A circuit for measuring a passive component includes a first comparator, a second comparator, a third comparator and a counter. The first comparator compares a first voltage and a first reference voltage to generate a first comparison signal. The second comparator compares a second voltage and a second reference voltage to generate a second comparison signal. The third comparator compares a third voltage and a third reference voltage to generate a third comparison signal. The counter counts the number of pulses according to the third comparison signal. The number of pulses corresponds to time for the third voltage to drop to the third reference voltage, and the first comparison signal and the second comparison signal are used to control the drop of the third voltage.