A Survey of Voltage-Controlled-Oscillator-Based $\mathrm{\Delta }\mathbf{}\mathrm{\Sigma }$ ADCs

The benefits of technology scaling have fueled interest in realizing time-domain oversampling ($\mathrm{\Delta }\mathsf{}\mathrm{\Sigma }$) of Analog-to-Digital Converters (ADCs). Voltage-Controlled Oscillators (VCO) are increasingly used to design $\mathrm{\Delta }\mathsf{}\mathrm{\Sigma }$ ADCs because of their simplicity, high digitization, and low-voltage tolerance, making them a promising candidate to replace the classical Operational Transconductance Amplifier (OTA) in $\mathrm{\Delta }\mathsf{}\mathrm{\Sigma }$ ADC design. This work aims to provide a summary of the fully VCO-based $\mathrm{\Delta }\mathsf{}\mathrm{\Sigma }$ ADCs that are highly digital and scaling-friendly. This work presents a review of first-order and high-order VCO-based $\mathrm{\Delta }\mathsf{}\mathrm{\Sigma }$ ADCs with several techniques and architectures to mitigate the nonidealities introduced by VCO, achieving outstanding power efficiency. The contributions and drawbacks of these techniques and architectures are also discussed.