PURPOSE: To reduce the variation of an analog input voltage by providing a switch which stays off at least while the analog input voltage is sampled by the voltage comparators of a second group between the switch group of plural pieces of the switches and the reference voltage impressing terminal of the voltage comparators of the second group.

CONSTITUTION: All reference voltage supply routes connected to the voltage comparators A21∼A2(K-1) of the second group through the switch group E1∼EL are provided with the switches SW1∼SWK-1, which turn on or off complementarily to the switches, included in the voltage comparators A2 of the second group respectively. Because these switches SW stay off at least while the analog input voltage is sampled by the voltage comparators A2 of the second group, the capacity of the switch group E is not transmitted to the input terminal of one side of the voltage comparators A2 of the second group, i.e., the analog voltage impressing terminal 16, and the variation of input impedance is reduced. Thus, the variation of the analog input voltage comes small.