Abstract: A current sensing buffer 26 for driving a digital signal line of a unit under test 12 is composed of current mirror circuits in series with output switching transistors supplying test signals to the digital signal line. The current mirror circuits 66, 72 and 80, 84 generate sample currents corresponding to the magnitude of current through the respective drivers 68 and 78 to detect drive faults caused by bus or component failures. In buses comprised of several digital signal lines, one current sensing buffer 26 is used per line, and each curent sensing buffer 26 is controlled by a programmable circuit tester 10. The current sensing buffer 26 can be incorporated into an interface pod 14 which is substituted for a control component of a unit under test 12.

Abstract: In a recirculation of remainder type analog to digital converter, successive remainders recirculated to develop an output digital signal are overlapped to cancel common error components.

Abstract: The ladder network of a recirculation of remainder analog to digital converter has pairs of controlled switches connected alternately to a voltage source and a ground, to establish unidirectional current flow through the switches. Unidirectional current flow eliminates ladder error caused by current flow direction resistances inherent in the switches.

Abstract: A calibration circuit for a recirculation of remainder analog-to-digital (A/D) converter heuristically solves a multiple variable conversion equation. Input analog calibration and analog reference signals are compared in a comparator, and when the two signals have a predetermined relationship, the comparator generates an indicator signal. A microprocessor determines a first pattern of digital reference signals that, together with digital calibration signals generated by the microprocessor, causes the comparator to generate the indicator signal and a second pattern of digital reference signals that, together with the input analog calibration signals, causes the comparator to generate the indicator signal. The difference between the first and second patterns of digital reference signals is stored as conversion coefficients.