SLOA059B October 2022 – March 2023 OPA2991 , TLC2654 , TLC4502 , TLE2021 , TLV2721
Most of the parameters are measured using a servo loop. Figure 5-1 shows a simplified circuit. This test loop is used for the major DC parameter measurements. VOS is measured with switches S1 and S2 closed, essentially providing a very low source impedance to ensure that input bias current offsets are negligible during the measurement. The inverting input of op amp A1 controls the output of the device under test (DUT) through the feedback loop containing RF and the 50-Ω resistor. When S3 is closed, A1 drives the output voltage of the DUT to zero by applying the necessary voltage to the positive terminal. Thus, the voltage across the 50-Ω resistor is equal to VOS, and the output of A1 is (RF/50)VOS. RF is adjusted depending on the expected offset voltage of the DUT so that the output of A1 is not saturated, yet is easily discerned.