Diego Ramírez Muñoz

Magnetic Field Sensors Based on Giant Magnetoresistance (GMR) Technology: Applications in Electrical Current Sensing.

The 2007 Nobel Prize in Physics can be understood as a global recognition to the rapid development of the Giant Magnetoresistance (GMR), from both the physics and engineering points of view. Behind the utilization of GMR structures as read heads for massive storage magnetic hard disks, important applications as solid state magnetic sensors have emerged. Low cost, compatibility with standard CMOS technologies and high sensitivity are common advantages of these sensors. This way, they have been successfully applied in a lot different environments. In this work, we are trying to collect the Spanish contributions to the progress of the research related to the GMR based sensors covering, among other subjects, the applications, the sensor design, the modelling and the electronic interfaces, focusing on electrical current sensing applications.

Full Wheatstone Bridge Spin-Valve Based Sensors for IC Currents Monitoring

Full Wheatstone bridge spin-valve-based electrical current sensors at the IC level are presented. Prototypes with different geometrical parameters have been designed, fabricated and fully characterized. DC characterization has been carried out, for measurement of insertion losses, linearity, voltage offset and sensitivity. Current ranges from 10 muA to 100 mA can be covered with these sensors with excellent linearity and sensitivities above 1 mV/(VmiddotmA) . AC characteristics have also been analyzed and bandwidths exceeding 100 kHz are demonstrated. Moreover, the temperature coefficients have been extracted in the range of -20degC to +60degC. In order to highlight the design properties, dependence of the sensors performance with external magnetic perturbations and self-heating have also been measured and quantified. The associated errors are in the range of 1%-2% of the full scale.

A Non-Invasive Thermal Drift Compensation Technique Applied to a Spin-Valve Magnetoresistive Current Sensor

A compensation method for the sensitivity drift of a magnetoresistive (MR) Wheatstone bridge current sensor is proposed. The technique was carried out by placing a ruthenium temperature sensor and the MR sensor to be compensated inside a generalized impedance converter circuit (GIC). No internal modification of the sensor bridge arms is required so that the circuit is capable of compensating practical industrial sensors. The method is based on the temperature modulation of the current supplied to the bridge, which improves previous solutions based on constant current compensation. Experimental results are shown using a microfabricated spin-valve MR current sensor. The temperature compensation has been solved in the interval from 0 °C to 70 °C measuring currents from −10 A to +10 A.

Temperature compensation of Wheatstone bridge magnetoresistive sensors based on generalized impedance converter with input reference current

A compensation method of the sensitivity drift of the Wheatstone bridge sensor is proposed. The technique was carried out by placing a temperature sensor and the bridge to be compensated inside a generalized impedance converter with input reference current. No internal modification of the bridge arms is required so that the circuit is capable to compensate practical industrial sensors. The method is based on the temperature modulation of the current supplied to the bridge, which improves previous solutions based on constant current compensation. Experimental results are shown using a commercial magnetoresistive bridge sensor.

Current loop generated from a generalized impedance converter: A new sensor signal conditioning circuit

A sensor signal conditioning circuit using a generalized impedance converter (GIC) is presented. The circuit is dc polarized and able to feed a constant current to one or more sensors. GIC input impedance is kept high to maintain the load current of its input reference voltage very low. Experimental results show the feasibility of the dc modified GIC as an electronic conditioning circuit for two resistive temperature detectors (RTD-Pt100) sharing the same current loop.

Fractional modeling of the AC large-signal frequency response in magnetoresistive current sensors.

Fractional calculus is considered when derivatives and integrals of non-integer order are applied over a specific function. In the electrical and electronic domain, the transfer function dependence of a fractional filter not only by the filter order n, but additionally, of the fractional order α is an example of a great number of systems where its input-output behavior could be more exactly modeled by a fractional behavior. Following this aim, the present work shows the experimental ac large-signal frequency response of a family of electrical current sensors based in different spintronic conduction mechanisms. Using an ac characterization set-up the sensor transimpedance function Zt(if) is obtained considering it as the relationship between sensor output voltage and input sensing current, Zt(jf)=Vo,sensor(jf)/Isensor(jf). The study has been extended to various magnetoresistance sensors based in different technologies like anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), spin-valve (GMR-SV) and tunnel magnetoresistance (TMR). The resulting modeling shows two predominant behaviors, the low-pass and the inverse low-pass with fractional index different from the classical integer response. The TMR technology with internal magnetization offers the best dynamic and sensitivity properties opening the way to develop actual industrial applications.

Generalized Impedance Converter as a New Sensor Signal Conditioning Circuit

Generalized impedance converters (GIC) are electronic circuits which have been widely used as controllable impedance in AC regime, especially in active filter synthesis. In the work presented, the GIC is used as a biasing circuit of a magnetoresistive current sensor (Wheatstone bridge) which is located inside the GIC itself. In this case, the GIC works under DC bias unlike its usual AC bias use

Current-to-current converter from a dc polarized generalized impedance converter circuit with input reference current

An enhanced dc generalized impedance converter (GIC) is presented. The circuit is driven by a constant current input source. Connecting a load resistance at the output port the GIC circuit works as a current-to-current converter and is capable to drive at a constant current source a grounded resistance sensor series loop. The experimental results are shown with a temperature and pressure sensors loop.

Constant Current Drive for Resistive Sensors Based on Generalized Impedance Converter

A generalized impedance converter (GIC) is presented in this paper as a programmable current source. A study of its properties and limitations is shown. Its current efficiency is compared with that of Howland topologies, and it is improved in this paper. The contribution of the operational amplifier static offsets to the current source is obtained by presenting typical values of these parameters, and an equivalent circuit with its differential and common modes is presented. The calculated and experimental GIC input impedances are obtained, showing its bandwidth limitation and proposing the possibility of extending the input impedance frequency response without varying its low-frequency value. In this paper, the GIC is used to provide constant current to a series sensor loop and a magnetoresistive current sensor (Wheatstone bridge), which are located within the GIC itself. In both cases, the GIC works under the DC regime, unlike usual AC use.

Transconductance Converters Based on Current Mirrors Applied to pH Measurement Using ISFET Sensors

New transconductance circuits (V/I converters) are designed based on current mirrors and operational amplifiers. Their input/output characteristics and the influence of DC parameters are obtained. Experimental results were performed, comparing them with the simulated results. An electronic instrumentation circuit is designed based on one of the transconductance converters analyzed. The purpose of this circuit was to condition the response of an ion-sensitive field-effect transistor (ISFET) to measure pH. In the proposed application, the V/I converter presented provides the stability and accuracy of the ISFET operating point, making an easy characterization procedure and the design of multichannel ISFET-based measurement systems possible. Experimental results of the ISFET main electrical parameters were obtained, showing the feasibility of applying these new converters to instrumentation and electrochemical measurements.