A 0.5–1.1-V Adaptive Bypassing SAR ADC Utilizing the Oscillation-Cycle Information of a VCO-Based Comparator

Abstract

A successive approximation register (SAR) analog-to-digital converter (ADC) with a voltage-controlled oscillator (VCO)-based comparator is presented in this paper. The relationship between the input voltage and the number of oscillation cycles (NOC) to reach a VCO-comparator decision is explored, implying an inherent coarse quantization in parallel with the normal comparison. The NOC as a design parameter is introduced and analyzed with noise, metastability, and tradeoff considerations. The NOC is exploited to bypass a certain number of SAR cycles for higher power efficiency of VCO-based SAR ADCs. To cope with the process, voltage, and temperature (PVT) variations, an adaptive bypassing technique is proposed, tracking and correcting window sizes in the background. Fabricated in a 40-nm CMOS process, the ADC achieves a peak effective number of bits of 9.71 b at 10 MS/s. Walden figure of merit (FoM) of 2.4–6.85 fJ/conv.-step is obtained over a wide range of supply voltages and sampling rates. Measurement has been carried out under typical, fast-fast, and slow-slow process corners and 0 °C–100 °C temperature range, showing that the proposed ADC is robust over PVT variations without any off-chip calibration or tuning.