SBOS893D August 2018 – June 2020 INA821
The input impedance of the INA821 is extremely high (approximately 100 GΩ.) However, a path must be provided for the input bias current of both inputs. This input bias current is typically 150 pA. High input impedance means that this input bias current changes little with varying input voltage.
For proper operation, Input circuitry must provide a path for this input bias current. Figure 68 shows various provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds the common-mode range of the INA821 and the input amplifiers saturate. If the differential source resistance is low, the bias current return path connects to one input (as shown in the thermocouple example in Figure 68). With a higher source impedance, using two equal resistors provides a balanced input with possible advantages of a lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.