A detailed time-domain analysis of noise due to thermal, 1/f and shot noise sources in CMOS active pixel sensors (APS) based on 3T and 4T pixel design is performed. We suggest that the conventional noise analysis based on the frequency domain, usually used for noise calculations of APS's cannot be strictly applied, because the switched APS circuitry under study cannot be represented as a linear time-invariant system. To calculate explicit noise expressions for noise performance we, therefore, resort to time-dependent circuit models and perform time-domain noise analysis, taking into account the stationary nature of the various noise processes. To accomplish that, we divide the operation cycle of APS to separate time phases, and in these periods of time the relevant circuits can be modeled as linear time-invariant systems represented by simple (time-domain) impulse response functions. Finally, we present and compare between the noise simulation and noise measurement results for both 3T and 4T pixels. The excellent correspondence between the analytic expressions and the measured results corroborate the validity of the theoretical results. (C) 2009 Elsevier Ltd. All rights reserved.