Novel Digital Voltage Ramp Generator for Use in Precision Current Sources in the Picoampere Range

We report on the development of a novel highly linear voltage ramp generator to be used for the traceable calibration of picoamperemeters. The generator is based on two digital-to-analog converters, one of them being used to compensate for the differential nonlinearity of the other one. The generator is completely controllable by a computer, its voltage slope dV/dt is adjustable between 1 and 1000 mV/s. During a ramp running between -10 and +10 V, the slope shows a relative variation of only 1.3 middot10^-5 (relative standard deviation). Due to a small output filter time constant of only 10 ms, we are, for the first time, able to conduct dynamic measurements of picoamperemeters with no sacrifice of linearity.