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. V_OS 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 R_F 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 V_OS, and the output of A₁ is (R_F/50)V_OS. R_F is adjusted depending on the expected offset voltage of the DUT so that the output of A₁ is not saturated, yet is easily discerned.

Figure 5-1 Simplified Servo Loop Test Circuit