Introduction
Accurate generation and precise measurements of low-currents below 100 pA are of great interest for the calibration of low-current detectors such as picoampermeters and electrometers [1]. A very common way to generate the sub-picoampere current range is based on the method of charging a capacitor C by a linearly varying voltage ramp dV/dt, I = C •dV/dt [1]-[3]. This voltage ramp technique limited by the voltage ramp and capacitor has many advantages in comparison with the high-value-resistor-based current sources, which are not suitable for the aforementioned current range due to diverse difficulties such as high voltage, temperature, humidity coefficients and low stability [3]. Therefore, a capacitor-based low-current source was formulated here. In addition, the results of the calibration of a commercial current meter and uncertainty analysis for the calibration setup are reported.