11. A High-Resolution Self-Oscillating Integrating Dual-Slope CDC for MEMS Sensors

An integrating dual-slope (DS) capacitance-to-digital converter (CDC), specifically designed for interfacing capacitive MEMS sensors, is presented. In particular, this work proposes a CDC that interfaces a MEMS sensor built with a bridge of capacitors. In this bridge, some capacitances are pressure sensitive, causing pressure-related changes in the bridge output. The voltage to digital conversion is then realized in two steps. First, a voltage amplifier boosts the output of the bridge. Second, an integrating DS ADC digitizes the output of the amplifier. The proposed ADC uses time instead of amplitude resolution to generate a multi-bit digital output stream. In addition, it performs noise shaping of the quantization error to reduce measurement time. These characteristics lead to the following properties: intrinsically low sensitivity to temperature and process variations, simplicity of trimming offset and gain to correct for sensor parameter spread, and an energy-efficient implementation. The effectiveness of the proposed architecture is demonstrated by measurements performed on a prototype, designed, and fabricated using standard 0.13 μm CMOS technology. Experimental results show that the proposed CDC achieves a maximum resolution of 17 bits, which corresponds to a capacitive resolution of 5.4aF, while consuming only 146 μA from a 1.5 V power supply, with an effective area of 0.317mm².