1.55mW 2GHz ERBW 7b 800MS/s 3-stage Pipelined SAR ADC in 28nm CMOS using a Kickback-Cancelling 7T-Dynamic Residue Amplifier with only 16fF Input Capacitance

Abstract

Achieving higher effective resolution bandwidth (ERBW) beyond Nyquist frequency is a key design requirement for the slice ADC design in a time-interleaved ADC (TIADC). While SAR ADC is a popular choice for a low-power ADC in advanced CMOS processes, the input capacitance $(\mathrm{C}_{in})$ presented by the front-end capacitive DAC (CDAC) to the ADC input limits achievable signal bandwidth. In contrast, pipelined SAR ADCs has more freedom in choosing C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</inf> because the resolution of the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> stage CDAC can be much lower than the total resolution. Therefore, it is possible to reduce C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</inf> to the thermal noise limit without being limited by the minimum unit capacitance. The downside of the pipelined SAR ADCs is the necessity of a residue amplifier, which often dominates the total power consumption. One can consider using a dynamic amplifier (DA) as a residue amplifier because achieving both high speed and low power is possible when the desired gain is modest [1–2]. Being an open-loop and fully-dynamic, however, the DA suffers from the gain inaccuracy and is vulnerable to the kickback noise. Furthermore, the gain varies significantly over process and temperature. [3] and [4] attempt to solve this issue by temperature-tracking bias but such compensation method requires off-chip and temperature-dependent voltage or resistor to tune the process uncertainty.