A 0.6-V 13-bit 20-MS/s Two-Step TDC-Assisted SAR ADC With PVT Tracking and Speed-Enhanced Techniques

Abstract

This article presents a low power-supplied 13-bit 20-MS/s time-to-digital converter (TDC)-assisted successive approximation register (SAR) analog-to-digital converter (ADC). In this hybrid architecture, the voltage-to-time converter (VTC) and TDC realize an inherent process, voltage, and temperature (PVT) robustness by inner tracking, thus inducing no extra power and circuit overheads. The voltage-domain and time-domain speed-enhanced techniques accelerate the first- and second-stage ADC conversions under a low power supply, respectively. Furthermore, in cooperation with a detect-and-skip switching scheme in the SAR ADC and an offset bit-shifting scheme in the two-step TDC, the ADC achieves linearity of 13 bit. The prototype ADC was fabricated in a 65-nm CMOS process with a 0.6-V power supply, achieving a 71.0-dB signal-to-noise and distortion ratio (SNDR) and an 89.5-dB spurious-free dynamic range with a Nyquist input at 20 MS/s, while exhibiting a Schreier figure-of-merit (FoM) of 181.9 dB. The ADC presents a sub-1-dB SNDR drop across a temperature range of −50 to 90 °C and ±5% power-supply variation.