We present a quantitative numerical analysis of dielectric microbead detection experiments with nanoribbons operated in the AC small signal regime. To this purpose, a comprehensive model of nanoribbon operation in electrolyte environment and with DC/AC signals is extended to include the effect of site-binding charges in DC, transient and AC conditions. The model is calibrated against DC measurements and pH-transients data from the literature. The impact of the microfluidic chamber size, the interconnects and the site-binding charge is investigated. The calibrated model suggests that the impedance response to micron-sized beads cannot be explained without also taking into account the bead surface charge.