  In this paper, we present a programmable Ultra Low DC Current Source (ULCS)
developed at T\"UB\.ITAK UME. The output current range is from 1 fA up to 100
pA with 100 aA resolution and is directly traceable to DC voltage, capacitance
and time units. The principle of the device is based on applying a linear ramp
voltage on standard capacitors with values between 1 pF and 1000 pF. These
standard capacitors are commercial standard capacitors which are kept in a
temperature controlled insulated box. Linear ramp voltage is generated using a
commercial DAQ card. Current source device is fully automated with computer
control and can be used to generate any currents with 100 aA resolution in its
full range. The uncertainty is 2.5 mA/A at 1 fA current.
