PURPOSE: To extract a very small change in analog data.

CONSTITUTION: A minimum value MIN and a maximum value MAX of analog data SA inputted within one period of a clock C1 are detected. An adder 17 subtracts the minimum value MIN from the maximum value MAX to calculate a dynamic range DR. It is fed to a ROM 20. The ROM 20 outputs a multiplication coefficient K1 inversely proportional to the dynamic range DR and it is multiplied with input data SB subject to level shift at a multiplier 19. The data SB are converted into digital data SD at an A/D converter 23 and the digital data are outputted. That is, when the amplitude of the input analog data SA is small, since the multiple coefficient K1 is increased, a change in the input analog data SA with a small amplitude is extracted. Since the dynamic range DR is A/D-converted in the unit of one clock in the A/D converter 10, the response is very fast and quantization error is prevented.