SBOSA75B July 2021 – November 2021 INA823
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 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 RG. This causes a signal current through RG, 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 VBE) 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.