Luc Hebrard

CMOS microsystem for AC current measurement with galvanic isolation

In this paper, we present an integrated AC current sensor based on sensitivity-optimised horizontal Hall effect devices (HHDs) and a differential readout chain. It has been designed for 5A nominal AC current measurement with 5 kV galvanic isolation and 0.5% accuracy over 1.5 kHz bandwidth, which allows up to 30/sup th/ (25 th) harmonic detection in 50 Hz (60 Hz) applications. From the sensing element to the instrumental chains output the signal conditioning is exclusively performed by low-noise standard CMOS analog blocks. Moreover the whole microsystem features a mixed signal structure dedicated to auto-balancing.

Horizontal Hall effect sensor with high maximum absolute sensitivity

Sensitivity of conventional Hall sensors is strongly limited by the well known short-circuit effect. Many researches were devoted to reduce offset and noise but few works were carried out to improve sensitivity. Here, a new shape of integrated horizontal Hall device is presented. This particular shape has been developed to minimize the short-circuit effect of the sensor, allowing one to shrink the device length and consequently to reduce the biasing resistance. Then the biasing current of this sensor can be significantly increased to obtain an absolute sensitivity higher than for conventional devices. Such a Hall effect device needs a specific biasing circuit which is also presented.

Compact Modeling of a Magnetic Tunnel Junction—Part I: Dynamic Magnetization Model

The potential application range of spintronic devices is wide. However, few works were carried out in the field of compact modeling of such devices. The lack of compact models dramatically increases the design complexity of circuits using spintronic devices. In this paper, focus is made on magnetic tunnel junctions (MTJs). It is presented in a set of two papers: the first part deals with the magnetic aspects of the MTJ, whereas the second one covers the electrical aspects. In this part, a dynamic magnetization model inspired from the micromagnetic formalism is presented. This vectorial model is able to describe the quiescent state, as well as the transient behavior of the magnetization vector of an anisotropic single-domain element. The dynamic magnetization model is implemented in VHDL-AMS and requires only eight parameters. Simulation results are also presented and compared with theoretical ones.

A chopper stabilized biasing circuit suitable for cascaded wheatstone-bridge-like sensors

Many sensitive devices are based on Wheatstone bridge structures or can be modeled as Wheatstone bridges like Hall effect magnetic sensors. These sensors require a biasing circuit, and many solutions were proposed. However, up to now, none of them gives the opportunity to cascade several sensors, while such a cascade can help in improving the signal-to-noise-ratio (SNR) or in removing some parasitic effects through the direct summing/subtraction of sensing/parasitic effects. The circuit this paper presents is based on an operational transconductance amplifier with n output stages, and allows to cascade n Wheatstone-bridge-like sensors. It is shown that the maximal number of bridges which can be efficiently cascaded is limited by the output resistance of the output stages. Nevertheless, this number remains sufficient in practical cases, easily up to n=10. To remove the 1/f noise coming from the output stages, a chopper stabilization is used. We also establish formulas which allow quick hand calculation of the main parameters of the circuit. A prototype where 10 Hall effect sensors are cascaded is presented as well as experimental results.

A Hall effect sensors network insensitive to mechanical stress

Hall effect devices are widely used as magnetic sensors in integrated technology. Since they are very sensitive to mechanical stress, they are not suitable for the design of systems subjected to vibrations (portable systems, automotive applications,...). To cancel this cross-sensitivity, the spinning current method may be used. Nevertheless, this method does not remove, but modulates, the mechanical signal. Vibrations whose frequencies are close to the spinning frequency are thus rejected in the base band. Here, we propose an alternative based on a Hall effect sensor network. This method totally removes the mechanical signal and may be combined with the spinning current method in order to reject the flicker noise of the sensor. Compared experimental results obtained with Hall effect sensors subjected to vibrations are presented and discussed.

Compact Modeling of a Magnetic Tunnel Junction—Part II: Tunneling Current Model

The potential application range of spintronic devices is wide. However, a few works were carried out in the field of compact modeling of such devices. The lack of compact models dramatically increases the design complexity of circuits using spintronic devices. In this paper, focus is made on magnetic tunnel junctions (MTJs). It is presented in a set of two papers: The first part deals with the magnetic aspects of the MTJ, whereas the second one covers the electrical aspects. In this part, the tunneling conduction across the MTJ is modeled using an analytical I-V equation, which is based on previous works on this topic and involves some assumptions that are discussed. The complete compact model is implemented in a very high speed integrated circuit (VHSIC) hardware description language analog mixed signal and includes magnetization aspects presented in the first part. The model requires 25 parameters (19 physical and 6 semiempirical parameters).

3D Hall probe integrated in 0.35 μm CMOS technology for magnetic field pulses measurements

This paper presents a 3 dimensional magnetometer based on Hall effect sensors integrated without any post processing in a standard low cost 0.35 mum CMOS technology. The system is dedicated to magnetic pulses measurements under a strong static field. Two vertical Hall devices (VHD) are sensitive to the components of the magnetic field oriented in the plane of the chip, while a horizontal Hall device (HHD) is sensitive to the component of the magnetic field orthogonally oriented to the plane of the chip. 3 identical instrumental chains are integrated to perform amplification of the 3 Hall voltages. The system implements a compensation of the static magnetic field and allows to measure magnetic fields pulses with a resolution of 79 muT over a [5 Hz - 1.6 kHz] bandwidth. Pulses are in the range from hundreds of muT to tens of mT in the frequency range from 1 Hz to 10 kHz. The static field is compensated up to 1.5 T. The spatial resolution is 44 mum. The system power consumption has been optimized to 15 mW.

Compact modeling of magnetic tunnel junction

A new compact model of a magnetic tunnel junction (MTJ) is presented in this paper. This model is intended to describe the behavior of a MTJ and to take the magnetic as well as the non-linear electronic transport phenomena into account. It should be suitable for circuits simulation and thus, it must be simple (no finite element approach, analytical current versus voltage characteristic only). For this purpose, some assumptions are made. The MTJ model is separated in two entities. The first one concerns the magnetization of a ferromagnetic thin film. The other focuses on the electrical conduction of a MTJ. Both models are implemented and coupled in VHDL-AMS in order to obtain the compact model of a MTJ, which is parameterized with 25 values (19 physical parameters and 6 semi-empirical ones). The first simulations are encouraging. They allow to retrieve classical results on MTJ and to predict interesting behaviors.

A two-axis magnetometer using a single magnetic tunnel junction

Based on a qualitative study of the Stoner-Wohlfarth model, we point out that driving a magnetic tunnel junction (MTJ) with an alternative two-dimensional magnetic field allows to measure simultaneously two components of an external magnetic field. Only one single MTJ without a pinning layer is needed to measure both components of a magnetic field parallel to the junction plane. The response of the magnetometer does not depend on the resistance of the junction or the amplitude of its variations. A prototype has been manufactured and encouraging experimental results are presented. Sensitivities higher than 500 V/T and a noise level of 2 /spl mu/T//spl radic/Hz are reported.

Compact modeling of a magnetic tunnel junction using VHDL-AMS: computer aided design of a two-axis magnetometer

Currently, the lack of compact magnetic tunnel junction (MTJ) model is a truly limiting factor for the design of spintronics circuits. In this paper, we present a compact MTJ model written in VHDL-AMS. This behavioral model is based on the Stoner-Wohlfarth model and takes most of the important phenomena such as magnetic coupling, capacitance, and magnetization dependent conductance into account. The method employed to model a two layer magnetic tunnel junction is detailed. Applications of this model such as the simulation of the operation of a magnetometer is also presented.