A Novel Capacitance-to-Time Converter for Differential-Type Capacitive Sensor Insensitive to Offset Mismatch and Parasitic Capacitance

A capacitance-to-time converter (CTC) that measures the change in capacitance
$\Delta C$
of a differential-type capacitive sensor (DCS) is presented in this letter. Most of the existing measurement circuits for DCS assumes its offset capacitance (
$C_{01}$
and
$C_{02}$
) be exactly matched. While in practical cases, it is difficult to have matched offset capacitance (
$C_{01} = C_{02}$
) due to sensor structure or manufacturing defects. This mismatch (
$C_{01} \ne C_{02}$
) will introduce a noticeable error in output of existing schemes. Moreover, the conversion time for most of these schemes are large, which is not suitable for high-speed applications. The proposed CTC implements a closed-loop system that generates a pulse width-modulated output whose
on
-time is proportional to
$\Delta C$
. It implements an automatic offset-mismatch nullification (AOFN) process that eliminates the effect of mismatch between the offset capacitances of DCS. The developed prototype of CTC showed a worst-case linearity error of 0.3% for
$\Delta C$
in a range of
$\pm$
180 pF and a conversion time of 2 ms. This result demonstrates the usefulness of the CTC for accurate and high-speed measurement applications.