To make constant nonuniformity of signal outputs owing to the fluctua tion of a power source voltage by inputting an analog signal by means of syn chronization with conversion which continues by means of fixed cycle and permitting a high speed wide band amplifier to repeat sampling by means of the same condition every time.

When an analog input signal is inputted to the high speed wide band amplifier 1, response is instantaneously started and current consumption is rapidly increased. Therefore, the power source current of the high speed wide band amplifier 1 is increased so that the fluctuation of the power source voltage occurs by resistance 4 intrinsic to a wiring and inductance 5. In this case, a high speed AD converter 2 continuously repeats conversion by fixed cycle and the analog input signal is inputted by synthesization with conversion so as to repeat the same operation. The high speed wide band amplifier 1 repeates sampling by the same condition every time in spite of the fluctuation of the power source voltage by the fixed cycle sampling so that an analog signal output receives same imfluence every time and AD conversion with precision is executed.