Low-noise Current Source Research Articles

A 20 A bipolar current source with 140 μA noise over 100 kHz bandwidth

The precise control of direct current (dc) magnetic fields is crucial in a wide range of experimental platforms, from ultracold quantum gases and nuclear magnetic resonance to precision measurements. In each of these cases, the Zeeman effect causes quantum states to shift in energy as a function of the magnetic field. The development of low-noise current sources is essential because electromagnets are the preferred tool to dynamically control the magnetic field. Here, we describe an ultra-low noise bipolar current source using pairs of complementary n- and p-channel metal–oxide–semiconductor field-effect transistors controlled by zero-drift operational amplifiers. Our source has a 90 kHz inherent bandwidth and provides current from −20 to 20 A with noise (0.1 Hz to 100 kHz) of 140 µA at ±20 A.

A Frontend for Magnetoresistive Sensors With a 2.2-pA/√Hz Low-Noise Current Source

In this letter, we present an integrated readout chip for magnetoresistive (MR) sensors consisting of a readout chain that comprises a dc-coupled fully differential difference amplifier (FDDA) followed by a programmable gain amplifier (PGA), as well as a low-noise current biasing scheme for the MR sensor. The current bias scheme features a 10-bit digital-to-analog converter (DAC) to compensate for process variations of the MR sensing element as well as to calibrate for variations in the dc bias field of the sensor. The bias current source achieves a very low current noise floor of 2.2 pA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sqrt {\mathrm {Hz}}$ </tex-math></inline-formula> for bias currents up to 1 mA. The readout chip is manufactured in 180-nm SOI CMOS and consumes a total power of 38 mW. The letter is an extended version of (Mohamed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , 2021) incorporating additional modeling details and measurement results.

A Deep Subelectron Temporal Noise CMOS Image Sensor With Adjustable Sinc-Type Filter to Achieve Photon-Counting Capability

This letter introduces a Gm-cell-based CMOS image sensor (CIS) achieving deep subelectron noise performance. The CIS presents a new compensation block and low-noise current source to improve the performance of the Gm pixel. Furthermore, an optional first-order IIR filter is implemented to improve the output swing. The conversion gain, full well capacity, and dynamic range of the CIS can be easily adjusted by the charging time and the filter mode for different applications. The prototype chip is fabricated in a standard 180 nm CIS process and has a deep subelectron read noise of 0.31e-rms at minimum (of the noise distribution) and 0.42e-rms at peak (of the noise distribution). A smooth and clear photon-counting histogram is observed.

A Low-RIN Spectrally Pure Whispering-Gallery-Mode Resonator-Based Semiconductor Laser

We demonstrate a high-spectral-purity self-injection locked distributed-feedback semiconductor laser with relative intensity noise approaching −143 dB/Hz at 1 kHz and −160 dB/Hz at 100-kHz frequency offset. The frequency noise of the laser is 2 Hz/Hz1/2 at 1 kHz and 1 Hz/Hz1/2 at 100-kHz frequency offset. The fiber-pigtailed output power exceeds 70 mW. The low-intensity-noise performance is achieved due to the utilization of a low-noise current source feeding the laser chip and a saturated semiconductor optical amplifier reducing the technical amplitude noise of the laser. Reduction in the laser frequency noise results from the self-injection locking of the laser chip via a high-quality-factor whispering-gallery-mode resonator. The laser linewidth is less than 100 Hz.

Programmable, very low noise current source.

We propose a new approach for the realization of very low noise programmable current sources mainly intended for application in the field of low frequency noise measurements. The design is based on a low noise Junction Field Effect Transistor (JFET) acting as a high impedance current source and programmability is obtained by resorting to a low noise, programmable floating voltage source that allows to set the sourced current at the desired value. The floating voltage source is obtained by exploiting the properties of a standard photovoltaic MOSFET driver. Proper filtering and a control network employing super-capacitors allow to reduce the low frequency output noise to that due to the low noise JFET down to frequencies as low as 100 mHz while allowing, at the same time, to set the desired current by means of a standard DA converter with an accuracy better than 1%. A prototype of the system capable of supplying currents from a few hundreds of μA up to a few mA demonstrates the effectiveness of the approach we propose. When delivering a DC current of about 2 mA, the power spectral density of the current fluctuations at the output is found to be less than 25 pA/√Hz at 100 mHz and less than 6 pA/√Hz for f> 1 Hz, resulting in an RMS noise in the bandwidth from 0.1 to 10 Hz of less than 14 pA.

Magnetic flux density measurement in magnetic resonance electrical impedance tomography using a low-noise current source

Current injected into an electrically conducting object induces distributions of magnetic flux density as well as voltage and current density. In magnetic resonance electrical impedance tomography (MREIT), we measure the induced magnetic flux density using an MRI scanner to reconstruct cross-sectional conductivity images of the object. The current injection must be synchronized with a chosen MRI pulse sequence and the current source needs to be programmable to accommodate various pulse sequences. For an injected current of 1 mA, for example, we may expect an induced magnetic flux density of a few nanoteslas, and noise reduction in measured magnetic flux density data is important for better conductivity image quality. In this paper, we describe how to reduce noise in measured magnetic flux density data based on analyses of noise sources in MREIT. Given an MRI scanner and an imaging object, we found that the current source including lead wires becomes a major noise source. We designed a low-noise programmable current source with optical links, batteries and electromagnetic shields. Equipped with lead switching capability, it automates MREIT imaging experiments using multiple pairs of electrodes. We found that the new current source improves the output current SNR by about 10 dB and the MR magnitude image SNR by about 30%. Placing it near an imaging object inside the shielded room, we could reduce the noise standard deviation in measured magnetic flux density data by 40%. We propose use of this low-noise current source for in vivo animal and human MREIT imaging studies.

Moderately shielded high-Tc SQUID system for rat MCG

Recently, we have developed a 5-channel high-Tc SQUID system with one signal channel intended for rat magnetocardiography (MCG) in moderately shielded or "quiet" real environment. This system is an adapted version of a human MCG system which has been improved with respect to user-friendliness and stability. A dewar with a cold-warm distance of 7 mm and a refill cycle time of up to one week is utilized. The implemented high-Tc SQUIDs are single-layer devices with grain boundary junctions fabricated at KRISS with laser ablation on 10 mm × 10 mm STO substrates. In order to cancel environmental magnetic noise, three of the five SQUIDs are arranged to build an axial software first-order or second-order gradiometer with a base line of 35 mm. The other two SQUIDs are used for balancing. To overcome previous system instabilities, we have implemented an Earth field compensation for each SQUID. For this, the SQUIDs were mounted in capsules containing integrated field compensation coils. The three Earth field components are measured with an additional triaxial fluxgate, and compensated at the SQUID locations using the low-noise current source of the SQUID readout electronics. This way, the SQUIDs can be cooled and operated in a low residual field that improves system stability and reduces low-frequency SQUID noise. It is even possible to slowly move the dewar in the Earth field (dynamic field compensation). Different noise cancellation procedures were optimized and compared employing a periodic signal source.

Development of a low noise MREIT current source

In MREIT conductivity imaging experiments of animal and human subjects, we should minimize the noise level in measured magnetic flux density data induced by injection currents with low amplitude. Since noise and artifact from an MREIT current source directly affect the quality of the data, a low-noise current source is desirable. In order to be compatible with various MREIT pulse sequences, it should be also programmable. We have developed a new MREIT current source, which is controlled by a PC program for flexibility. We designed it in such a way that it is located inside the shield room of an MRI system. To minimize noise and artifact, we adopted an optical link for the connection to the PC outside the shield room. The enclosure of the new current source provides a magnetic as well as electric shielding to prevent high frequency switching noise of the current source from interfering with the scanner. It is powered by a rechargeable battery so that the entire current source is isolated from the ground. Equipped with automatic lead switching capability, it simplifies and automates MREIT imaging experiments. Our experimental results show that its performance is superior to the previous version, which is located outside the shield room.

High-T/sub c/ SQUID sensors with integrated earth field compensation

An encapsulation with integrated field compensation coils for superconducting quantum interference device (SQUID) magnetometers is presented. The capsule is fabricated from printed-circuit board material. In spite of the small capsule size of 18 mm/spl times/16 mm/spl times/4.8 mm, the compensation field at nominal capsule dimensions is homogeneous to about /spl plusmn/2% over the area of the SQUID magnetometer pickup coil of 9 mm/spl times/9 mm. The field homogeneity degrades by about a factor of two for worst-case tolerances in the thickness of the board material or in the positions of the field coils. A low-noise current source was integrated into our microcontroller-based SQUID readout electronics in order to supply the field coils. Compensation fields of up to /spl plusmn/71 /spl mu/T can be produced with only 19 fT//spl radic/Hz excess noise above the 1/f corner at about 2 Hz. The current source has a 10%-90% settling time of 2.2 ms allowing one to compensate the background field dynamically in environments with very large low-frequency field fluctuations or even in slowly moved sensor systems.

Ultra low-noise current sources

The solid state DC current sources available on the market are not suitable for applications in low noise measurement systems because of the high level of low-frequency noise introduced in the measurement chain. The most important cause of low-frequency noise in such instruments is the solid state device used as a voltage reference (usually a Zener diode). This problem has been solved, in the instrument described in this paper, by using a new circuit topology in which the solid-state voltage reference has been substituted by a low-noise battery. The instrument, capable of supplying a current as high as 100 mA, is characterized by a low-frequency noise level some orders of magnitude lower than that of similar commercial instrumentation.

Wafer level measurement system for sarf characterization of metal lines

A feasibility study concerning the application of the SARF technique at wafer level has been performed. The main problem of this type of measurements is the excess noise generated at the point contacts used to supply the lines under investigation with the test current. This problem has been successfully solved by setting up a probe station entirely contained in a shielded box and by supplying the samples under test by means of purposely designed low noise current sources. The background noise of the whole measurement system coincides with that of the system used until now for packaged samples.

Testing of full size high current superconductors in SULTAN III

The high field test facility SULTAN III in operation at PSI/Switzerland tests full size industrial prototype superconductors for fusion applications such as ITER. The facility provides a background field of up to 11 T over a length of 58 cm. A 50 kA superconducting transformer works as a very low noise current source which allows a criterion of 0.1 /spl mu/V/cm to determine the superconducting to normal transition. Three 3.6 m long cable-in-conduit conductors based on both NbTi and Nb/sub 3/Sn, developed by different manufacturers, suitable for the central solenoid and toroidal field coils of ITER, have been tested so far. This paper presents the results of extensive measurements of critical current and current sharing temperature of the Nb/sub 3/Sn conductors in the 8-11 T range for temperatures between 4.5 K and 11 K. Voltage versus current curves have been analyzed with respect to the n value. The manufacturing of a high quality joint between two Nb/sub 3/Sn conductors after heat treatment is reported, together with some measurements of the joint resistance. >

A 0.8nV/√Hz CMOS preamplifier for IC-magneto-resistive read elements

State-of-the-art tape drives now employ MR heads to achieve increased bit density and speed-independent signal amplitude. In such systems, the preamplifier (PA) has to support simultaneous reading of up to four signals from up to eight magneto-resistive (MR) read elements. Hence, thermal considerations require a minimum power consumption architecture. Due to manufacturing and wear out, head impedances from 20 to 150 /spl Omega/ must be allowed. The need for optimizing the read elements operating point also requires the integration of four programmable low-noise current sources. To allow for backward compatibility, the PA should amplify very large input signals at low distortion levels. Previous PA designs make use of expensive BiCMOS technology, have no on-chip low-noise current DACs, and do all the signal processing in the voltage-mode. This paper describes how CMOS technology can be used to achieve sub-nVspl radic/Hz noise levels counting contributions from both the amplifier and the bias-current sources. It also describes how large dynamic range and high PSRR are achieved by combining current and voltage-mode signal processing. The actual chip designed contains four preamps, four bias DACs for the read elements, an 8:4 input multiplexer, and a serial interface. It is realized in 1.2 /spl mu/m CMOS technology. >