Behavioral Modeling of Opamp Gain and Dynamic Effects for Power Optimization of Delta-Sigma Modulators and Pipelined ADCs

Abstract

This paper proposes a simple, yet accurate, analytical model for the effect of opamp gain and dynamics (slew rate and bandwidth) on the transfer function of switched-capacitor (SC) amplifiers and integrators. Furthermore, it demonstrates the detrimental effects of: a) the nonlinear variation in the opamp dc gain; and b) the feedforward transmission of the feedback capacitor, on the harmonic distortion and settling behavior of these SC stages. These effects, typically ignored in the behavioral simulations of SC stages, are analyzed and modeled. Thus, accurate behavioral simulations of ΔΣ modulators or pipelined analog-to-digital converters (ADCs) can be performed in SIMULINK, using the proposed models for their SC building blocks (integrators or amplifiers). The proposed behavioral models are validated in HSPICE. Behavioral simulation examples are presented to illustrate the importance of such accurate modeling for low-power design.