Ultrastable low-noise current amplifier

Abstract

An ultrastable low-noise current amplifier (ULCA) i s presented. The ULCA is a non-cryogenic instrument based on specially designed operational amplifiers and resis tor networks. It involves two stages, the first pro viding a 1000-fold current gain and the second performing a current-to -voltage conversion via an internal 1 MΩ 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/ √Hz helps to keep averaging times low at small input currents. A cryogenic current co mparator is used to calibrate both input current ga in and output transresistance, providing traceability to the quan tum Hall effect. Typically, within one day after ca libration, the uncertainty contribution from short-term fluctuatio ns 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/ √Hz. The ULCA also allows the traceable