Figures-of-Merit to Evaluate the Significance of Switching Noise in Analog Circuits

Abstract

An analysis flow is proposed to determine the significance of induced (switching) noise in analog circuits. The proposed flow is exemplified through two commonly used amplifier topologies. Specifically, input-referred switching noise is introduced as the first figure-of-merit and compared with the well-known equivalent input device noise through analytic expressions. The comparison is achieved as a function of multiple parameters that characterize switching noise in the time domain (modeled as a decaying sine wave), such as peak amplitude, period, oscillation frequency within each period, and damping coefficient. The analytic expressions used to obtain input-referred switching and device noise are verified with SPICE simulations. These expressions are utilized to develop dominance regions for both noise sources. Furthermore, time-domain switching noise amplitude (at the bulk node) at which the input device and switching noise magnitude are equal (in the frequency domain) is determined as the second figure-of-merit, providing guidelines for the signal isolation process. Reverse body biasing is also proposed to alleviate the effect of switching noise by weakening the bulk-to-input transfer function as opposed to reducing the switching noise amplitude at the bulk nodes. It is demonstrated that this method has a negligible effect on primary design objectives of the victim circuit while reducing the input-referred switching noise by up to 10 dB. As a case study, the proposed flow is applied to a potentiostat circuitry where input sensitivity is of primary importance.