PURPOSE:To eliminate a digitizing error caused by an error of switching time of an integration current by applying high speed integration and low speed integration intermittently to switch the integration when no transient phenomenon takes place, counting the number of times of discharging in digitizing process. CONSTITUTION:A timing generating circuit 47 generates a sampling pulses PS with a pulse width t1 for a prescribed period to control a sampling switch SW1 of an integration circuit 1 to be turned on for a period t1. Thus, the electric charge stored in a capacitor CX is discharged in the unit of comparatively large electric charge and stepwise integration is applied. The number of times of the unit discharge is counted by a counter 51 in this case to decide a high- order m-bit in the digitizing process. The integration is executed in the unit discharge and the number of times of unit discharge is counted to digitize the analog input signal, then the integration time is not affected at all.
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