The typical input bias current of the OPAx397 is extremely low (typically 10fA). Input bias current is dominated by leakage current from the ESD protection diodes, which is proportional to the area of the diode. The OPAx397 is able to achieve ultra-low input bias current as a result of modern process technology and advanced electrostatic discharge (ESD) protection design that minimizes the area of the diode.

In overdriven conditions, the bias current can increase significantly. The most common cause of an overdriven condition occurs when the operational amplifier is outside of the linear range of operation. When the output of the operational amplifier is driven to one of the supply rails, the feedback loop requirements cannot be satisfied and a differential input voltage develops across the input pins. This differential input voltage results in the forward-biasing of the ESD cells. Figure 6-1 shows the equivalent circuit.