Capacitive sensor interfacing using sigma-delta techniques

A monolithic precision capacitive sensor interface based on a Sigma-Delta core is presented. All necessary functions such as sensor excitation, temperature sensor, voltage reference, etc. are included. Traditionally the most widely used method for digitizing the output of a capacitive sensor was to first convert the capacitance to a voltage, followed by a regular ADC. The presented interface circuit directly converts a capacitance to a digital word, at high resolution (2aF/sqrtHz), high linearity (100ppm) and high accuracy (4fF). The circuit is based on a novel modification to a standard switched-cap Sigma-Delta modulator. The fixed input capacitor in the switched capacitor front-end is replaced by the external variable sensor capacitance, and a fixed voltage is applied in place of the normal variable voltage input. A fully differential circuit is used in conjunction with a switching technique which allows for cancellation of leakage current errors. The circuit is also insensitive to parasitic capacitance to ground