The input impedance of the INA819 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 very little with varying input voltage.

For proper operation, input circuitry must provide a path for input bias current. Figure 9-4 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 INA819, and the input amplifiers saturate. If the differential source resistance is low, the bias current return path can connect to one input (as shown in the thermocouple example in Figure 9-4). 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.

Figure 9-4 Providing an Input Common-Mode Current Path