1. Technical Field
The present disclosure relates to an electrical biasing circuit for a magnetic-field sensor, in particular an anisotropic magnetoresistive (AMR) magnetic sensor, and to a corresponding electrical-biasing method.
2. Description of the Related Art
Magnetic-field sensors, in particular AMR sensors, are used in a plurality of applications and systems, for example in compasses, in systems for detecting ferrous materials, in the detection of currents, and in a wide range of other applications, thanks to their capacity for detecting natural magnetic fields (for example, the Earth's magnetic fields) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
In a known way, the phenomenon of anisotropic magnetoresistivity occurs within particular ferrous materials, which, when subjected to an external magnetic field, undergo a variation of resistivity as a function of the characteristics of the external magnetic field. Usually, these materials are applied in the form of thin strips so as to form magnetoresistive elements, and the magnetoresistive elements thus formed are electrically connected to form a bridge structure (typically a Wheatstone bridge).
Manufacturing of AMR sensors with standard techniques of semiconductor micromachining is also known, as described, for example, in U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example Permalloy (a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate of semiconductor material, for example silicon.
When an electric current I is made to flow through a magnetoresistive element (see FIG. 1a), the angle θ between the direction of magnetization M of the magnetoresistive element and the direction of flow of the electric current I affects the effective value of resistivity of the magnetoresistive element, so that, as the value of the angle θ varies, the value of electrical resistance also varies (in detail, this variation follows a law of the cos2θ type). The magnetoresistive elements are generally brought into an initial condition of magnetization, via appropriate biasing means; next, an external magnetic field He causes a variation of the direction of magnetization M of the magnetoresistive elements (and a variation of the value of angle θ), and consequently a variation of resistance. In particular, it is the component of the external magnetic field He perpendicular to the direction of magnetization M, acting along the so-called axis of sensitivity or detection, that affects the value of the angle θ.
The Wheatstone-bridge detection structure of an AMR sensor includes magnetoresistive elements that have ideally the same value of resistance and are such as to form diagonal pairs of equal elements, which react in a way opposite with respect to one another to the external magnetic fields, as shown schematically in FIG. 1b (where I is once again the electric current flowing in the magnetoresistive elements and R is the common resistance value). If a supply (or electrical biasing) voltage Vss is applied at input to the bridge detection structure (in particular to first two terminals of the bridge, operating as input terminals), in the presence of an external magnetic field He, a resistance variation ΔR of the magnetoresistive elements and a corresponding variation of the value of voltage drop on the magnetoresistive elements occur. There follows an unbalancing of the bridge, causing a voltage variation ΔV at output (in particular between the remaining two terminals of the bridge, operating as output terminals). Given that the initial direction of magnetization of the magnetoresistive elements is known beforehand, as a function of this voltage variation ΔV it is possible to determine the component of the external magnetic field acting along the axis of sensitivity of the detection structure.
In particular, in order to detect the unbalancing of the Wheatstone bridge and generate an electrical output signal indicating the characteristics of the external magnetic field to be measured, a reading circuit (or front-end) is normally used, which is coupled to the output of the detection structure of the AMR sensor and includes a signal-conditioning stage, comprising amplification and filtering units, and possibly an analog-to-digital-converter stage, which supplies to the outside the output signals.
A measurement technique proposed and widely used moreover envisages the use of coils or straps, which are integrated in the same AMR sensors, and are designed to generate, when traversed by current, a magnetic field with pre-defined direction and sense; these coils are known as “set/reset straps”. For example, the set/reset straps are provided on the same substrate as that on which the magnetoresistive elements of the sensor are provided, being electrically insulated from, and set in the proximity of, the magnetoresistive elements.
During operation, the initial orientation of the magnetization direction is obtained by applying to the magnetoresistive elements, via the set/reset straps, an intense magnetic field for a short period of time, of a value such as to force and align the orientation of the magnetic dipoles of the magnetoresistive element in a first pre-defined direction (in the case of a “set” pulse) of a magnetization axis (the so-called “easy axis”), or else in a second pre-defined direction, opposite to the first direction (in the case of a “reset” pulse) along the same magnetization axis, according to the sense of the magnetic field generated, and hence in a way coherent with the sense of the current that circulates in the same set/reset straps. The aforementioned set and reset operations are known and described in detail, for example, in U.S. Pat. No. 5,247,278.
The reversal of orientation of the magnetic dipoles causes inversion of sign of the signal at output from the Wheatstone bridge, in the presence of an external magnetic field He. Instead, any possible offset signals superimposed on the useful signal in the output signal do not reverse their own polarity, since they are due exclusively to mismatch between the components internal to the sensor and are hence independent of the characteristics of the external magnetic field He.
Consequently, the measuring procedure envisages applying a set pulse and, after waiting an appropriate relaxation time such as to eliminate possible current tails and allow settling of the magnetic dipoles of the ferromagnetic material, acquiring a first sample of the output signal (for example, a voltage signal, Vout) in the presence of the external magnetic field He; the first sample of the output signal, designated by Voutset is given by:Voutset=He·S+Voutoff where S is the sensitivity of the magnetic sensor and Voutoff the offset signal superimposed on the output.
Next, a reset pulse is applied and, after waiting an appropriate relaxation time such as to eliminate possible current tails and allow settling of the magnetic dipoles of the ferromagnetic material, a second sample of the output signal is acquired, to obtain:Voutreset=He·(−S)+Voutoff where −S is the value of sensitivity of the magnetic sensor, having in this case a value equal and opposite to the value of sensitivity S during the set operation, on account of the reversal of sense of the magnetic dipoles of the magnetoresistive elements of the sensor.
A subtraction is then made between the first sample and the second sample that have been acquired, which makes it possible to derive the useful signal, cancelling out the effects of the offset contribution on the output signal, as follows:(Voutset−Voutreset)=He·S+Voutoff−(He·(−S)+Voutoff)=2He·S 
Moreover known to the art is the joint use of three Wheatstone-bridge detection structures, each constituted by appropriately oriented magnetoresistive elements, having axes of sensitivity orthogonal to one another, for determining the magnitude and direction of an external magnetic field acting in any direction of space (thus determining the corresponding components along the three axes of sensitivity). This configuration is used, for example, in the manufacturing of compasses or magnetometers, for example in mobile-phone devices or in other mobile devices.
In this kind of applications, the magnetic sensor hence comprises, as a whole, three detection structures, each corresponding to a respective axis of sensitivity, x, y or z, and moreover a biasing and reading circuit, typically provided as application-specific integrated circuit (ASIC), configured so as to detect and process the electrical signals supplied by the various detection structures, and supply an appropriate biasing voltage to the same detection structures, during the entire operation of the magnetic-sensor device (the biasing voltage, see FIG. 1b, is used for supplying the Wheatstone bridge and determining unbalancing thereof in the presence of external fields).
Also due to the low values of electrical resistance of the various magnetoresistive elements constituting the detection structures, the consumption of each Wheatstone-bridge detection structure is rather high (even of the order of some milliamps). In addition, the same reading circuit, which comprises amplifiers and in general active electronic components, has considerable consumption levels, which adds to the consumption associated to the detection structures, determining as a whole a considerable energy expenditure. The energy consumption is still higher in the case where a number of reading circuits, which operate in parallel, are used, each coupled to a respective detection structure; a solution of this sort moreover entails a considerable increase in the area occupation for the integrated implementation.
The problem of electrical consumption of magnetic sensors is hence particularly felt, especially in applications in which a number of detection structures is used for detecting components of magnetic fields oriented along respective axes of sensitivity. This problem, of course, is even more important in applications in which the energy usage represent a determining design constraint, such as, for example, in battery-supplied portable applications.