An ultrastable low-noise current amplifier (ULCA) is presented. The ULCA is anon-cryogenic instrument based on specially designed operational amplifiers andresistor networks. It involves two stages, the first providing a 1000-foldcurrent gain and the second performing a current-to-voltage conversion via aninternal 1 MOhm reference resistor or, optionally, an external standardresistor. The ULCA's transfer coefficient is extremely stable versus time,temperature and current amplitude within the full dynamic range of +/-5 nA. Alow noise level of 2.4 fA/sqrt(Hz) helps to keep averaging times low at smallinput currents. A cryogenic current comparator is used to calibrate both inputcurrent gain and output transresistance, providing traceability to the quantumHall effect. Typically, within one day after calibration, the uncertaintycontribution from short-term fluctuations of the transresistance is below onepart in 10^7. The long-term stability is expected to be better than one part in10^5 over a year. A high-precision variant is available that shows asubstantially improved stability at the expense of a higher noise level of 7.5fA/sqrt(Hz). The ULCA also allows the traceable generation of small electricalcurrents or the calibration of high-ohmic resistors.
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