A 12.5-MHz Bandwidth 77-dB SNDR SAR-Assisted Noise Shaping Pipeline ADC

Abstract

This article presents a successive approximation register (SAR)-assisted noise-shaping (NS) pipeline architecture which breaks the speed bottleneck of the existing SAR or SAR-assisted-type NS analog-to-digital converters (ADCs). Rather than only for residue amplification and pipeline operation, the multiplying digital-to-analog converter (MDAC) is also reused as unity buffer and analog adder to realize the NS with error feedback (EF) structure in this design. While incorporating the proposed alternative loading capacitor (ALC) technique, an ideal first-order noise transfer function (NTF) is realized without additional feedback phase and only with a small analog circuit overhead. Unlike other NS SAR ADCs that involved amplification, the inter-stage gain attenuates the noise from the second-stage comparator, thus leading to both high speed and resolution. Fabricated in a 65-nm CMOS process, the prototype achieves a signal-to-noise-and-distortion ratio (SNDR) of 77.1 dB over 12.5-MHz bandwidth (BW) with only over-sampling ratio (OSR) of 8. Under a 1.2-V supply voltage, the ADC consumes 4.5 mW and exhibits a Scherier figure of merit (FoM) of 171.5 dB.