A/D CONVERSION CIRCUIT

PURPOSE:To prevent fault of a servo mechanism or the like by providing an error detection circuit detecting an output of plural level discrimination signals from a change point detection circuit to a parallel comparison A/D conversion circuit so as to make a digital output signal outputted from the A/D conversion circuit invalid selectively. CONSTITUTION:If two or more of level discrimination signals e1...en go to a high level, a high level error detection signal ED is outputted to a digital signal processing unit DSP. In this case, the digital signal processing unit DSP makes the digital output signals D0...Di outputted from the A/D conversion circuit invalid and uses the preceding sampling data as it is. Even when the output signal of voltage comparator circuits VC1...VCn is changed stepwise through the superimposition of a pulse noise onto the power voltage, input reference voltage or the input analog signal Vin, the control by the digital signal processing unit DSP is suppressed and the malfunction of the servo mechanism is prevented.