Summing Node Test Method: Simultaneous Multiple AC Characteristics Testing of Multiple Operational Amplifiers

This paper proposes a summing node test method for the operational amplifier and shows the followings: (i) It can be used for parallel testing of multiple AC characteristics (such as open loop gain (AOL), PSRR and CMRR) of one operational amplifier simultaneously with the equivalent accuracy but much faster compared to the NULL method. Also it can measure them even for multiple operational amplifiers at the same time. (ii) It can measure THD, SNR and THD+N of the operational amplifier with the comparable accuracy to the audio analyzer usage case, by applying proper analog filters. In other words, it can measure them with remarkable accuracy at very low cost. These have been verified with simulations and experiments. The proposed summing node test method uses an inverting operational amplifier under test and its negative input is amplified by an auxiliary non-inverting operational amplifier. The input and power supply voltages for the operational amplifier under test are modulated by AC signals with different frequencies. The auxiliary amplifier output is digitized after analog filtering and FFT is performed to the digitized data. This proposed method can reduce operational amplifier test time with good accuracy but without expensive instruments at mass production shipping, to meet the requirements for IoT and automotive as well as audio applications.