Abstract
A new type of the ultrastable low-noise current amplifier (ULCA) is presented. It involves thick-film resistors to achieve a high feedback resistance of 185 GΩ at the input amplifier. An improved noise level of 0.4 fA/Hz with a 1/f corner of about 30 μHz and an effective input bias current well below 100 aA are demonstrated. For small direct currents, measurement uncertainties below 10 aA are achievable even without current reversal or on/off switching. Above about 1 pA, the stability of the ULCA's resistor network limits the relative measurement uncertainty to about 10 parts per million. The new setup is used to characterize and optimize the noise in the wiring installed on a dilution refrigerator for current measurements on single-electron transport pumps. In a test configuration connected to the wiring in a pulse tube refrigerator, a total noise floor of 0.44 fA/Hz was achieved including the contributions of amplifier and cryogenic wiring.
Highlights
The interest in accurate direct current measurements below 1 nA becomes increasingly significant
We present results obtained with a new ultrastable low-noise current amplifier (ULCA) type that is tailored for sub-picoampere direct currents and is well suited for operation without current modulation
2 · 0.4√fA/ Hz = 0.57 fA/ Hz and (0.57/ 0.9) fA/ Hz = 0.6 fA/ Hz are expected for the cases without and with current reversal
Summary
The interest in accurate direct current (dc) measurements below 1 nA becomes increasingly significant. A capacitor may be charged by the current to be measured in order to avoid the contribution of thermal noise in the resistor In both cases the accuracy is limited, either by the stability of the high-value resistor or by the frequency dependence of the capacitance which causes an uncertainty contribution in the calibration of about 10 parts per million (ppm) at best.. We present results obtained with a new ULCA type that is tailored for sub-picoampere direct currents and is well suited for operation without current modulation.. We present results obtained with a new ULCA type that is tailored for sub-picoampere direct currents and is well suited for operation without current modulation.12 Both the current noise and the effective input bias current are considerably improved compared to the ULCA with R = 3 GΩ.
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