Ultrastable Low-Noise Current Amplifier: A New Tool for Small Current Metrology

Abstract

The Ultra stable Low-noise Current Amplifier (ULCA) is a user-friendly and superior alternative to existing instruments for small direct currents in the range between about 1 fA and 5 μA. The principle of the portable laboratory table-top device, operated at room temperature, is based on a novel dual-stage transimpedance amplifier concept. The total transimpedance of 1 GO is calibrated with a cryogenic current comparator with an uncertainty < 0.1 μO/O traceable to the quantum Hall resistance. The output voltage is measured with a voltmeter calibrated traced to the Josephson voltage standard. In addition to its electrometer function, in combination with a voltage source the ULCA also can be used as a current generator. Therefore, it represents a new tool for ultra-accurate small-current measurement and generation traceable to quantum electrical standards. It outperforms commercial devices and calibration setups used in metrology institutes by up to two orders of magnitude in accuracy. The unique features of the ULCA are the excellent stability of its transimpedance (drift less than 5 μO/O per year, short-term fluctuations over one week< 0.1 μO/O), its small temperature coefficient (typically about 0.2 μO/O per Kelvin), fast settling(difference to final value < 0.1 μA/A after 3 s), and the low input current noise of 2.4 fA/vHz. This enables measuring a direct current of 100 pA with a total relative uncertainty of 0.1 μA/A in about 10 h. Besides being excellently suited for R&D in small-current metrology (e.g. for research on single-electron pumps) the ULCA is also widely applicable for calibrations, for instance for electrometers, small-current sources, or high-value resistors. Corresponding fields (and specific examples) are electronic industry (ICs), medicine and biotechnology (dosimetry, radiation protection, DNA sequencers) as well as environmental monitoring (concentration measurements of small particles in air or aerosols), and lighting industry (photo current measurement). Framed by two patent applications, the technology was transferred from the Physikalisch-Technische Bundesanstalt (PTB) to a German company (Magnicon GmbH, Hamburg), which manufactures and markets the ULCA since 2016 licensed by PTB.