Summing Node and False Summing Node Methods: Accurate Operational Amplifier AC Characteristics Testing without Audio Analyzer

This paper demonstrates the harmonic distortion measurement of operational amplifiers by applying our proposed summing node method; we show that it can provide low-cost and high-accuracy testing at the mass production shipping stage. Experiments show that measurement accuracy below -130 dBc is possible without expensive test equipment such as an audio analyzer. We show by theory, simulations, and experiments that the summing node method makes the measurement accuracy robust to the harmonics of the signal source providing the sinusoidal input signal to the operational amplifier under test. In other words, a high precision signal generator is not required. Furthermore, we propose the false summing node method, which does not require direct probing of the summing node, in order to avoid oscillation and instability of the video-band operational amplifier under test. Simulations and experiments verify that it can achieve accurate testing without probing of the summing node and it is robust against the ratio variation of the two resistors in the false summing node testing circuit.