A 118.6fJ/Conversion-Step Two-Step Time-Domain RC-to-Digital Converter With 33nF/10MΩ Range and 53aFrms Resolution

Sensor readout ICs for internet-of-things (IoT) systems require not only high energy efficiency and resolution but also a wide input range to cover a variety of sensors with different output types and characteristics [1], [2]. Readout methods based on delta-sigma modulation (DSM) [3], [4] and two-step conversion (successive approximation + time-domain (TD) DSM) [5] have been proposed to achieve high resolution and energy efficiency, but these structures suffer from limited input ranges as they convert the sensor output to voltage, whose range is strictly constrained by the given supply rails. Alternatively, a TD readout method converts the sensor output into a TD signal, eliminating the range constraint [1], [2], [6]. However, its resolution is limited by the jitter performance of the oscillator of the counting clock [1]. This resolution issue can be addressed together with a wide input range by a dual-oscillator-based structure utilizing a large oversampling ratio (OSR) [2]. However, a power-hungry high-frequency reference oscillator (R-OSC) should continuously operate to reduce the quantization noise (Q-noise), degrading energy efficiency greatly. Although this structure can reduce both the Q-noise and random noise by sacrificing the readout time, the signal-to-noise ratio (SNR) per energy efficiency is limited by the performance of the oscillator itself. Here, we present a TD 2-stepconversion readout IC achieving a wide input range, high resolution, and high energy efficiency altogether.