Analysis of Passive Charge Sharing-Based Segmented SAR ADCs

Abstract

This article presents the theoretical analysis of passive charge sharing-based segmented successive-approximation-register (SAR) analog-to-digital converter (ADC), where the precise reference source in a capacitive digital-to-analog converter (CDAC) is replaced by a capacitor that is β times larger than its bit capacitor and precharged to the reference level, known as a reference charge reservoir (RCR). A segmented SAR-ADC uses a coarse SAR-ADC to compute some most significant bits (MSBs). Four methods, namely aligned switching (AS) with bitwise RCRs, AS with a subsample-wise RCR, detect-and-skip aligned switching (DAS-AS) with bitwise RCRs, and DAS-AS with a subsample-wise RCR are introduced for setting fine MSBs. Closed-form analytic expressions of the reference error due to the finite reference capacitance are derived and validated by behavioral modeling and circuit simulation of an 11-bit 50 MS/s segmented SAR ADC in 65-nm CMOS technology. The error expressions can be used to select one of the four methods for setting the fine MSBs and to determine β for the required linearity or for implementing digital circuitry for precise error correction.