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

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.