Scientists at e-SI-Amp partner Physikalisch-Technische Bundesanstalt (PTB) have developed a stable current amplifier for the sub-picoampere measurement range. The low noise and stability makes direct current measurements with uncertainties of less than 10 attoamperes (1 attoampere = 10-18 A) readily achievable.

Uncertainty versus averaging time for
the ultra-low current ULCA variant

The “ultrastable low-noise current amplifier” (ULCA) enables traceable measurement and generation of small currents with uncertainties down to 0.1 ppm. The standard ULCA has input current noise ~2.4 fA/√Hz, a 1/f noise corner of about 1 mHz, combined with an extremely low temperature coefficient and stable gain.

PTB has developed a new ULCA tailored for sub-picoampere (<10-12 A) direct currents. These levels are typical in sensors for radiometry or environmental monitoring where the current is related to a small numbers of ionisation events or detected particles. This ULCA variant has a reduced current noise of 0.4 fA/√Hz and 1/f noise corner of ~0.032 mHz, making sensitive measurements possible in a much shorter time.

The improved stability means that continuously averaging for 30 minutes can give a measurement uncertainty of only 10 aA without the polarity reversal or on/off switching that would normally be required to account for offset drift. Because the input bias current (<100 aA) is very stable in time and temperature, measurements can be made in an environment without temperature stabilization.

This research was published open-access in the Review of Scientific Instruments a journal of the American Institute of Physics (AIP):
C. Krause, D. Drung, and H. Scherer,
“Measurement of sub-picoampere direct currents with uncertainties below ten attoamperes”,
Rev. Sci. Instrum. 88, 024711 (2017);