All materials are prone to polarization in the presence of an electric field. The molecules of the given material within the electric field become aligned at varying rates; a phenomena known as polarization. The rate depends on the strength of the electric field and the susceptibility of the material. When the electric field is removed, the molecules in the material return to the original alignment and random distribution, a phenomena known as relaxation. The rate at which the molecules return to normal alignment depends on the permittivity and resistivity of the material. In conductors, polarization and relaxation happens nearly instantaneously. In dielectrics, the time delay for polarization and relaxation can be significant.

In most applications, dielectric relaxation is not a major design concern. However, for femtoampere leakage current, dielectric relaxation becomes a major concern. The realignment of molecules causes a small displacement current to appear across the material. The displacement current from the dielectric relaxation is often greater than the input bias current level of the OPA928. The time required for the displacement current in common FR-4 PCB materials to dissipate under the input bias current level of the OPA928 can take well over an hour. The ingression of moisture into the dielectric material can significantly increase the relaxation time. To minimize the dielectric relaxation time and the leakage effects, use ceramic-based PCB materials such as Rogers 4350B and consider implementing a baking process to remove excess moisture.