The following publications offer detailed information about the ULCA-1 current amplifier (licensed by PTB, available now):

Ultrastable low-noise current amplifier: A novel device for measuring small electric currents with high accuracy
D. Drung, C. Krause, U. Becker, H. Scherer, and F. J. Ahlers
Review of Scientific Instruments 86, 024703 (2015)
doi: 10.1063/1.4907358

Validation of the ultrastable low-noise current amplifier as travelling standard for small direct currents
D Drung, C Krause , S P Giblin , S Djordjevic, F Piquemal, O Séron, F Rengnez, M Götz, E Pesel and H Scherer
Metrologia 52 (2015) 756–763
doi: 10.1088/0026-1394/52/6/756

Improving the Traceable Measurement and Generation of Small Direct Currents
Dietmar Drung, Martin Götz, Eckart Pesel, and Hansjörg Scherer
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 11, pp. 3021-3030, NOVEMBER 2015 (PDF)

Validation of a quantized-current source with 0.2 ppm uncertainty
Friederike Stein, Dietmar Drung, Lukas Fricke, Hansjörg Scherer, Frank Hohls, Christoph Leicht, Martin Götz, Christian Krause, Ralf Behr, Eckart Pesel, Klaus Pierz, Uwe Siegner, Franz J. Ahlers, and Hans W. Schumacher
 Applied Physics Letters 107, 103501 (2015)
doi: 10.1063/1.4930142

Ultrastable Low-Noise Current Amplifiers With Extended Range and Improved Accuracy
Dietmar Drung, Christian Krause
IEEE Transactions on Instrumentation and Measurement (2016)
doi: 10.1109/TIM.2016.2611298

Measurement of sub-picoampere direct currents with uncertainties below ten attoamperes
C. Krause, D. Drung, and H. Scherer
 Rev. Sci. Instrum. 88, 024711 (2017)
doi: 10.1063/1.4975826

Noise-optimized ultrastable low-noise current amplifier
C. Krause, D. Drung, M. Götz, and H. Scherer
 Rev. Sci. Instrum. 90, 014706 (2019)
doi: https://doi.org/10.1063/1.5078572

Calibration of ultrastable low-noise current amplifiers without direct use of a cryogenic current comparator
C. M Götz, D Drung, C Krause and Mortara A
Metrologia 57 (2020) 055008 (9pp)
doi: https://doi.org/10.1088/1681-7575/ab8f7a