3.4 A 0.01mm2 25µW 2MS/s 74dB-SNDR Continuous-Time Pipelined-SAR ADC with 120fF Input Capacitor

The kT/C noise poses a fundamental SNR limit for discrete-time (DT) ADCs with front-end sampling operations. To achieve a high resolution, the sampling capacitor needs to be sufficiently large (e.g., 3pF for 80dB SNR assuming $2\mathrm{V}_{pp}$ swing) and it has to be quadrupled for every 1b increase in the resolution. A large sampling capacitor poses critical challenges for both the ADC input buffer and the reference buffer (Fig. 3.4.1). To meet the stringent linearity requirement of high-resolution ADCs, these buffers consume a significant amount of power, which can be comparable or even higher than the ADC itself. In addition, a large capacitor makes it difficult to design the ADC front-end S/H circuit, leading to considerable power/area costs and increased design complexity.