Accurate subpicoampere current source based on a differentiating capacitor with software-controlled nonlinearity compensation

A high accuracy current source has been developed for the generation of very small currents (below 200 pA) based on applying a linear voltage ramp to a capacitor. Using software-controlled compensation for the nonlinearity of the voltage ramp, a stable current can be reached within a few seconds. A voltage ramp within typically 10/spl middot/10/sup -6/ of any desired value between less than 1 mV/s and several hundreds of mV/s can be realized. Using a high quality air-dielectric capacitor, currents below 200 pA can be generated with an uncertainty of 20 /spl mu/A/A (k = 2), with a lower limit of 2.0 aA (k = 2).