Electrometer calibration with sub-part-per-million uncertainty

We performed calibrations of four different commercial picoammeters using the Ultrastable Low-noise Current Amplifier (ULCA) as a calibrator current source operated in the range between 1 femtoampere and 1 microampere. The results allow the comprehensive characterization of the devices under test regarding noise, settling and burden voltage behavior as well as stability of the gain factor, and confirm the performance of the ULCA for use as small-current calibrator standard. Also, we present a further advanced setup for the calibration of transimpedance amplifiers. Accuracy limits for best electrometer calibrations in the current range between 1 femtoampere and 1 microampere and possible implications on corresponding calibration and measurement capabilities are discussed.