Sampling a waveform in an IC device to which a repeating test pattern is applied, includes the steps of: a) defining a portion of the test signal containing a feature of interest; b) applying a sampling signal to an AFM device adjacent a surface of the IC device at a predetermined point during the portion for a series of consecutive repetitions of the test signal pattern, the sampling signal having substantially shorter duration than the feature of interest; c) measuring deflection of a cantilever in the AFM device on application of the sampling signal; and d) determining the voltage at the predetermined point from the measured deflection of the cantilever. The steps can be applied at several points in the portion of interest and the measurements integrated and displayed.