Abstract: The measurement of substrate noise waveforms in mixed-signal integrated circuits is described in this paper.This method uses voltage comparators as on-chip noise detectors.From the influence of substrate noise on the comparator's state,the noise can be measured statistically.The substrate noise waveform can be reconstructed by using the measured results.The block diagram of a test system is designed.The formula of the equivalently sampled noise voltage and the test error are deduced. The experimental result through SPICE simulation is given based on the experimental circuits set up along with their substrate models.