Ultrastable low-noise current amplifier

TL;DR

The paper introduces an ultrastable, low-noise current amplifier (ULCA) that offers high stability, low noise, and traceability for precise current measurements and resistor calibration, suitable for quantum standards and metrology.

Contribution

It presents a novel ULCA design with exceptional stability, low noise, and traceability, enabling improved precision in electrical current measurements and resistor calibration.

Findings

Transfer coefficient stable over time, temperature, and current.

Low noise level of 2.4 fA/√Hz facilitates quick averaging.

Calibration uncertainty below 10^-7 within one day.

Abstract

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.