An instantaneous differential non-linearity DNL can be determined with a high accuracy with a reduced volume of computation and independently from a testing frequency while allowing an evaluation of factors in a compounded fault. A sine signal is applied to an AD converter 14 under test, a conversion output of which is divided into a sine component and a cosine component, with local maxima or minima aligned with each other. A square sum of the individual samples is formed, and a square root of the square sum is formed to determine an instantaneous amplitude (21). The amplitude of the sine wave signal is interleaved into a series of instantaneous amplitudes (20), and a first stage of the wavelet transform (46) is applied to the interleaved series of instantaneous amplitudes, with its output being oversampled to perform a second stage of wavelet transform (46′). A maximum amplitude of the transform output is detected by peak detector 23′. A detected value is used to estimate the DNL.