Amplifier Slew Rate Research Articles

Arbitrary magnetic field gradient waveform correction using an impulse response based pre-equalization technique

The time-varying magnetic fields used in magnetic resonance applications result in the induction of eddy currents on conductive structures in the vicinity of both the sample under investigation and the gradient coils. These eddy currents typically result in undesired degradations of image quality for MRI applications. Their ubiquitous nature has resulted in the development of various approaches to characterize and minimize their impact on image quality.This paper outlines a method that utilizes the magnetic field gradient waveform monitor method to directly measure the temporal evolution of the magnetic field gradient from a step-like input function and extracts the system impulse response. With the basic assumption that the gradient system is sufficiently linear and time invariant to permit system theory analysis, the impulse response is used to determine a pre-equalized (optimized) input waveform that provides a desired gradient response at the output of the system. An algorithm has been developed that calculates a pre-equalized waveform that may be accurately reproduced by the amplifier (is physically realizable) and accounts for system limitations including system bandwidth, amplifier slew rate capabilities, and noise inherent in the initial measurement. Significant improvements in magnetic field gradient waveform fidelity after pre-equalization have been realized and are summarized.

Modeling of a Second Order Sigma-Delta Modulator with Imperfections

Abdelghani Dendouga, Nour-Eddine Bouguechal, Souhil Kouda and Samir Barra Advanced Electronic Laboratory, Batna University, Algeria 05 Avenue Chahid Boukhlouf, 05000 BATNA, ALGERIE E-mail: dendouga_gh@hotmail.com Abstract—Sigma delta modulators (ΣΔMs) form part of the core of today’s mixed-signal designs. The ongoing research on these devices shows the potential of ΣΔ data converters as a promising candidate for high-speed, high-resolution, and low-power mixed-signal interfaces. This work presents a new accurate behavioral model of a second order law-pass Switched Capacitor (SC) Sigma-Delta modulator. Our main contribution consists to predict the effect of almost sources of noise on the operation of sigma delta modulator such as non idealities of op-amp and switches. The purpose of this work is the presentation of a behavioral model of a second order switched capacitor ΣΔ modulator considering (Error due to Clock Jitter, Thermal noise Amplifier Noise, Amplifier Slew-Rate, Non linearity of amplifiers, Gain error, Charge Injection, Clock Feed-through, and Nonlinear onresistance). A comparison between the use of MOS switches and the use transmission gate switches use is analyzed.

Quo Vadis Automation?

The methods of handling small signals from multi-decade sensors are discussed. A simple, fast and accurate preprocessing amplification unit (PPU) with the capabilities of automatic gain switching is presented. The unit could be used in the analog channels of a computer based data acquisition and process control system. The use of such a unit with some of the analog sensors would increase the resolution of the measuring subsystem involved through the combined action of a shared analog-to-digital converter (ADC). The PPU consists of an operational amplifier with suitable performance, a resistance feedback network whose value could be changed by switching, and FET switches. The switching control is done by a reversible counter, decoder, and state control logic all using TTL chips. A digital code representing the gain constant is available together with the amplified signal. An experimental unit was built for use in a wide range, neutron flux, measuring channel. This channel is connected to the control computer's analog subsystem which has a 10 bit ADC. Accuracies better than 0.1% of full scale on the most sensitive range (10 <R xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</R> amp.) and switching speeds less than 1 u. sec. are obtainable. The accuracy limits are imposed by the quality of a single amplifier, and the speed limits by the amplifier slew rate and the delay in a few IC logic chips.

A Preprocessing Unit for Wide-Range Measuring Channels

The methods of handling small signals from multidecade sensors are discussed. A simple, fast and accurate preprocessing amplification unit (PPU) with the capabilities of automatic gain switching is presented. The unit could be used in the analog channels of a computer based data acquisition and process control system. The use of such a unit with some of the analog sensors would increase the resolution of the measuring subsystem involved through the combined action of a shared analog-to-digital converter (ADC). The PPU consists of an operational amplifier with suitable performance, a resistance feedback network whose value could be changed by switching, and FET switches. The switching control is done by a reversible counter, decoder, and state control logic all using TTL chips. A digital code representing the gain constant is available together with the amplified signal. An experimental unit was built for use in a wide range, neutron flux, measuring channel. This channel is connected to the control computer's analog subsystem which has a 10 bit ADC. Accuracies better than 0.1% of full scale on the most sensitive range (10-9 amp.) and switching speeds less than 1 ?.sec. are obtainable. The accuracy limits are imposed by the quality of a single amplifier, and the speed limits by the amplifier slew rate and the delay in a few IC logic chips.