The input impedance of the INA851 is very high at approximately 1 GΩ. However, a path must be provided for the input bias current of both inputs. This input bias current is typically 5 nA. 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 9-1 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 INA851, and the input amplifiers saturate. If the differential source resistance is low, the thermocouple example in Figure 9-1 shows that the bias current return path can connect to one input. With a higher source impedance, using two equal resistors provides a balanced input with the possible advantages of a lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.

For more details about why a valid input bias current return path is necessary, see the Importance of Input Bias Current Return Paths in Instrumentation Amplifier Applications application report.

Figure 9-1 Providing an Input Common-Mode Current Path