensitivity is achieved by the conversion mechanism itself. It is a combination of a SC relaxation oscillator and an integrating type analog-to-digital converter whose integration time is made an integer multiple of the time period, Tintr, of the interfering signal. Although in a typical dual-slope converter, the integration time can be set as a suitable multiple of Tintr, the deintegration time is a function of the measurand. Thus, the extent of interference rejection that can be achieved, using such converters, is limited. The SC relaxation oscillator in the proposed CDC is operated for a fixed duration to make a measurement. This duration is set as a multiple of the Tintr. In this scheme, the final output is proportional to the number of transitions in the output of the SC relaxation oscillator which is directly proportional to the value of the sensor capacitance and has negligible sensitivity to the presence of the interference signal. The proposed scheme has been realized and tested using a SPICE tool. The non-linearity of the scheme is found to be less than 0.1% for a sensor capacitance, varied from 15 pF to 25 pF. The study conducted, to evaluate the effect of coupled interference from a distribution power line, showed that the sensitivity of the output to the interference is negligible for a large range. This paper also proposes a method to further increase this range and to improve the resolution of the proposed conversion method. The proposed converter is suitable for capacitive sensors that are used in the consumer applications.