The resistance of insulators is substantially affected by both temperature and humidity. Humidity can significantly lower the effective resistance of insulators and cause an increase in leakage current around the affected material. When water molecules settle on the surface of a given material, such as the plastic packaging and PCB, a parallel conductive path is created. Effective guarding techniques can help mitigate this behavior in sensitive applications.

In some cases, water molecules can also penetrate the surface of a given material. The water molecules in the material increase the conductivity of the body of the material and a reduction of resistance is established across all adjacent nodes. Contrary to surface level leakage paths, leakage through the material cannot be mitigated with guarding techniques. Use PCB materials with low humidity absorption properties to reduce moisture related errors.

Figure 7-6 shows the input bias performance of the OPA928 across temperature under different levels of humidity. Relative humidity is inversely proportional to temperature. The temperature range for high relative humidity levels is limited to maintain reliable measurements.

Figure 7-6 Input Bias Current vs Temperature at Various Levels of Relative Humidity

The measurements were made using a calibrated humidity chamber and an air wired circuit. Although air wiring the input pins eliminates many potential leakage paths, the feedback components remain sensitive to large changes in relative humidity. External components are typically the dominant source of leakage current when compared to the input bias current of the OPA928. To achieve outstanding input bias current performance, use low leakage components such as PTFE or polypropylene capacitors.

Moisture absorption and desorption is a strong function of both time and temperature. Baking is demonstrated to be an effective method of removing excess moisture. Baking time depends on several variables, including PCB and component material and bake temperature. If a baking procedure is implemented, a one-hour bake at 125°C is a good starting point.