A 0.1-nW–1-$\mu$ W Energy-Efficient All-Dynamic Versatile Capacitance-to-Digital Converter

Abstract

A versatile, low-power, and energy-efficient capacitance-to-digital converter (CDC) for Internet-of-Things (IoT) is presented, based on an all-dynamic architecture with adaptable speed, resolution, and range. The proposed CDC includes a single-armed capacitive bridge and a differential switched-capacitor 10-b asynchronous successive approximation register (SAR) analog-to-digital-converter (ADC). The bridge output is directly sampled by the ADC through fully passive correlated-double-sampling (CDS) approach, which enables a fully dynamic operation. The design is fabricated in a 65-nm CMOS technology. Thanks to the dynamic nature of the CDC, sampling rates from 1 S/s up to 100 kS/s are supported and capacitances from 1.23 to 24.59 pF can be digitized, while the power scales inherently from 0.1 nW to $1~\mu \text{W}$ . Optionally, the range can be further extended to >100 pF, and oversampling can be used to enhance resolution. This makes the design versatile to efficiently deal with a variety of sensors having different speed and resolution requirements and different capacitance values. The 0.1 nW lowest absolute power is $> 20\times $ smaller than the prior art, and the figure of merit (FoM) from 18 to 59 fJ/conv-step is also the lowest among prior designs. To provide application examples, this chip is further verified with a microelectromechanical (MEMS) pressure sensor and a MEMS accelerometer. It can measure environmental pressure consuming only 0.8 nW at a speed of 100 S/s, and measure acceleration using 1.4 nW at a speed of 200 S/s.