8.1 Overview

The INA818 is a monolithic, precision instrumentation amplifier incorporating a current-feedback input stage and a four-resistor difference amplifier output stage. The functional block diagram in the next section shows how the differential input voltage is buffered by transistors Q₁ and Q₂ and is forced across resistor R_G, which causes a signal current to flow through resistors R_G, R₁, and R₂. The output difference amplifier, A₃, removes the common-mode component of the input signal and refers the output signal to the REF pin. The V_BE and voltage drop across R₁ and R₂ produce output voltages on A₁ and A₂ that are approximately 0.8 V lower than the input voltages.

Each input is protected by two field-effect transistors (FETs) that provide a low series resistance under normal signal conditions, and preserve excellent noise performance. When excessive voltage is applied, these transistors limit input current to approximately 8 mA.