An input analog signal is fed to an amplifier circuit and an AD converter circuit. The AD converter circuit converts the input analog signal into a digital value of a predetermined number of bits, and outputs the digital value to an encoder (not shown) A DA converter circuit converts the digital value of a predetermined number of bits output from the AD converter circuit into an analog signal. The amplifier circuit samples and amplifies the input analog signal by a factor of .alpha. (greater than 1). A subtracter circuit subtracts an output of the DA converter circuit amplified by a factor of .alpha. from an output of the amplifier circuit.