Abstract: An improved testing circuit is provided which includes a pair of comparator circuits each of which is disposed for comparing two minimum threshold amplitudes of responses from a device under test, and a multiplexer circuit coupled to the outputs of the pair of comparator circuits for switching between the outputs of the pair of comparator circuits. The use of a pair of comparator circuits and a multiplexer circuit allows continuous operation of the comparators thereby providing accurate testing at high speeds of operation.The improved testing circuit of this invention also includes a skew adjustment circuit which provides for adjustment of the leading and trailing edges of a test stimuli signal from one testing circuit with respect to the leading and trailing edges of test stimuli signals from other testing circuits incorporated in a circuit testing system.