Logo of the Physikalisch-Technische Bundesanstalt
symbolic picture: "magazines"

New picoamperemeter: handy, simple, extremely accurate

The Ultrastable Low-noise Current Amplifier measures pA currents with excellent accuracy

PTB-News 1.2015
Especially interesting for
  • metrology institutes and fundamental research
  • calibration laboratories
  • manufacturers of high-precision electronics

A handy picoamperemeter has been developed at PTB which is able to traceably measure extremely small currents with unprecedented accuracy. The Ultrastable Low-noise Current Amplifier (ULCA) exceeds the accuracy of previously available picoamperemeters by about two orders of magnitude.

Two-channel ULCA prototype (front view with SMA connectors)

Measuring currents in the range of 100 pA accurately is increasingly gaining in importance – not only for the calibration of picoamperemeters for applications in medical engineering or in the semiconductor industry, but also for fundamental research in the field of current generation based on single electrons. The accuracy of conventional commercially available measuring instruments is limited to relative uncertainties of approx. 10 parts in a million (i.e. 10–5). Better accuracy could previously only be achieved by using extremely complex metrological procedures and apparatuses.

A new development, which is currently being expedited at PTB, opens up new measurement capabilities with unprecedented accuracy. The ULCA is based on a new picoamperemeter concept with a two-stage set-up whose patent is pending. The first stage amplifies the input current by a factor of 1000 while the second stage converts current to voltage. Besides being relatively easy to use, this handy device is characterized by excellent amplifier properties. The second ULCA prototype generation already showed a very low input current noise of 2.4 fA/√Hz with a white spectrum down to very low frequencies of about 1 mHz. The optimized circuit design allows the realization of an amplification factor with maximum stability and linearity (the effective transimpedance is 1 GΩ). In addition, the amplification factor can be calibrated with a relative uncertainty of better than 10–7 at averaging times below one hour, hereby ensuring traceability to the quantum resistance standard (quantum Hall effect) by means of a cryogenic current comparator (as used for electric resistance metrology at PTB).

The combination of these excellent properties thus allows the absolute measurement of a 100 pA current within a measuring time of only 10 hours with a relative uncertainty of one part in ten million. The ULCA hereby exceeds the accuracy of previously available picoamperemeters by about two orders of magnitude.


Dietmar Drung
Department 7.2 Cryophysics and Spectrometry
Phone: +49 (0)30 3481-7342
E-mail: dietmar.drung(at)ptb.de

Scientific publication

D. Drung, C. Krause, U. Becker, H. Scherer, F. J. Ahlers: Ultrastable lownoise current amplifier: a novel device for measuring small electric currents with high accuracy. Rev. Sci. Instrum. 86, 024703 (2015)