of course the same purpose can be performed by adding the circuits 28 30 to the first example since the level detector circuit 23 is made up of some operational amplifiers it is expected that some offset voltage is superimposed on the detected lever s1 this offset voltage makes stricter the tolerance for the input output characteristics of the expansion circuit at low levels of signal to prevent this the level offset correcting circuit 28 is provided in the present example as shown in fig1 this level offset correcting circuit 28 controls the reference voltage vbot applied to the bottom side of the a d converter 35 by means of digital data coff to change the reference voltage vbot by the offset voltage thus canceling the offset voltage it is possible to know whether the offset voltage has been canceled by reading the level of the offset check output toff from the nor gate circuit 29 i e judging whether the level is high or low more specifically the input to the level detector circuit 23 is removed under this condition the digital data coff to the level offset correcting circuit 28 is changed in a stepwise fashion while monitoring the digital control data sd i e the level of the offset check output toff if the.