Errors in Low Current Measurements (1)

One of the most common causes of error when measuring low currents (<1nA) is offset current, which can come from the test setup or the measuring instrument.

Potential Cause: Insulating Material

Current can leak through an insulating material or over its surface. The insulating material may itself store or generate charge.

Remedies

A. Choose a good insulator

Several properties are important when evaluating an insulator material:

1. Volume Resistivity—Leakage of current directly through the material.
2. Surface Resistivity—Leakage across the surface, a function primarily of surface contaminants.
3. Water Absorption—Leakage dependent on the amount of water that has been absorbed by the insulator.
4. Piezoelectric or stored charge effects— The creation of charge unbalances (and thus current flow) or voltage shift due to mechanical stress.
5. Triboelectric effects—The creation of charge unbalance due to frictional effects when materials rub against each other.
6. Dielectric Absorption—The tendency of an insulator to store/release charge over long periods of time. For a listing of common insulating materials and their characteristics, see the Keithley Low Level Measurements handbook, Section 2.2.2.

B. Keep the insulator clean

Oils and salts from the skin can degrade insulator performance, and contaminants in the air can be deposited on the insulator surface, reducing its resistance. Clean the insulator with a cotton swab dipped in methanol. After cleaning allow the insulator to dry for several hours at low humidity before use, or dry it with dry nitrogen gas.

C. Guard the insulator

Use a driven guard that surrounds the insulator; a guard that surrounds the entire high impedance area is even better.