Capacitance deviation-to-time interval converter using current-tunable Schmitt triggers

A capacitance deviation-to-time interval converter is presented for interfacing capacitive sensors. It consists of two ramp integrators, two current-tunable Schmitt triggers, and two logic gates. A prototype circuit built using discrete components exhibits a resolution as high as 13 bits and a linearity error less than +/- 0.2% when the output pulse width is counted by a 20 MHz clock signal. Its application to a humidity sensor is also presented