The INA823 is a monolithic precision instrumentation amplifier that incorporates a current-feedback input stage and a four-resistor difference-amplifier output stage. One of the features of an instrumentation amplifier (IA) is that the gain is set by placing an external resistor across the RG pins, as described in Section 8.3.1. The three-op-amp IA topology in the INA823 limits the maximum input voltage applied to the input terminal. The maximum input voltage depends on the common-mode voltage, differential voltage, gain, and the reference voltage; for more information, see Section 8.3.2. The INA823 also features protection at each input by two junction field-effect transistors (JFETs) that provide a low series resistance under normal signal conditions, and preserve excellent noise performance. When excessive voltage is applied, these transistors limit the input current, as described in Section 8.3.3.

The INA823 is developed for medical-sector applications such as infusion pumps (see Section 9.2.1), and industrial applications such as programmable logic controllers (see Section 9.2.2)

The schematic in Figure 8-1 shows how the INA823 operates. A differential input voltage is buffered by the input transistors, Q₁ and Q₂, and is forced across R_G. This causes a signal current through 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 threshold voltage of Q₁ and Q₂ (defined as V_BE) along with the voltage drop across R₁ and R₂ produce output voltages on A₁ and A₂, respectively, that are approximately 0.8 V less than the input voltages.