Subpicoampere current source based on a digital-to-analog converter
A current source based on a voltage ramp generated by a 16-bit digital-to-analog converter and a differentiating capacitor has been developed in MIKES. The system is suitable for generating currents from 1 fA to 100 pA. Standard deviation of average dU/dt:s of 70 ramps during an overnight measurement is as low as 1 ppm.
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).
Application of Single Electron Tunneling: Precision Capacitance Ratio Measurements
A metrological application is reported of the single electron tunneling (SET) phenomena: a precise measurement of the ratio of two cryogenic capacitors. The measurement used a superconducting SET electrometer as the null detector for a capacitance bridge. A 3‐ppm level of imprecision has been achieved in the measurement of the capacitance ratio from 100 to 1000 Hz. Further improvements can be made in the attempt to obtain an imprecision of 10−8 at lower frequencies, sufficient for the metrological measurement of capacitance or the fine‐structure constant using a SET pump.