Low leakage current designs require special considerations. As described in Section 7.1.5, polarization of dielectric material and capacitors can have severe adverse effects on low leakage measurements. Even small electric potentials can polarize dielectrics enough to result in residual leakage current greater than the input bias current of the OPA928. Depending on the severity, the dielectric relaxation of insulators and dielectric absorption of capacitance at the input can make measurements unreliable for a prolonged time period.

Dielectric polarization can be created unintentionally in some common applications. In particular, the transimpedance amplifier configuration is prone to dielectric polarization. The dynamic range of interfacing sensors can vary widely, and a current input beyond the expected range can cause the output to slam to the supply rail. When the output is slammed, the amplifier is unable to maintain a virtual short and the high impedance node voltage increases significantly. The voltage increase not only causes current flow into the input, but also polarizes the material enough to create dielectric relaxation related leakage.

A diode clamp in the feedback path can be used to limit the output voltage swing and prevent the op amp from saturating. The leakage from the diode, however, can be quite large and is unusable in low leakage circuits. A better design can be made using the internal guard buffer of the OPA928. A Zener diode can be connected from the output to the guard, bypassing the high impedance node altogether as shown in Figure 7-16.

Figure 7-16 Improved Diode Limiter

During normal operation, the small leakage current from the Zener diode, I_Leakage, is handled by the internal guard buffer as shown in Figure 7-17. In the overrange condition shown in Figure 7-18, the Zener diode begins to conduct more current, I_R, and creates a voltage drop of across the 1kΩ resistor. The voltage that develops at the guard pin causes the internal protection diodes to conduct and source the remaining sensor current, I_SENSOR. The guarded diode limiter circuit regulates the voltage at the inverting node even during an overrange condition.

Figure 7-17 Guarded Diode Limiter During Normal Operation

Figure 7-18 Guarded Diode Limiter During Overrange