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A self-actuating and detecting (SAD) circuit loop that drives a micro-cantilever in a gas sensor, in which the resonance frequency shifting of the beam is monitored to detect its mass change caused by adsorption of certain gas molecules, is presented. The loop cannot only drive the cantilever but also trace the fundamental resonance frequency shift of the beam. The motivation of the SAD circuit is simplifying structure and process of the microbeam. This paper presents simulation and experiments of the mechanical resonance of the microbeam and resonance of the electric signal in the SAD loop. The relations between the two resonance frequencies are discussed and analyzed. While the beam is driven in resonating mode, the resonance frequency shift of the signal in the SAD circuit is approximately equal to the mechanical resonant frequency shift of the cantilever. The frequency resolution of the SAD with the microbeam is 0.6-0.7 Hz in our work, and theoretical measurement range of the system is 18750 Hz.