PURPOSE: To constitute an input circuit where the amplitude of an input signal of a switched capacitor filter is made constant for an optional time within an optional switching period by constituting the input circuit with an on/off switch and a two-terminal pair circuit so as to bring the impedance viewed from a terminal pair B to zero or nearly zero.

CONSTITUTION: When the on/off switch SW is turned on, a capacitive element C is charged until its terminal voltage is equal to an input voltage VIN. Thus, an output voltage of an operational amplifier, that is, the input voltage VIN of the switched capacitor filter is equal to the input voltage VIN. When the on/off switch SW is in OFF state next, the terminal voltage of a capacitive element L is kept to the final value of the voltage across the capacitive element according to the law of conservation of electric charge. The capacitance of the capacitive element L required in the operation above may be the minimum value of each capacitor constituting the switched capacitor filter.