Ultrastable low-noise current amplifier
  An ultrastable low-noise current amplifier (ULCA) is presented. The ULCA is a non-cryogenic instrument based on specially designed operational amplifiers and resistor networks. It involves two stages, the first providing a 1000-fold current gain and the second performing a current-to-voltage conversion via an internal 1 MOhm reference resistor or, optionally, an external standard resistor. The ULCA's transfer coefficient is extremely stable versus time, temperature and current amplitude within the full dynamic range of +/-5 nA. A low noise level of 2.4 fA/sqrt(Hz) helps to keep averaging times low at small input currents. A cryogenic current comparator is used to calibrate both input current gain and output transresistance, providing traceability to the quantum Hall effect. Typically, within one day after calibration, the uncertainty contribution from short-term fluctuations of the transresistance is below one part in 10^7. The long-term stability is expected to be better than one part in 10^5 over a year. A high-precision variant is available that shows a substantially improved stability at the expense of a higher noise level of 7.5 fA/sqrt(Hz). The ULCA also allows the traceable generation of small electrical currents or the calibration of high-ohmic resistors. 
Dietmar Drung, Christian Krause, Ulrich Becker, Hansj\"org Scherer and   Franz Josef Ahlers
physics.ins-det