PURPOSE: To measure the error of an A/D converter accurately without being affected by noise by providing a noise elimination circuit and a change point detection circuit obtaining the change point of an output data of the A/D converter.
CONSTITUTION: A noise elimination circuit 13 samples N1 sets of output data 19 of an A/D converter 12,eliminates K1 sets of data from the maximum value and the minimum value of sampled values and calculates a mean value of remaining (N1-2K1) sets of data. Then the mean value is determined as the output data of the A/D converter 12 to eliminate noise. A change point detection circuit 15 detects the number of points when a noise eliminating data 20 changes from '0' to '1', the noise elimination circuit 15 recognizes N2 pcs. of leading points, and averages the mean value of the remaining data after K2 pcs. of data from the maximum and minimum values of the N2 pcs. of data. Thus, the result of measurement not affected by noise is obtained from an error calculation circuit 16 and the evaluation quality of the A/D converter 12 is improved.