A Silicon-Based Closed-Loop 256-Pixel Near-Field Capacitive Sensing Array with 3-ppm Sensitivity and Selectable Frequency Shift Gain

This paper presents a two-dimensional capacitive sensor circuit implemented in 28-nm silicon technology for material characterization. The circuit is based on a 4.5-GHz quadrature oscillator with a single inductor and distributed capacitor array with 12.6-um pitch. A mixer down-converts the oscillation frequency to an intermediate frequency around 10 MHz. A digital processor controls the sensor operation and processes the frequency information. The 16x16 sensor array is designed to enable scalability. A novel trans-linear loop embedding the quadrature oscillator provides frequency shift amplification up to 36 times with a wide acquisition bandwidth of 2.4 MHz. The readout time for single pixel breaks the limit set by the quantization noise floor and reaches near optimal window, while the sensor maintains a sensitivity of 3 ppm.