PURPOSE: To reduce the number of parameters which affects on the accuracy of an output signal, by using a capacitor which supplies plural input signals to the input of an operational amplifier in common.

CONSTITUTION: The output of input signal switches are connected in common, and a second capacitor 22 connected between those output and the input of the input switch of the operational amplifier is provided in common for plural printer signals. Second short-circuitted switches 13 and 14 which connect both terminals of the second capacitor to common potential, and a control circuit 61 which supplies a clock to a first short-circuitted switch 11, the second short- circuitted switches 13 and 14, the input switch 12 of the operational amplifier, and the input signal switches 15W17 and performs ON/OFF control on them are provided. In such a way, it is possible to reduce the number of parameters which affects on the accuracy of the output signal.