27.1 A 250kHz-BW 93dB-SNDR 4th-Order Noise-Shaping SAR Using Capacitor Stacking and Dynamic Buffering

The noise-shaping (NS) SAR is an emerging hybrid architecture that aims to combine the benefits of both SAR and ΔΣ ADCs [1-8]. The key in an NS SAR is the NS filter. As shown in Fig. 27.1.1, prior NS filter techniques can be classified into two types. The first way is to use a closed-loop amplifier-based integrator [1-3]. With sufficient gain of the amplifier, this type of integrator can realize a sharp noise transfer function (NTF). However, the high-gain multi-stage amplifier produces large noise, is power-consuming and unfriendly to technology scaling [2,3]. The second way is to use passive charge sharing to perform error feedback [4-6] or integration [7,8]. The fully passive filter avoids using closed-loop amplifiers, but it has signal attenuation issue and the resulting NTF is mild [4]. Also, due to the lack of effective gain, it suffers from large comparator noise. To improve the NTF and noise suppression capability, some works place an open-loop amplifier before the passive filter [5-7], or implement a passive gain after it [8]. However, the gain of an open-loop amplifier is sensitive to PVT variations, and the passive gain method suffers from severe parasitic effects when a large gain is required.