This paper deals with the image noise behavior due to photon fluctuation in a CT system based on RC low-pass filter for projection data acquisition. The system configuration considered consists of a stationary detector ring and a rotating x-ray source with convolution-based detector fan image reconstruction. The current generated in individual detectors by transmitted photons during x-ray source rotation is fed into a RC filter and its output is sampled at regular time intervals matched to a basic spatial sampling distance. A projection ray sample is formed by either one or multiple time samples depending upon scan speed. An analytical model of photon noise propagation has been established for this system that characterizes its noise behavior and compares the behavior to the case with active integrators. In general, RC sampling introduces positive correlation between basic time samples, and in turn, between projection samples. This results in varying noise behavior for different scan speeds under a constant dose. The noise behavior is also affected by the reconstruction filters through the positive or negative correlations, which depend on the filter coefficients. Basically, it acts as a smoothing hardware filter which suppresses high frequency noise at an early stage of data acquisition. The impact of RC filtering on spatial resolution is also mentioned.