A 72-dB SNDR 130-MS/s 0.8-mW Pipelined-SAR ADC Using a Distributed Averaging Correlated Level Shifting Ring Amplifier

Abstract

This article presents a 14-b 130-MS/s two-stage pipelined-SAR analog-to-digital converter (ADC) using a distributed averaging correlated level shifting (DACLS) ring amplifier as its residue amplifier (RA). Compared to the prior CLS and ACLS techniques that reduce the RA gain error due to their finite open-loop gain, the proposed DACLS ring amplifier no longer requires an extra level-shifting capacitor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{\mathrm {LS}}$ </tex-math></inline-formula> ) at the RA output, such that the RA bandwidth can be improved. Furthermore, instead of a single-level shift in the prior arts, the DACLS ring amplifier provides an option for multiple level shifts and thereby accomplishes a greater RA gain error reduction. In addition, a customized bypass-window scheme is applied, which can skip some power-wasting digital-to-analog converter (DAC) switchings by window detection and thus reduces the power consumption of the ADC. To reduce the bit-cycling time of the SAR conversion, a delay-reduced (DR) SAR logic is introduced to increase the ADC speed. The ADC chip is fabricated in 28-nm CMOS technology and occupies an active area of 0.013 mm2. At 130-MS/s and the Nyquist-rate input frequency, the measured SNDR is 72.5 dB, while the power consumption is only 0.82 mW. Without any calibration, the proposed ADC achieves a Walden and a Schreier figure-of-merit (FoM) of 1.8 fJ/conversion step and 181.5 dB, respectively. Compared to the prior-art ADCs with an input bandwidth ≥ 40 MHz and a SNDR > 68 dB, this work shows the best FoMs.