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. The 16×16 sensor array is designed with minimal signal energy loss to enable scalable designs. The quadrature oscillator is embedded in a trans-linear loop for frequency shift amplification up to 36 times with an acquisition bandwidth of 2.4 MHz. The readout time for single pixel drops below the limit set by the quantization noise floor and reaches near optimal window, while the sensor maintains a sensitivity of 3 ppm. A mixer is used to down-convert the high-frequency component to an intermediate frequency. A digital core is used to acquire and process the data.