A Digital Readout IC with Digital Offset Canceller for Capacitive Sensors

Abstract

A digital readout IC for capacitive sensors is presented. Digital capacitance readout circuits suffer from static capacitance of sensors, especially single-ended sensors, and require large passive elements to cancel such DC offset signal. For this reason, to maximize a dynamic range with a small die area, the proposed circuit features digital filters having a coarse and fine compensation steps. Moreover, by employing switched-capacitor circuit for the front-end, correlated double sampling (CDS) technique can be adopted to minimize low-frequency device noise. The proposed circuit targeted 8-kHz signal bandwidth and oversampling ratio (OSR) of 64, thus a 3rd-order Delta Sigma modulator operating at 1 MHz was used for pulse-density-modulated (PDM) output. The proposed IC was designed in a 0.18-mu m CMOS mixed-mode process, and occupied 0.86 x 1.33 mm(2). The measurement results shows suppressed DC power under about -30 dBFS with minimized device flicker noise.