The setup of the current source was built on the basis of the same principle described in Ref. 1. The core element in this setup is an analogue-based voltage ramp generator, in which the non-linearity of a real capacitor in the circuit caused by dielectric absorption and leakage resistance is compensated by an analog feedback network. The integrator can generate a stable voltage ramp from -10 V to +10 V within 2000 s and therefore has a fixed voltage slope of 9.93 mV/s +/- 83 ppm. The variation of the voltage
measured current ID(t) is obtained by a DUT using a capacitor with an offset current of 1.47 pA (generated nominal current: 100 fA). Essentially, the offset