8.3.6 Input Bias Current Return Path

The INA827 input impedance is extremely high—approximately 20 GΩ. However, a path must be provided for the input bias current of both inputs. This input bias current is typically 35 nA. High input impedance means that this input bias current changes very little with varying input voltage.

Input circuitry must provide a path for this input bias current for proper operation. Figure 58 shows various provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds the INA827 common-mode range, and the input amplifiers saturate. If the differential source resistance is low, the bias current return path can be connected to one input (as shown in the thermocouple example in Figure 58). With higher source impedance, using two equal resistors provides a balanced input with possible advantages of lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.

Figure 58. Providing an Input Common-Mode Current Path