The detection circuit for small differential capacitances can converts the differential capacitivevalues into electrical signals such as voltage or current, which is widely used in high-precision measuringinstruments such as micro-displacement detection and inertial sensors. Based on the principle of bridgedetection ,it designs a bridge detection circuit for small differential capacitances.First, the transfer function ofits AC amplifier circuit and wo-stage low-pass filter are measured ,and then conducts a calibration experimenton the sensitivity coeffcient of the circuit ,and finally based on the designed detection circuit, the differentialcapacitances single pendulum is made up as a pair of differential capacitances to study the voltage output of thedifferential capacitances single pendulum without the ball, with the ball but without excitation ,and with the balland excitation ,and then confirms the movement period of the pendulum through the frequency domain analysisAs an innovative physics experiment for undergraduates, this experiment could cultivate students’ practicalability ,and deepens the understanding of the related knowledge involved in the experiment such as mechanics.kinematics ,vibration and wave ,electromagnetism ,digital circuits and analog circuits.