Design and Implementation of a 46-kS/s CMOS SC Dual-Mode Capacitive Sensor Interface With 50-dB SNR and 0.7% Nonlinearity

This paper presents the design and implementation of a 46-kS/s CMOS switch-capacitor dual-mode capacitive sensor interface circuit for inkjet-printed capacitive humidity sensors. The specifications of the interface circuit, which includes a capacitance-to-voltage (C2V) converter combined with an analog-to-digital converter (ADC), are optimized at system level, emphasizing the C2V operation followed by the data converter. A closed form of the maximum output range of a single-stage C2V is provided to prevent cascade amplification. The gain-boosting technique is utilized in the operational transconductance amplifier design to improve the closed-loop linearity. The correlated double sampling technique attenuates the dc offset and low-frequency flicker noise from C2V. A 10-b successive approximation register ADC digitizes the output of C2V. The total area of the digital-to-analog (DAC) array is limited not only by the matching behavior, but also by the noise performance of C2V. The rail-to-rail ability is required to render the compatibility with various possible sensor inputs. A single-ended cascaded binary-weighted capacitive DAC is used to implement the charge redistribution binary search algorithm. The circuit is implemented in a 0.18-μm CMOS technology and occupies an area of 1.2 mm². The tested prototype shows 0.69% nonlinearity in mode 1 and 1.38% nonlinearity in mode 2. The SNR of mode 1 is 50.1 dB and that of mode 2 is 36.5 dB, which meets the specification of 7.32 b in mode 1 and 5.32 b in mode 2. The total power consumption of the capacitive sensor interface is 70 μW.