Small electrical currents are of increasing interest for fundamental and applied purposes in metrology:
- "Single-electron pumps" enable the generation of currents of the order of 100 pA by controlled transport of single charge quanta (electrons). Their application in metrology for the purpose of a future quantum current standard (related to the impending re-definition of the ampere in the international system of units, SI) requires further research, to be supported by highly precise and accurate measurements of small currents. Here is a video on the topic.
- Semiconductor industry, for instance in the branches of micro- and nanoelectronics as well as for medical and environmental measurements – for instance in dosimetry and imission control – increasingly require methods for accurate small current measurements. This is connected to an increasing demand in instrument calibrations (picoammeters and small current sources).
The working group, in close cooperation with the Department 7.2, has developed a novel instrument for the highly accurate measurement and generation of small currents, the "Ultrastable Low-noise Current Amplifier" (ULCA). The highly stable low-noise current amplifier is a handy non-cryogenic instrument, based on specially developed operational amplifiers and resistor networks. The two-stage concept combines a power amplifier (amplification factor of 1000) and a current-to-voltage converter (transresistance 1 MΩ). The amplifier’s transfer coefficient, i.e. the effective total transresistance of 1 GΩ, is extremely stable with respect to drift (typically about 5.10-6 of relative change per year) as well as to temperature and input current. The low noise level of about 2.4 fA/√Hz helps to keep measurement times relatively short even at low input currents. The calibration of the transfer coefficient fully traceable to the quantum Hall resistance is performed by using a cryogenic current comparator with a relative uncertainty of less than 0.1 µΩ/Ω.
The UCLA enables, for example, measuring a current of 100 pA with a relative uncertainty of one part in ten million within one day. The instrument is also suitable for the traceable generation of currents at highest precision and accuracy.
Prototype of a two-channel Ultrastable Low-noise Current Amplifier (ULCA), developed by PTB (right picture with opened housing).
Commercially available version of the two-channel ULCA with battery box for power supply (Magnicon GmbH, Hamburg).
The instrument is to be produced and distributed by Magnicon GmbH (Hamburg), licensed by the PTB.
Publications of the working group in this field