SBOS893D August 2018 – June 2020 INA821
The INA821 is a monolithic precision instrumentation amplifier that incorporates 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 Q1 and Q2 and is forced across RG, which causes a signal current to flow 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 VBE and voltage drop across R1 and R2 produces 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.