Abstract: Internal servo loop circuitry is included on the same chip (10C) with an ADC (10B). Automatic test equipment (12) operates with the internal servo loop circuitry and external servo loop circuitry (1B) to test the ADC. The internal servo loop circuitry includes a target register (14), a digital comparator (18), and a crossover counter (22). An integrator (32) responsive to the digital comparator (18) produces an input signal (Vin) to the ADC, which generates a corresponding digital output sample (Dout). The comparator compares the output sample with a target code in the target register and causes the direction of the input signal to reverse each time the digital output sample crosses over the target code. The counter causes a voltmeter to measure a transition voltage value of the input voltage after a predetermined number of crossovers.