A Capacitance-Ratio-Modulated Current Front-End Circuit With Pulsewidth Modulation Output for a Capacitive Sensor Interface

This paper presents a front-end circuit with pulsewidth modulation (PWM) output for a capacitive sensor interface. A single-MOSFET-based capacitance-ratio-modulated current (CRMC) circuit is proposed to transform the sensed capacitance into a current proportional to a capacitance ratio. The proposed single-MOSFET-based CRMC circuit not only saves a lot of chip area but also lowers power consumption. Then, a dual-slope integration circuit transforms further the modulated current into PWM output. A prototype chip is designed and fabricated with a 0.35-mu m complementary metal-oxide-semiconductor process and has a core area of 0.09 mm(2). The measurement results of the prototype chip demonstrate an accuracy value of up to 9.3 bit with 54-mu W power consumption. The microwatt power consumption and the small chip area of the proposed front-end circuit make it very suitable for a capacitive sensor interface in smart sensor chips.