Commercial Preamplifiers Research Articles

Signal feedback applications in low-field NMR and MRI.

Tuned pick-up coils with high quality factors Q are used in NMR and MRI for high-sensitivity and low-noise detection. However, large Q-factors introduce bandwidth issues at low frequency and the associated enhanced currents may cause significant radiation damping effects, especially with hyperpolarised samples. Signal feedback can be used to actively control these currents and adjust the detection bandwidth without resistive losses. Capacitive and inductive coupling methods are compared using detailed models and the operating conditions for efficient feedback with negligible noise penalty are discussed. Several high-impedance commercial preamplifiers have been found to affect the resonance characteristics of tuned coils in a gain-dependent way, or could not be used in low-frequency NMR because of oscillations at large positive gain. This is attributed to an undocumented internal feedback, and could be neutralised using external feedback. The implementation of an inductive coupling scheme to feed a suitably amplified phase-adjusted signal back into the PU coils of low-field NMR systems is described, and three experimental applications are reported. One system is used for NMR studies of distant dipolar field effects in highly polarised liquid 3He without or with radiation damping. The moderate intrinsic Q-factor (≈7) could be reduced (down to 1) or increased (up to 100) to control transient maser oscillations. Another system was used for MRI of water samples around 2 mT with Q≈190 litz-wire detection coils. The detection bandwidth was increased by actively reducing the Q-factor to obtain uniform sensitivities in images and avoid artifacts introduced by intensity corrections. Finally, parallel acquisition in MRI was performed using two separately tuned detection coils placed above and below the sample. They were actively decoupled using two feedback systems. For an imaging field of view smaller than the sample, artifact-free unfolded images demonstrate the efficiency of this active coil decoupling scheme.

A low-cost, ultra-fast and ultra-low noise preamplifier for silicon avalanche photodiodes

An ultra-fast and ultra-low noise preamplifier for amplifying the fast and weak electrical signals generated by silicon avalanche photodiodes has been designed and developed. It is characterized by its simplicity, compactness, reliability and low cost of construction. A very wide bandwidth of 300 MHz, a very good linearity from 1 kHz to 280 MHz, an ultra-low noise level at the input of only 1.7 nV Hz−1/2 and a very good stability are its key features. The compact size (70 mm × 90 mm) and light weight (45 g), as well as its excellent characteristics, make this preamplifier very competitive compared to any commercial preamplifier. The preamplifier, which is a main part of the detection system of a homemade laser remote sensing system, has been successfully tested. In addition, it is versatile and can be used in any optical detection system requiring high speed and very low noise electronics.

Fast transimpedance preamplifier for a boron-coated multiwire proportional chamber neutron detector

A low-noise and fast preamplifier is needed for the new boron-coated multiwire proportional chamber (MWPC) neutron detector with a delay line readout system to improve position resolution. A transimpedance preamplifier with a rise time of 30ns, a signal-to-noise ratio higher than 40dB, and an automatic gain control function are designed to meet the aforementioned requirements. On the other hand, we also compare it to commercial preamplifiers. Then, the advantages and disadvantages of commercial and in-house preamplifiers are analyzed. The preamplifier is used and tested on a MWPC neutron detector, and results are presented.

Undergraduate Experiment on Thermal Noise with a Lock-in Amplifier: Determination of Boltzmann's Constant

An undergraduate experiment on the thermal noise of resistors, which can be used to measure Boltzmann’s constant, is described. A lock-in amplifier is employed to amplify, filter and detect the thermal noise so that a commercial pre-amplifier and a band-pass filter are unnecessary. Therefore, the method does not require calibration of the frequency-dependent gain. The thermal noise of a resistor is measured as functions of the resistance and the bandwidth, giving values of the Boltzmann constant of kB = (1.362 ± 0.020) ◊ 10 23 J/K and kB = (1.373 ± 0.025) ◊ 10 23 J/K, respectively. The experiment, thus, permits reasonably precise measurement of the Boltzmann constant with uncertainties of less than 2% while allowing the undergraduate students to gain a hands-on understanding of both a fundamental source of noise in electronic circuits and the operation of the lock-in detection technique.

A low-cost multielectrode system for data acquisition enabling real-time closed-loop processing with rapid recovery from stimulation artifacts

Commercially available data acquisition systems for multielectrode recording from freely moving animals are expensive, often rely on proprietary software, and do not provide detailed, modifiable circuit schematics. When used in conjunction with electrical stimulation, they are prone to prolonged, saturating stimulation artifacts that prevent the recording of short-latency evoked responses. Yet electrical stimulation is integral to many experimental designs, and critical for emerging brain-computer interfacing and neuroprosthetic applications. To address these issues, we developed an easy-to-use, modifiable, and inexpensive system for multielectrode neural recording and stimulation. Setup costs are less than US$10,000 for 64 channels, an order of magnitude lower than comparable commercial systems. Unlike commercial equipment, the system recovers rapidly from stimulation and allows short-latency action potentials (<1 ms post-stimulus) to be detected, facilitating closed-loop applications and exposing neural activity that would otherwise remain hidden. To illustrate this capability, evoked activity from microstimulation of the rodent hippocampus is presented. System noise levels are similar to existing platforms, and extracellular action potentials and local field potentials can be recorded simultaneously. The system is modular, in banks of 16 channels, and flexible in usage: while primarily designed for in vivo use, it can be combined with commercial preamplifiers to record from in vitro multielectrode arrays. The system's open-source control software, NeuroRighter, is implemented in C#, with an easy-to-use graphical interface. As C# functions in a managed code environment, which may impact performance, analysis was conducted to ensure comparable speed to C++ for this application. Hardware schematics, layout files, and software are freely available. Since maintaining wired headstage connections with freely moving animals is difficult, we describe a new method of electrode-headstage coupling using neodymium magnets.

Comparative tests between commercial and in-house transimpedance preamplifiers for multiwire detectors at synchrotron facilities

New gas filled detectors able to accept events at higher rates than 10 6 photons/s are under study within a collaboration agreement between European Synchrotron Radiation Facility (ESRF), France, and Laboratorio de Luz Sincrotrón (LLS), Spain. The aim of these agreement is to build new detectors able to perform on-time resolved X-ray experiments in the sub-millisecond time scale whilst at the same time have single photon counting capabilities and adequate spatial resolution (less than 300 μm). These detectors will require preamplifiers with low noise, good signal/noise ratio (better than 20 dB), high gain (better than 0.3 μV per electron), short rise time (lower than 10 ns) and pulse duration shorter than 120 ns. We present the design and the results of the evaluation of a fast, low noise charge preamplifiers developed at LLS, Barcelona, Spain. On the other hand, we also present the analogous preamplifier developed at ESRF and we compare both to commercial preamplifiers. The advantages and disadvantages of commercial and in-house preamplifiers are analyzed. The application of these preamplifiers on a multiwire detector is discussed and results are presented.

137Cs Gamma-ray detection at Summit, Greenland

AbstractGlobal fall-out from atmospheric testing of thermonuclear weapons produced horizon markers corresponding to the initiation of testing in 1953 and the maximum fall-out in 1963. The radioactive isotope137Cs associated with these events has a half-life of 30.2 years. Therefore, with the appropriate radiation detectors, this fall-out can be used as a long-term temporal indicator in glaciers and snowpack. A prototype γ-ray detector system was successfully tested and was used to make in-situ measurements of the137Cs marker in a borehole at Summit, Greenland. The system consisted of a 7.6 cm by 7.6 cm NaI(Tl) scintillation crystal/photomultiplier detector, commercial pre-amplifier, amplifier and power supplies, and a microcomputer-based pulse-height analyzer. The measurements were made in boreholes of 25.4 cm and 12.7 cm diameter to depths of 22 m. Based on the results reported here, the γ-ray detection technique promises to be a powerful way to locate quickly horizon markers in the field.

137Cs Gamma-ray detection at Summit, Greenland

AbstractGlobal fall-out from atmospheric testing of thermonuclear weapons produced horizon markers corresponding to the initiation of testing in 1953 and the maximum fall-out in 1963. The radioactive isotope 137Cs associated with these events has a half-life of 30.2 years. Therefore, with the appropriate radiation detectors, this fall-out can be used as a long-term temporal indicator in glaciers and snowpack. A prototype γ-ray detector system was successfully tested and was used to make in-situ measurements of the 137Cs marker in a borehole at Summit, Greenland. The system consisted of a 7.6 cm by 7.6 cm NaI(Tl) scintillation crystal/photomultiplier detector, commercial pre-amplifier, amplifier and power supplies, and a microcomputer-based pulse-height analyzer. The measurements were made in boreholes of 25.4 cm and 12.7 cm diameter to depths of 22 m. Based on the results reported here, the γ-ray detection technique promises to be a powerful way to locate quickly horizon markers in the field.

Ultra low-noise preamplifier for low-frequency noise measurements in electron devices

The design and realization of an ultra-low-noise, high-input-impedance amplifier for low-frequency noise measurements in electronic devices is presented. Special care is devoted to the solution of typical problems encountered in the design of low-noise low-frequency equipment, such as power supply noise and temperature fluctuations. The ultra-low-noise preamplifier has a bandwidth of over seven decades with a low-frequency roll-off of 4 mHz. The noise characteristics obtained are sensibly better than those of commercial preamplifiers commonly adopted in low-frequency noise measurements. The application of this preamplifier to the realization of standard 1/f/sup gamma / noise generators is presented. >

A three-channel electromyograph with synchronized slow-motion photography.

Muscle transplants are performed to strengthen knee extension in polio patients. The time of contraction of muscles in relationship to gait is under investigation. Relative activity is evaluated by electromyographic recording of muscle action potentials. Voltages are recorded from three pairs of flexible wire electrodes implanted in the working muscles. The electromyograph incorporates three commercial preamplifiers, a three-channel electronic switch, a dc amplifier, and a long persistent cathode-ray tube with 3-second sweep. An image of the cathode-ray screen is projected optically via a half-silvered mirror into the lens of a 16-mm movie camera. The camera shoots through the mirror to photograph the walking patient on the same frame with the three muscle traces. Recording speed is 64 frames per second. Frame-by-frame analysis is accomplished by means of a hand-crank projector. This system of recording thus provides simultaneous visualization of muscle electrical activity and the mechanical aspects of gait.