picoammeter

A picoammeter is an electronic instrument designed to measure very small currents, typically in the picoampere (10^-12 A) range, and sometimes extending into femtoamps with specialized equipment. It is a specialized form of electrometer or current amplifier and is optimized for low leakage, low input bias, and high stability.

Most picoammeters employ a transimpedance amplifier configuration: the input current flows into the inverting input of an operational amplifier through an extremely high-value feedback resistor. The amplifier maintains a virtual ground at its input, converting I_in into a voltage V_out ≈ I_in × R_f. This high gain makes the device sensitive enough to detect minute currents. To minimize error sources, the input is guarded and shielded, and the front end uses low-leakage components and, in many cases, a chopper or auto-zero scheme to reduce offset drift.

Measurement ranges typically cover from tens of femtoamps up to a few nanoamps or more, with a corresponding change in gain settings or feedback resistance. Noise, input bias current, leakage paths, and temperature can affect accuracy. Calibration is performed with known current sources or calibrated resistors, and results are traceable to standards.

Applications include characterizing photodiodes and solar cells, measuring leakage currents in dielectric materials or integrated circuits, and experiments in solid-state physics and vacuum electronics where currents are extremely small. Picoammeters may be standalone instruments or integrated into electrometers or semiconductor test systems, and they are often used in conjunction with shielding, guarding, and careful circuit layouts to minimize parasitic currents.