Abstract: A semiconductor test instrument (12) provides an AC interrogating signal to an element under test, such as a single pin of an IC. Horizontal and vertical signals are developed and applied to a display (33) which produces an analog signature signal representative of the operating condition of the test element. The horizontal and vertical signals are converted to digital signals by an A-D converter (34) and stored in memory (39). An analog signature is also obtained from the same element as the test element but which is known to be good. The horizontal and vertical components of this signature are also converted into digital signals and stored in memory (42). The test and reference digital signals are then compared and those test digital signals which are different identified. The analog signatures corresponding to both the test and reference digital values are then reconstructed for display and superimposed, for viewing by an operator.