A low DC current source based on the method of charging a capacitor using a linear voltage ramp was built for the calibration of picoammeters and the metrological features of the setup were tested. The measurement uncertainty was improved by offset current compensation and a galvanic isolation between the personal computer and the ramp generator via opto-coupling. Using this setup, a low-current meter was calibrated in the range from 10 pA to 1 fA with an uncertainty of 3x10-5 to 6x10