The proposed frequency-locked loop (FLL) utilizes the flat frequency response characteristics of the moving window filter (MWF) in closed-loop and adaptive sampling pulse adjustment for capacitance measurement. An operational amplifier (op-amp) based relaxation oscillator generates a square wave whose fundamental sine wave is extracted by MWF. Then, the fundamental sine wave is tracked by the FLL to estimate its frequency which is inversely proportional to the unknown capacitance. The FLL employs another MWF to track the center frequency and its variation due to capacitance. The MWF offers an almost flat frequency response around the center frequency in closed-loop. However, the small magnitude and phase errors observed in the flat frequency response had been corrected by adjusting the sampling pulses adaptively. Experimental investigation demonstrates the capabilities of the scheme for wider and accurate capacitance measurement.