Ultrastable Low-noise Current Amplifier Research Articles

Series connection of quantum Hall resistance array and programmable Josephson voltage standard for current generation at one microampere

An accurate current can be realized by a series connection of an invariant voltage and resistance with zero interconnection resistance in the framework of Ohm’s law. In practice, however, accurate current realization is limited by the temporal drift and instability of employed resistors, solid-state voltage sources and interconnection wires against environmental fluctuations, such as temperature or atmospheric pressure. Here, we generated a current of 1 μA, determined by the Josephson voltage and quantized Hall resistance array (QHRA). A programmable Josephson voltage standard (PJVS), 1 MΩ QHRA and ultrastable low noise current amplifier (ULCA) are connected in series to generate and measure a current of 1 μA. The expected current is calculated as the ratio of the Josephson voltage to the QHRA plus the output resistance of the PJVS. The relative difference between the current measured by the ULCA and the calculated current is as small as 40 nA A−1 with a combined measurement uncertainty (k = 1) of 0.13 μA A−1.

Improvements of the programmable quantum current generator for better traceability of electrical current measurements

A programmable quantum current generator based on the application of Ohm’s law to quantum voltage and resistance standards has demonstrated a realization of the ampere from the elementary charge with a 10−8 relative uncertainty (Brun-Picard et al 2016 Phys. Rev. X 6 041051). Here, we report on improvements of the device leading to a noise reduction of the generated quantized current. The improved quantum current generator is used to calibrate different ammeters with lower measurement uncertainties. Besides, measurements of its quantized current using a calibrated ultrastable low-noise current amplifier have shown that the realizations of the ampere at PTB (Physikalisch-Technische Bundesanstalt) and LNE (Laboratoire national de Métrologie et d’essais) in the range ±50 μA agreed to −3.7 parts in 107 with a combined standard uncertainty of 3.1 parts in 107 (coverage factor k c = 1).

Calibration of ultrastable low-noise current amplifiers without direct use of a cryogenic current comparator

The established calibration of the transresistance of the ultrastable low-noise current amplifier (ULCA) directly involves a cryogenic current comparator (CCC). Here, we present an alternative approach based on a calibrated set of three resistors and three digital voltmeters. This approach was experimentally verified for a two-channel ULCA with nominally 1 GΩ transresistance using PTB’s travelling 12 bit CCC setup in an on-site comparison at METAS. Series of interleaved experiments included calibrations of the input and output stages of two different ULCA channels. The consistency found between the conventional calibration with PTB’s CCC and calibration with METAS’ set of three resistors—each traced to the quantum Hall resistance with METAS’ CCC—confirms that the alternative traceability chain provides valuable results. If the standard resistors are calibrated without using a CCC, the new method even opens the possibility to calibrate the ULCA without any CCC in the traceability chain involved, however, typically at the expense of increased uncertainty.

The next generation of current measurement for ionization chambers

Re-entrant ionization chambers (ICs) are essential to radionuclide metrology and nuclear medicine for maintaining standards and measuring half-lives. The requirements of top-level metrology demand that systems must be precise and stable to 0.1 % over many years, and linear from 10−14 A to 10−8 A. Thus, laboratories depend on bespoke current measurement systems and often rely on sealed sources to generate reference currents. To maintain and improve present capabilities, metrologists need to overcome two looming challenges: ageing electronics and decreasing availability of sealed sources. Possible solutions using Ultrastable Low-Noise Current Amplifiers (ULCAs), resistive-feedback electrometers, and (quantum) single-electron pumps are reviewed. Broader discussions of IC design and methodology are discussed. ULCAs show promise and resistive-feedback systems which take advantage of standard resistor calibrations offer an alternative.

Interlaboratory Nanoamp Current Comparison With Subpart-Per-Million Uncertainty

An ultrastable low-noise current amplifier (ULCA) has been transferred between two laboratories, NPL and PTB, four times, with three of the transfers yielding a relative change in the trans-resistance gain of less than $2 \times 10^{-7}$. This is a new bench-mark for inter-laboratory transfer of small current. We describe in detail the use of a cryogenic current comparator to calibrate the ULCA at NPL, and the use of the ULCA to measure $1$ G$\Omega$ resistors with relative uncertainties at the $10^{-7}$ level.

Calibrating Ultrastable Low-Noise Current Amplifiers of the Second Generation With a Cryogenic Current Comparator

Input and output stages of the second generation of ultrastable low-noise current amplifiers can be conveniently calibrated by means of a cryogenic current comparator. The uncertainty achievable for measurement durations of less than 2 h makes our calibration setup a helpful tool for the selection of hardware in the development of instrumentation tailored to meet the requirements of special applications.

Electrometer Calibration With Sub-Part-Per-Million Uncertainty

We performed calibrations of four different commercial picoammeters using the ultrastable low-noise current amplifier (ULCA) as a calibrator current source operated in the range between 1 fA and $1~\mu \text{A}$ . The results allow the comprehensive characterization of the devices under test regarding noise, settling, and burden voltage behavior as well as stability of the gain factor, and confirm the performance of the ULCA for use as small-current calibrator standard. Also, we present a further advanced setup for the calibration of transimpedance amplifiers. Accuracy limits for the best electrometer calibrations in the current range between 1 fA and $1~\mu \text{A}$ and possible implications on corresponding calibration and measurement capabilities are discussed.

Noise-optimized ultrastable low-noise current amplifier.

We have developed a noise-optimized ultrastable low-noise current amplifier (ULCA) aimed at reducing the uncertainty at low currents. It involves a thin-film resistor network with 6.75 GΩ at the high-ohmic path which reduces the noise level to 1.6 fA/Hz. Noise investigations as well as short-term and long-term stability studies were carried out. The stability of the input current gain was measured using a cryogenic current comparator at ±6.1 nA. Methods for investigating the measurement accuracy at low input currents of about 100 pA at a level of below one part in 107 are introduced and experimentally verified. The performance of the noise-optimized ULCA is compared with that of the standard variant introduced in 2014. It is shown that the reduced noise floor is achieved without impairing the stability of the transresistance.

Measurement of sub-picoampere direct currents with uncertainties below ten attoamperes.

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.

Improving the Traceable Measurement and Generation of Small Direct Currents

We present the latest improvements in the traceable measurement and generation of small electric currents. A central tool in our traceability chain for small direct currents is a new binary cryogenic current comparator (CCC) with a total of 18 276 turns. This 14-bit CCC is well suited for the calibration of high-value resistors and current amplifiers, but also for the direct amplification of small currents. A noise level of 5 fA/ $\surd $ Hz at 0.05 Hz is routinely achieved. The systematic uncertainty due to noise rectification was exemplarily investigated in a ratio-error test configuration, showing that a total uncertainty of about one part in $10^{6}$ can be achieved at 100 pA. For further improvement, a new instrument was developed, the ultrastable low-noise current amplifier (ULCA). Its transfer coefficient is highly stable versus time, temperature, and current amplitude within a full dynamic range of ±5 nA. The ULCA is calibrated with the 14-bit CCC at high current amplitude, and allows the measurement or generation of 100-pA direct current with an uncertainty of one part in $10^{7}$ . The novel setup was successfully used to investigate the uncertainty of the established capacitor charging method. A quantum metrology triangle experiment based on the presented instruments is proposed.

Validation of the ultrastable low-noise current amplifier as travelling standard for small direct currents

An interlaboratory comparison of small-current generation and measurement capability is presented with the ultrastable low-noise current amplifier (ULCA) acting as travelling standard. Various measurements at direct currents between 0.16 nA and 13 nA were performed to verify the degree of agreement between the three national metrology institutes involved in the study. Consistency well within one part per million (ppm) was found. Due to harsh environmental conditions during shipment, the ULCA’s transfer accuracy had been limited to about ±0.4 ppm. Supplemental measurements performed at PTB indicate that further improvements in accuracy are possible. Relative uncertainties of 0.1 ppm are achieved by applying on-site calibration of the ULCA with a suitable cryogenic current comparator.

Ultrastable low-noise current amplifier: A novel device for measuring small electric currents with high accuracy

An ultrastable low-noise current amplifier (ULCA) is presented. The ULCA is a non-cryogenic instrument based on specially designed operational amplifiers and resistor networks. It involves two stages, the first providing a 1000-fold current gain and the second performing a current-to-voltage conversion via an internal 1 MΩ reference resistor or, optionally, an external standard resistor. The ULCA's transfer coefficient is highly stable versus time, temperature, and current amplitude within the full dynamic range of ±5 nA. The low noise level of 2.4 fA/√Hz helps to keep averaging times short at small input currents. A cryogenic current comparator is used to calibrate both input current gain and output transresistance, providing traceability to the quantum Hall effect. Within one week after calibration, the uncertainty contribution from short-term fluctuations and drift of the transresistance is about 0.1 parts per million (ppm). The long-term drift is typically 5 ppm/yr. A high-accuracy variant is available that shows improved stability of the input gain at the expense of a higher noise level of 7.5 fA/√Hz. The ULCA also allows the traceable generation of small electric currents or the calibration of high-ohmic resistors.

Ultrastable Low-Noise Current Amplifiers With Extended Range and Improved Accuracy

The ultrastable low-noise current amplifier (ULCA) was developed as an advanced instrument for the improved measurement of currents generated by single-electron transport (SET) devices. It was optimized for direct currents in the picoampere range, and achieves an uncertainty of $ /A for a current of 100 pA typically generated by SET pumps. This paper summarizes our efforts in extending the ULCA’s current range and minimizing the measurement uncertainty over a wide range. Two ULCA variants with improved short-term uncertainty of about 0.02 $\mu \text{A}$ /A or improved noise level of 1.4 fA/ $\sqrt {\text {Hz}}$ are presented. Combining these devices allows the construction of a novel standard for direct currents below 50 $\mu \text{A}$ , which has superior performance to previous methods for small-current generation or measurement. ULCAs involving thick-film resistors achieve noise levels down to 0.43 fA/ $\sqrt {\text {Hz}}$ . A low-bias variant features an effective input bias current within ±100 aA and a temperature coefficient within ±10 aA/K, which enables uncertainties below 10 aA in direct current measurements without reversing or switching ON/OFF the signal current.