An Easy-to-Interface CDC With an Efficient Automatic Calibration

Abstract

A new capacitance-to-digital converter (CDC) that gives a digital output proportional to the measurand $kx$ , independent of its nominal capacitance, is reported in this paper. Various CDCs are available in the market, but their output is a function of the nominal value $C_{0}$ of the sensor. Thus, a manual correction is required in the output, whenever a new capacitive sensor is interfaced to such CDCs. Moreover, the $C_{0}$ of the sensors is usually a large value compared with the change in capacitance due to $kx$ . Thus, in a typical CDC, a large percentage of the output counts/bits is used to measure and represent the $C_{0}$ leading to an underutilization of the hardware as far as the measurement of $kx$ is concerned. The proposed CDC uses a new automatic calibration (AC), employing a switched-capacitor (SC) circuit-based voltage source in a feedback topology, and provides an output independent of the $C_{0}$ of the sensor employed. The proposed CDC is based on an SC dual-slope technique and possesses the advantages of the dual-slope conversion method. The CDC circuit is designed such that the entire full scale can be used for representing the change in the $kx$ . In addition to this advantage, the AC helps to remove the dependence on the accuracy of the reference capacitor employed in the CDC. A detailed analysis of the effect of various sources of errors in the CDC has been conducted and presented in this paper. A prototype of the proposed CDC has been developed and tested. The results obtained show that the output of the CDC is not a function of the $C_{0}~(C_{0}$ varied from 60 pF to 1.46 nF) of the sensors interfaced.