High Linearity Front-End Circuit for RF Sampling ADCs with Nonlinear Junction Capacitor Cancellation

Abstract

This paper presents a high linearity front-end circuit for RF sampling ADCs, including an input buffer and a sampling network. The input buffer uses a two-stage NMOS cascode structure and is powered by a separate LDO to support a larger signal swing input with high power supply rejection (PSR) and linearity. We use bootstrap switch with bulk-switching techniques to ensure sampling linearity, while a feed-through compensation technique with self-cancellation of nonlinear junction capacitor is applied to achieve better performance at high-frequency inputs. The above techniques are validated at a 1GS/s ADC in 65nm process, and the simulation results show that the low-frequency PSR of the input buffer reaches over 120dB, and the SNR, SNDR and SFDR of the overall front-end circuit are 78.74dB, 72.37dB and 75.37dB at 2GHz frequency 1.6Vpp input. The −3dB bandwidth of the front-end circuit achieves 4.4GHz.