System and method for suppressing low-frequency noise from magnetoresistive sensors

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to a system and method for suppressing low-frequency noise from a magnetoresistive type sensor. Magnetoresistive sensors cover Giant Magnetoresistance (GMR) sensors and Tunnel Magnetoresistance (TMR) sensors, but the method can be extended to any magnetoresistive type magnetic field sensor. ART ANTERIEUR

[0002] The low-frequency noise of magnetoresistive sensors such as GMRs or TMRs is considered today as the major obstacle for their use in certain applications requiring very high detectivity in low frequencies. An example of these applications is the measurement of magnetic fields in biological environments, such as magnetic fields associated with physiological signals and in particular neuronal signals. These signals vary slowly, with frequencies lower than 1 KHz and their detection is affected by the low-frequency noise of the sensor used during the measurement.

[0003] For anisotropic magnetoresistance (AMR) sensors, a current direction change method can suppress part of their 1 / f noise (see for example I. Mateos et al. “Low-frequency noise characterization of a magnetic field monitoring system using an anisotropic magnetoresistance”, published in Sensors and Actuators A, Volume 235, 2015). This technique cannot be applied to GMRs and TMRs because their resistance and resistance change do not depend on the current direction.

[0004] Techniques for modulating the field seen by the sensor can be applied. These techniques move the operating point of the sensor out of its low-frequency noise. Examples of such techniques are described in the papers "Towards picoTesla Magnetic Field Detection Using a GMR-MEMS Hybrid Device" by A. Guedes et al., published in IEEE TRANSACTIONS ON MAGNETICS, Vol. 48, No. 11 Pages 4115-4118, 2012, and "Minimizing 1 / f noise in magnetic sensor using a microelectromechanical system flux concentrator", by A. S. Edelstein et al., published in Journal of Applied Physics, Vol. 91, page 7795, 2002.

[0005] These publications propose using frequency-modulated flux concentrators, but this technique gives modest results and requires the use of MEMS (Micro Electro-mechanical systems) to perform mechanical modulation.

[0006] In the particular case of sensors coupled to superconducting current loops, supercurrent modulation can relatively effectively suppress low-frequency noise by also shifting the sensor's operating point to higher frequencies. Such solutions are described in patent documents EP2165206 and EP2165210.

[0007] Publication US2008224695 A1 discloses another magnetoresistive sensor, including modulation means for reducing low frequency noise.

[0008] However, these techniques have several drawbacks such as implementation difficulties, a limitation to miniaturization, low efficiency or the use of superconducting materials which require specific conditions of use and can have high costs. RESUME DE L'INVENTION

[0009] The invention aims to solve the problems mentioned above by proposing a system for suppressing low-frequency noise from magneto-resistive sensors, this system being reliable, compact and feasible from any type of magneto-resistive sensor.

[0010] To this end, a first object of the invention is a system for suppressing low-frequency noise from magnetoresistive sensors, said suppression system comprising: A device for measuring a magnetic field, said device comprising at least one magnetoresistive sensor, said magnetoresistive sensor having a first sensitivity at a first operating point and a second sensitivity at a second operating point, the sensitivity at the second operating point being low or zero; Modulation means adapted to switch the magnetoresistive sensor from the first operating point to the second operating point and from the second operating point to the first operating point, said modulation means having a first configuration corresponding to the first operating point and a second configuration corresponding to the second operating point;Means for processing the signal from the device for measuring a magnetic field, said processing means being adapted to produce a linear combination of a first response of the measuring device in the presence of the magnetic field at the first operating point corresponding to the first configuration of the modulation means and a second response of the measuring device in the presence of the magnetic field at the second operating point corresponding to the second configuration of the modulation means.;

[0011] A magnetoresistive sensor C is any element with an electrical resistance dependent on the external magnetic field. By measuring the variation in resistance across the terminals of element C, it is possible to measure the external magnetic field. The external magnetic field or magnetic field is the magnetic field that we want to measure.

[0012] Tunnel magnetoresistance (TMR) sensors and giant magnetoresistance (GMR) sensors can be used in the present invention.

[0013] According to one embodiment of the invention, the GMR or TMR magnetoresistive sensors used are sensors without hysteresis.

[0014] Each magnetoresistive sensor used in the present invention has different operating points having different sensitivity.

[0015] Modulation means M are understood to mean means for periodically modifying the sensitivity of the magnetoresistive sensors used. The means M may comprise a current or voltage generator. The modulation carried out by the means M is, for example, a periodic variation in the sensitivity of the magnetoresistive sensors C. The frequency of this modulation is higher than the frequency of the low-frequency noise that is to be eliminated.

[0016] Low or zero sensitivity of the magnetoresistive sensor C means a sensitivity of less than 0.05% / mT in the case of a GMR having a typical sensitivity of 1% / mT and 1% / mT for a TMR having a sensitivity of 20% / mT. Signal processing means T means used to select the response M1 of the device D when the device D is at the first operating point and the response M2 of the device D when the device is at the second operating point. The signal processing means T are also adapted to produce a linear combination of the responses M1 and M2. The processing means T may comprise analog circuits, digital circuits or a mixture of analog and digital circuits.

[0017] The low-frequency noise associated with a magnetoresistive C sensor is, as in all conductors, a noise of resistance fluctuations. Furthermore, the external field also creates a change in resistance. When using a magnetoresistive sensor to measure a slowly varying magnetic field, the two resistance variations cannot therefore be separated by a single measurement.

[0018] The proposed invention is based on a different principle from those proposed so far. It consists of oscillating the magnetoresistive sensor between two different operating points. The two points are chosen so that the response to an external magnetic field is different. The response of the sensor to the external magnetic field is also called the sensitivity of the sensor. The oscillation of the sensor between two different operating points is also called modulation of the sensitivity of the sensor.

[0019] In other words, the invention consists of oscillating the sensor between these two measurement points at a frequency faster than the 1 / f noise domain as indicated in the figure 1 and therefore to measure at high frequency the response of the sensor in each state. We thus obtain two independent curves, M1 and M2, the two curves being functions of time. A linear combination of these two curves makes it possible to obtain a curve dependent only on the external field and a curve giving the internal resistance fluctuations.

[0020] The reconstruction of these two curves, M1 and M2, can be done either digitally or analogically.

[0021] The invention will be all the more effective as the two points correspond to very different sensitivities.

[0022] In particular, the fact of having chosen a second operating point with low or zero sensitivity makes it possible to retain, in the measurement M2 corresponding to this operating point, only the resistance variations due mainly to low-frequency noise, the sensor no longer being sensitive to the external field B. This measurement can then be subtracted from the measurement M1 carried out at the first operating point, so as to isolate the resistance fluctuations due essentially to the variation in the external magnetic field B and therefore corresponding to a measurement of this external field.

[0023] The device according to the invention therefore makes it possible to separate the resistance variations of a magnetoresistive sensor due to low-frequency noise and the resistance variations due to the variation of the external magnetic field B. In other words, the device according to the invention makes it possible to eliminate the low-frequency noise of magnetoresistive sensors.

[0024] The device according to the invention may also have one or more of the characteristics below, considered individually or in all technically possible combinations, as long as they fall within the scope of the appended claims: the device D for measuring a magnetic field B comprises two magneto-resistive sensors 301, 302 arranged in a half-bridge arrangement and a low-noise preamplifier PA, the two magneto-resistive sensors having an inverted response to the magnetic field, the half-bridge arrangement comprising a first arm B1 and a second arm B2, the two arms being connected in parallel, each of the arms comprising a resistor R and one of the magneto-resistive sensors 301, 302, the half-bridge arrangement further comprising a first V+ and a second V-output, the two outputs being connected to the low-noise preamplifier PA, each output V+, V- being the junction point between one of the resistors R and one of the magneto-resistive sensors 301, 302;the measuring system D comprises a direct voltage source V for supplying the half-bridge assembly, the direct voltage V being connected to the junction point between the two resistors R or to the junction point between the two magneto-resistive sensors 301, 302;the measuring device D comprises a first 401, 401a and a second 402, 402a pair of magneto-resistive sensors and a low-noise preamplifier PA, the sensors of the first pair 401, 401a having an inverted response compared to the sensors of the second pair 402, 402a, the magneto-resistive sensors 401, 401a, 402, 402a being arranged in a bridge arrangement, the bridge arrangement comprising a first arm B1 and a second arm B2, the two arms being connected in parallel, each arm comprising a magneto-resistive sensor of the first pair 401, 401a and a magneto-resistive sensor of the second pair 402, 402a, the bridge arrangement further comprising a first V+ and a second V- output, the two outputs being connected to the low-noise preamplifier, each output V+, V- being the junction point between a magneto-resistive sensor of the first pair 401, 401a and a magneto-resistive sensor of the second pair 402, 402a;the measuring system D comprises a DC voltage source V for powering the bridge assembly, the DC voltage source V being connected to a junction point between a magneto-resistive sensor of the first pair and a magneto-resistive sensor of the second pair; the measuring device D comprises local heating means so as to turn over the reference layer 703, 704 of at least one of the magneto-resistive sensors; the measuring device D comprises current lines 301, 302, 403, 404 for applying a magnetic field in the plane of the layers of the magneto-resistive sensors, so that in the presence of current in the lines each magneto-resistive sensor C is at the second operating point 202 and in the absence of current in the lines each magneto-resistive sensor C is at the first operating point 201;when the current is applied, the magnetoresistive sensors having inverted responses are at the second operating point (202) having low or zero sensitivity (Ssat), while having the same resistance; the modulation means M comprise a high-frequency general clock for generating a switching signal L1 / L2 between the first operating point 201 and the second operating point 202; the switching signal L1 / L2 comprises current pulses flowing in the current lines 301, 302, 403, 404 for switching each magnetoresistive sensor between the first operating point 201 and the second operating point 202; the processing means T of the signal from the device D for measuring a magnetic field B comprise a device for rapid digital acquisition of the signal from the preamplifier PA;the processing means T of the signal from the device D for measuring a magnetic field B comprise: A first Sample and Hold circuit intended to record the signal M1 measured by the measuring device D at the first operating point 201; A second Sample and Hold circuit intended to record the signal M2 measured by the measuring device D at the second operating point 202; A digital DSP or analog acquisition system 1301 for the linear combination of the signals from the first and second Sample and Hold circuits; the general clock further generates a first signal SH1 for controlling the first Sample and Hold circuit and a second signal SH2 for controlling the second Sample and Hold circuit. ;

[0025] Another object of the invention is a method for suppressing low-frequency noise associated with the measurement of a magnetic field by a measuring device comprising at least one magnetoresistive sensor, said method comprising the following steps: Identifying a first and a second operating point of the at least one magnetoresistive sensor, the magnetoresistive sensor having a first sensitivity at the first operating point and a second sensitivity at the second operating point, the sensitivity at the second operating point being low or zero; Modulating the sensitivity of the magnetoresistive sensor by switching the magnetoresistive sensor from the first operating point having the first sensitivity to the second operating point having the second sensitivity and from the second operating point to the first operating point; During the modulation, measuring a first response of the measuring device D in the presence of the magnetic field at the first operating point S1 and a second response M2 of the measuring device in the presence of the magnetic field at the second operating point;Calculate a linear combination of the first response M1 and the second response M2 of the measurement system D.;

[0026] The first step of the method according to the invention makes it possible to identify the two operating points of the magnetoresistive sensors included in the device D, the two operating points having two different sensitivities.

[0027] Advantageously, the second operating point is chosen so as to have low or zero sensitivity to the external magnetic field. This makes it possible to differentiate resistance fluctuations due to low-frequency noise from sensor C from resistance fluctuations due to the external magnetic field.

[0028] The sensitivity of the sensor is then modulated, for example using modulation means M, so as to switch the magneto-resistive sensors of the device D between the two operating points.

[0029] It is therefore possible to measure the response of device D at the first and second operating points, so as to identify resistance fluctuations due mainly to low-frequency noise in conditions of low or zero sensitivity of the magnetoresistive elements.

[0030] The method according to the invention may also have one or more of the characteristics below, considered individually or in all technically possible combinations, as long as they fall within the scope of the appended claims: the modulation frequency of the sensitivity of the sensor is greater than the frequency 101 at which the low frequency noise becomes lower than the thermal noise associated with the sensor C; the modulation frequency MOD of the sensitivity of the sensor is at least twice as high as the frequency 101 at which the low frequency noise becomes lower than the thermal noise associated with the sensor C;the measuring device D comprises two magneto-resistive sensors 301, 302 arranged in a half-bridge arrangement and a preamplifier, the two magneto-resistive sensors having an inverted response to the magnetic field, the half-bridge arrangement comprising a first arm B1 and a second arm B2, the two arms being connected in parallel, each arm comprising a resistor R and one of the magneto-resistive sensors 301, 302, the half-bridge arrangement further comprising a first V+ and a second V- output, the two outputs being connected to the low-noise preamplifier, each output V+, V- being the junction point between one of the resistors R and one of the magneto-resistive sensors 301, 302;the measuring device D comprises a first 401, 401a and a second 402, 402a pair of magneto-resistive sensors and a low-noise preamplifier, the sensors of the first pair 401, 401a having an inverted response compared to the sensors of the second pair 402, 402a, the magneto-resistive sensors 401, 401a, 402, 402a being arranged in a bridge arrangement, the bridge arrangement comprising a first arm B1 and a second arm B2, the two arms being connected in parallel, each arm comprising a magneto-resistive sensor of the first pair 401, 401a and a magneto-resistive sensor of the second pair 402, 402a, the bridge arrangement further comprising a first V+ and a second V- output, the two outputs being connected to the low-noise preamplifier, each output V+, V- being the point junction between a magneto-resistive sensor of the first pair 401, 401a and a magneto-resistive sensor of the second pair 402, 402a;the step MOD of modulating the sensitivity of at least one magneto-resistive sensor is carried out using a current line arranged near the at least one magneto-resistive sensor and adapted to generate a magnetic field in the plane of the layers of the magneto-resistive sensor, so as to saturate the magneto-resistive sensor by reducing or canceling its sensitivity when the sensor is at the second operating point; the step MES of measuring the first response M1 of the measuring device D and the second response M2 of the measuring device D is carried out using a digital or analog acquisition system 1101; the linear combination step LIN is carried out using a digital or analog acquisition system. ; LISTE DES FIGURES

[0031] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which: there figure 1 shows a diagram of the low-frequency noise reduction system S according to an object of the invention; figure 2 shows the noise spectral density in 1 / f or low frequency noise; the figure 3 shows a typical response of a GMR or TMR sensor; the figure 4 shows a first example of measuring device D associated with the low-frequency noise reduction system S as given in figure 1 ; in this case a half-bridge assembly with two magneto-resistive elements is represented; the figure 5 shows a second example of measuring device D associated with the low-frequency noise reduction system S as given in figure 1 ; in this case a full bridge assembly with four magneto-resistive elements is represented; the figure 6 shows a physical example of a C-shaped GMR sensor, with integrated current lines and allowing a magnetic field to be applied for modulation of the sensor sensitivity; this system can be used in the magnetic field measurement device D as given in figure 1 ; there figure 7 shows an example of a bridge output time curve as given in figure 5 ; there figure 8 shows a typical stack of a tunnel magnetoresistance TMR; such a sensor can be used in device D shown in figure 1 ; there figure 9 schematically illustrates an embodiment of the low-frequency noise suppression system S with digital processing of the signal from the measuring device D; figure 10 shows the electronic diagram of an embodiment of the low-frequency noise suppression system S making it possible to obtain the two curves M1 and M2 in an analog manner and to carry out a digital linear combination; the figure 11 shows the electronic diagram of an embodiment of the low-frequency noise suppression system S making it possible to obtain the two curves M1 and M2 in an analog manner and to carry out an analog linear combination; the figure 12 shows an example of signals from the modulation means M and allowing the sensitivity of the GMR or TMR sensors to be modulated; figure 12 also shows the signals used to drive the analog signal processing circuits M1 and M2; the figure 13 shows the steps of the method for implementing the noise reduction system according to the invention. DESCRIPTION DETAILLEE DE L'INVENTION

[0032] There figure 1 shows an example of the low-frequency noise reduction system S according to the invention. The system S comprises: modulation means M used to modulate the sensitivity of at least one magnetoresistive sensor; the modulation means comprise for example a direct voltage generator V and a current or voltage pulse generator GI; a device D for measuring an external magnetic field B; the device D comprises at least one magnetoresistive sensor C, a low-noise amplifier PA for amplifying the signal from the magnetoresistive sensor C and a bandpass filter FPB for eliminating the low and high frequency components of the signal measured by D; it is important to note that each magnetoresistive sensor forming part of the device D has different operating points with different sensitivities to the external magnetic field B; the device D provides a first measurement M1 and a second measurement M2 from the device D;a signal processing device T for recording the two measurements M1 and M2 and / or performing the linear combination of the first measurement M1 and the second measurement M2.;

[0033] There figure 2 shows an example of the 1 / f noise spectral density of a magnetoresistive sensor. In this figure we see that the low-frequency noise becomes lower than the thermal noise from frequency 101. In this case, the oscillation frequency between the two operating points must be higher than frequency 101 and if possible, at least twice the frequency corresponding to point 101.

[0034] The oscillation frequency between the two operating points with different sensitivities is also called the sensitivity modulation frequency of magnetoresistive sensors.

[0035] Advantageously, it is better to choose a sufficiently high modulation frequency, that is to say beyond the frequency where the resistance fluctuations become equal to the thermal noise.

[0036] There figure 3 shows the typical response of a GMR or TRM type magnetoresistive sensor linearized to have a correct response in zero field. The ordinate axis represents the voltage measured at the terminals of the magnetoresistive sensor and the abscissa axis represents the external magnetic field. The two operating points 201 and 202 are two points having a first sensitivity S1 and a second sensitivity S2.

[0037] The sensitivity being proportional to the slope of the curve illustrated in the figure 3 , we see that point 201 corresponds to a high sensitivity S1 and point S2 to a low or zero sensitivity. In particular, point 201 is located in the operating zone or sensitivity zone of the sensor. Point 202 is located in the saturation zone where its sensitivity becomes low or zero. The system S according to the invention makes it possible to oscillate the magneto-resistive sensor between the two points 201 and 202.

[0038] For example, when a magnetic field applied to each magnetoresistive element exceeds a value Hs in the plane of the stack's sensitivity axis, the sensor is in the saturation region.

[0039] There figure 4 shows a first embodiment of the measuring device D of the system S according to the invention. According to this embodiment, the measuring device D comprises two magneto-resistive sensors 301 and 302 and two identical resistors R. These four elements are connected according to a half-bridge assembly illustrated in figure 4 .

[0040] In particular, the half-bridge circuit comprises a supply voltage V connected to a first arm B1 and to a second arm B2. The two arms are connected in parallel. The end of arms B1 and B2 opposite the end connected to voltage V is connected to ground. Each of the two arms B1 and B2 comprises a resistor R and a magnetoresistive element. In the example illustrated in figure 4 , the supply voltage V is connected to the junction point between the two resistors R. Alternatively, the voltage V can be connected to the junction point between the two magneto-resistive sensors 301 and 302.

[0041] Each arm B1, B2 of the assembly of the figure 4 includes a V-, V+ output. Both V-, V+ outputs are connected to a low noise preamplifier PA.

[0042] The two magnetoresistive sensors 301 and 302 have an inverted response to the external field. In other words, under the action of the external magnetic field and uniform in the volume occupied by the bridge of the figure 4 , an increase in magnetoresistance of the first magnetoresistive sensor 301 corresponds to a decrease in magnetoresistance of the second magnetoresistive sensor 302.

[0043] Advantageously, this makes it possible to measure a potential difference between the two outputs V+ and V- which is proportional to the external magnetic field to be measured.

[0044] To implement the invention, it is necessary to modulate the sensitivity of the magnetoresistive sensors 301 and 302 between the first operating point 201 having a high sensitivity S1 and the second operating point Ssat having zero sensitivity. This modulation can be obtained by applying a sufficiently intense magnetic field in the plane of the layers of the magnetoresistive sensors to saturate the sensors by moving them into the saturation zone.

[0045] Since both elements 301 and 302 have an inverted response, the saturation field to be applied to each element must be inverted.

[0046] According to the embodiment illustrated in the figure 4 , the application of the saturation magnetic field on each element 301, 302 is carried out using the current lines 303 and 304. The strong field-zero field oscillation is carried out by integrated current lines 303 and 304 as proposed for example on the figure 6 The current must be chosen in such a way that the sensor saturates in the applied current mode, i.e. it allows the creation of a magnetic field on the elements greater than Hs+Hp where Hp is the desired field operating range for the sensor and Hs is the saturation field of the element. As the current applied for the same field will vary inversely with the width of the sensors, it is preferable to use sensors with a small lateral dimension, typically of the order of 3 to 5µm, but widths outside this range can be chosen.

[0047] The first operating point of the sensor corresponds to a current in the lines of zero. The second operating point corresponds to a current in the lines which saturates the sensors by creating a sufficiently intense saturation magnetic field. It should be noted that the current in the lines must be applied in such a way that the second operating point 202 corresponds to the same resistance value for both sensors and must therefore be physically reversed given their inverted response. The arrangement of the current lines 303, 304 illustrated in figure 4 allows for the same current flowing in the lines to obtain inverted magnetic fields in correspondence of elements 301 and 302 having inverted responses. There may be small rapid transients during the application of the current and its removal.

[0048] Advantageously, the current lines 303 and 304 can be integrated on the measuring device D, reducing the size of the low-frequency noise reduction system S.

[0049] Alternatively, if elements 301 and 302 are sufficiently spaced, two independent coils can be used for the application of the saturation magnetic field.

[0050] Advantageously, the half-bridge assembly allows for an output independent of the operating point of the magneto-resistive sensors. In other words, the differential voltage at the output of the bridge of the figure 4 when the magnetoresistive elements are at the first operating point 201 is close to the differential voltage when the magnetoresistive elements are at the saturation point 202.

[0051] This configuration is very advantageous because in both cases it is possible to amplify the output voltage of the bridge without saturating the low noise preamplifier PA.

[0052] There figure 5 shows a second embodiment of the measuring device D for the low-frequency noise reduction system S. In this case, there is a full-bridge circuit with four magneto-resistive elements. Elements 401 and 401a have an inverted response compared to elements 402 and 402a.

[0053] The operation of the circuit of the figure 5 is similar to the operation of the figure 4 The saturation field of the magnetoresistive elements can be applied through the current lines 403 and 404. These current lines can be integrated into the device D to reduce the size of the system S.

[0054] Alternatively, if the elements 401, 401a, 402, 402a are sufficiently spaced apart, four independent coils can be used to generate the magnetic field to saturate the sensors.

[0055] Advantageously, the bridge assembly allows on the one hand to have an output independent of the operating point of the magneto-resistive sensors and on the other hand allows to gain a factor of 2 on the output amplitude.

[0056] Both in the case of the half-bridge assembly illustrated in the figure 4 and in the case of the bridge assembly illustrated in figure 5 , the magnetoresistive elements must have an inverted response. This inverted response can be obtained using already known methods: the first consists of mounting 4 identical but physically reversed sensors. This method is simple to use but requires having two independent silicon dies and therefore has a higher commercial cost. The second method consists of turning over by local heating means the reference layer of the two magnetoresistive elements 402, 402a by local heating under field. The third method consists of depositing two slightly different stacks which have inverted but very close responses. In a preferred mode, we apply the second method which allows to have an industrializable process at low cost.

[0057] There figure 6 shows a physical example of a C-shaped GMR sensor with integrated current lines to create a sufficient field to saturate the sensors while having low current consumption.

[0058] There figure 7a shows an example of the output time curve of a device D such as that shown in figure 5 . This curve represents the output voltage of the bridge of the figure 5 , the bridge being subjected to a sinusoidal external magnetic field. Curve 7b shows the current in the current lines 403 and 404. The current sent in the current lines corresponds to the magnetic field applied to the sensors 401, 401a, 402 and 402a and capable of saturating them. In other words, when the current in the lines is zero, the sensors are at the first operating point 201 with high sensitivity.

[0059] In the presence of current in the lines, the applied magnetic field saturates the C sensors, moving their operating point to points 202 with low or zero sensitivity.

[0060] Advantageously, the current in the current lines 403, 404 is for example controlled using the modulation means M of figure 1 The absence of current in the lines corresponds to the first configuration of the modulation means M and the presence of current in the lines corresponds to the second configuration of the modulation means M.

[0061] Advantageously, the operating point of the magneto-resistive elements of the measuring device D can be controlled using the modulation means M.

[0062] There figure 7c gives the output of the bridge in the presence of the external magnetic field and the current excitation of the figure 7b . On this curve we see that in the absence of current in the current lines 403, 404 the device D is sensitive to the external magnetic field and the output of the bridge follows the variations of this field. This is, for example, the measurement at point 601 of figure 7b .

[0063] In the presence of current in the current lines 403, 404 the magnetoresistive sensors are saturated and the device D is no longer sensitive to the external magnetic field. This is for example the measurement at point 602.

[0064] The device D of the system S according to the invention is therefore capable of providing a first measurement M1 corresponding to the first operating point of the magnetoresistive sensors. This first measurement M1 corresponds to the points of type 601 on the figure 7c . Device D is also capable of providing a second measurement M2 corresponding to the second operating point of the magnetoresistive sensors. The second measurement M2 corresponds to the points of type 602 on the figure 7c .

[0065] Advantageously, the second measure M2 associated with the points of type 602 of the figure 7c contains resistance fluctuations mainly due to low-frequency noise from magneto-resistive elements.

[0066] By performing a linear combination of the M1 and M2 measurements it is therefore possible to eliminate low frequency noise.

[0067] There figure 8 shows a typical stack of a TMR tunnel magnetoresistance. Layer 701, often a CuN-type Cu alloy, serves as the lower electrode. Layer 702 serves as the growth layer. Layers 703, an antiferromagnetic of the PtMn or IrMn type coupled to a CoFeB-type layer 704, serve as a reference. The barrier is formed of Al2O3 or preferably MgO 705. Layers 706 and 707 form a classic free layer. That is, a layer that follows the external field. Layer 708 serves as protection and as a starting point for the upper contact of the tunnel junction.

[0068] There are many stacking variations known from the literature.

[0069] There figure 9 shows an example of implementation of the system S according to the invention in the case of direct digital acquisition and digital reconstruction of the noise-free signal.

[0070] The modulation means M generate a supply signal V as well as two periodic signals L1 and L2 of frequency f and adjustable pulse width. The typical frequency f for GMRs is around 100kHz. For small TMRs it can go up to 10MHz. The signal V is a DC voltage which feeds the GMR or TMR bridge. The periodic signals L1 and L2 feed the current lines 403, 404. A typical pulse width value is 50% of the total cycle. The two pulses are in phase. At the output of the bridge, a low noise preamplifier PA as well as an FPB filter cutting the frequencies above f and well below conditions the signal which is acquired, digitally converted and processed by the DSP digital processing means.

[0071] The modulation means M according to embodiment 9 comprise for example a direct voltage generator V and a pulse generator or a function generator GI.

[0072] The ultra-low noise preamplifier PA must have a bandwidth of at least 5 times the switching speed.

[0073] DSP digital signal processing systems perform very fast acquisition, typically at a sampling rate of around 10 MHz. The signal is acquired directly at the output of the FPB filter. In this case, all processing is done digitally. The points after each applied field transition are averaged. This reconstructs the two curves M1 and M2. These two curves are then subtracted to obtain the noise-free signal.

[0074] Advantageously, this embodiment is easy to implement, thanks to the use of DSP digital signal processing means.

[0075] Other embodiments rely on the use of analog signal processing means.

[0076] There figure 10 shows an example of an electrical diagram allowing to obtain the two independent curves M1 and M2 in an analog way and a linear combination carried out digitally thanks to the DSP digital processing means. In addition to the modules already present on the figure 9 , a double Sample & Hold 1101 is inserted. It is controlled by the modulation means M which at this moment generate 4 signals. Two signals, L1 and L2, are intended for the current lines of the bridge according to the figure 4 and two signals with a smaller pulse width of approximately 40% and in phase opposition are sent to the Sample & Hold 1101. Thus the two S&H 1101 separate the signals measured when the magneto-resistive sensors are not saturated, measurement M1, and when the magneto-resistive sensors are saturated, measurement M2. The two signals M1 and M2 are stored, converted and subtracted to obtain the noise-free signal. These operations of storage and subtraction of the measurements M1 and M2 are carried out by the digital signal processing means DSP.

[0077] More specifically, the operation of the two S&H 1101 circuits is explained in relation to the figure 12 which shows the signals generated by the GI function generator.

[0078] The function generator acts as a general high-frequency clock (typically 1 MHz) and generates three signals L1 / L2, SH1, SH2. The L1 / L2 signal controls the implementation of the zero-current-high-current switching and allows switching from detection mode to saturated mode. The SH1 signal controls a first S&H 1101, the SH2 signal controls the second S&H 1101.

[0079] There figure 12 shows that a first S&H circuit is in acquisition mode during the absence of current in the current lines, namely when the L1 / L2 signal is zero. This first S&H circuit allows us to go back to measurement M1. On the contrary, the second S&H circuit is in acquisition mode when a current flows in the current lines, namely when the L1 / L2 signal is non-zero. This second S&H circuit allows us to go back to measurement M2, essentially containing the resistance fluctuations due to low-frequency noise.

[0080] The first configuration of the modulation means M corresponds to the absence of the L1 / L2 signal: the magneto-resistive sensors are in the sensitivity zone. The second configuration of the modulation means M corresponds to the presence of the L1 / L2 signal: the magneto-resistive sensors are in the saturation zone.

[0081] Advantageously, this way of controlling the S&H circuits makes it possible to separate the M1 and M2 measurements using simple and low-cost electronics.

[0082] The time shift between the L1 / L2 signal and the two SH1 and SH2 signals is such that the S&H circuits are put into acquisition mode after a short time at the end of switching and into memory mode before the next switching as shown in the figure 12 .

[0083] Advantageously, this time shift makes it possible to eliminate transients following a switching and to make the elimination of low-frequency noise more precise.

[0084] There figure 11 shows a third example of embodiment of the system S according to the invention, with a fully analog signal processing step. Unlike the device of the figure 10 , this time the digital signal processing means DSP are replaced by a subtractor circuit 1301 capable of carrying out the subtraction of the two measurements M2 and M1 in analog form.

[0085] Advantageously the system of the figure 11 is a fully analog system and potentially integrable at the sensor level making these modulation and subtraction steps transparent to the user.

[0086] There figure 13 shows the steps of the method for implementing the system S according to the invention.

[0087] During a first step ID, the two operating points 201 and 202 of the at least one magnetoresistive sensor forming part of the device D for measuring the external field B are chosen. The points 201 and 202 are chosen so as to have two very different sensitivities to the external magnetic field B. The sensitivity Ssat at the second operating point is very low or zero.

[0088] During the second step MOD the modulation means M are used to switch the at least one magnetoresistive sensor C from the first operating point 201 having the first sensitivity S1 to the second operating point 202 having the second sensitivity Ssat and from the second operating point 202 to the first operating point 201.

[0089] In the MES step, the response of the magnetoresistive sensors of device D in the sensitivity zone and in the saturation zone is recorded, the sensitivity modulation of the magnetoresistive sensors still being in progress. The response of the magnetoresistive sensors in the saturation zone, or measurement M2, essentially contains the resistance fluctuations due to low-frequency noise. The response of the magnetoresistive sensors in the sensitivity zone, or measurement M1, contains the resistance variations due to the variations of the external magnetic field, in addition to the fluctuations due to the low-frequency noise. This gives two time-dependent and independent curves.

[0090] It is therefore possible, during the LIN step, to carry out a linear combination of the M1 and M2 measurements to obtain the noise-free signal and possibly a curve giving only the fluctuations in internal resistances.

[0091] According to one embodiment, the linear combination of the LIN step consists of subtracting the two measurements M1 and M2.

[0092] According to another embodiment, during the LIN step the measurements M1 and M2 are combined linearly according to a formula of the type M1 - α M2. The parameter α essentially depends on the residual sensitivity during the measurement M2. If this is zero, α is zero otherwise it will be approximately equal to the sensitivity ratio.

[0093] According to one embodiment of the method according to the invention, the modulation frequency of the sensitivity of the magnetoresistive sensors is greater than the frequency 101 at which the low-frequency noise becomes lower than the thermal noise associated with the magnetoresistive sensors.

[0094] According to one embodiment of the method, the modulation frequency of the sensitivity of the sensor is at least twice the frequency 101 at which the low-frequency noise becomes lower than the thermal noise associated with magnetoresistive sensors.

[0095] The MES step can be carried out using the measuring device D. The device D can be produced according to one of the configurations illustrated in relation to the system S according to the invention.

[0096] The steps MES and LIN can be carried out using the signal processing means T. The processing means T can be digital, analog or partly digital and partly analog according to one of the configurations explained in relation to the system S according to the invention.

Claims

1. System (S) for suppressing low frequency noise of magnetoresistive sensors, said suppression system (S) including a device (D) for measuring a magnetic field (B) and modulation means (M), said system (S) for suppressing comprising: - said device (D) including at least one magnetoresistive sensor (C), said magnetoresistive sensor (C) having a first sensitivity (S1) at a first operating point (201) and a second sensitivity (Ssat) at a second operating point (202), the sensitivity (S1) at the first operating point being high and the sensitivity (Ssat) at the second operating point being low or zero; said system being characterized in that it further comprises: - the modulation means (M) being suited to switching the magnetoresistive sensor (C) from the first operating point (201) to the second operating point (202) and from the second operating point (202) to the first operating point, said modulation means (M) having a first configuration corresponding to the first operating point (201) and a second configuration corresponding to the second operating point (202); - Means (T) for processing the signal derived from the device (D) for measuring a magnetic field (B), said processing means (T) being suited to making a linear combination of a first response (M1) of the measuring device (D) in the presence of the magnetic field (B) at the first operating point (S1) corresponding to the first configuration of the modulation means (M) and a second response (M2) of the measuring device (D) in the presence of the magnetic field (B) at the second operating point (Ssat) corresponding to the second configuration of the modulation means (M).

2. System (S) for suppressing low frequency noise of magnetoresistive sensors according to the preceding claim characterised in that the device (D) for measuring a magnetic field (B) includes two magnetoresistive sensors (301, 302) arranged according to a half-bridge arrangement and a low noise preamplifier (PA), the two magnetoresistive sensors having an inverse response to the magnetic field, the half-bridge arrangement including a first arm (B1) and a second arm (B2), the two arms being connected in parallel, each of the arms including a resistor (R) and one of the magnetoresistive sensors (301,302), the half-bridge arrangement further including a first (V+) and a second (V-) output, the two outputs being connected to the low noise preamplifier (PA), each output (V+, V-) being the junction point between one of the resistors (R) and one of the magnetoresistive sensors (301, 302).

3. System (S) for suppressing low frequency noise of magnetoresistive sensors according to the preceding claim characterised in that the measuring system (D) includes a DC voltage source (V) for the supply of the half-bridge arrangement, the DC voltage (V) being connected to the junction point between the two resistors (R) or to the junction point between the two magnetoresistive sensors (301, 302).

4. System (S) for suppressing low frequency noise of magnetoresistive sensors according to claim 1 characterised in that the measuring device (D) includes a first (401, 401a) and a second (402, 402a) pair of magnetoresistive sensors and a low noise preamplifier (PA), the sensors of the first pair (401, 401a) having an inverse response compared to the sensors of the second pair (402, 402a), the magnetoresistive sensors (401,401a, 402, 402a) being arranged according to a bridge arrangement, the bridge arrangement including a first arm (B1) and a second arm (B2), the two arms being connected in parallel, each of the arms including a magnetoresistive sensor of the first pair (401,401a) and a magnetoresistive sensor of the second pair (402, 402a), the bridge arrangement further including a first (V+) and a second (V-) output, the two outputs being connected to the low noise preamplifier, each output (V+, V-) being the junction point between a magnetoresistive sensor of the first pair (401, 401a) and a magnetoresistive sensor of the second pair (402, 402a).

5. System (S) for suppressing low frequency noise of magnetoresistive sensors according to the preceding claim characterised in that the measuring system (D) includes a DC voltage source (V) for the supply of the bridge arrangement, the DC voltage source (V) being connected to a junction point between a magnetoresistive sensor of the first pair and a magnetoresistive sensor of the second pair.

6. System (S) for suppressing low frequency noise of magnetoresistive sensors according to one of the preceding claims characterised in that the measuring device (D) includes current lines (301, 302, 403, 404) to apply a magnetic field in the plane of the layers of the magnetoresistive sensors, such that in the presence of current in the lines each magnetoresistive sensor (C) is at the second operating point (202) and in the absence of current in the lines each magnetoresistive sensor (C) is at the first operating point (201).

7. System (S) for suppressing low frequency noise of magnetoresistive sensors according to one of the preceding claims, characterised in that the modulation means (M) include a high frequency master clock for the generation of a switching signal (L1 / L2) between the first operating point (201) and the second operating point (202).

8. System (S) for suppressing low frequency noise according to one of the preceding claims characterised in that the switching signal (L1 / L2) includes current pulses circulating in the current lines (301, 302, 403, 404) for the switching of each magnetoresistive sensor between the first operating point (201) and the second operating point (202).

9. System (S) for suppressing low frequency noise of magnetoresistive sensors according to one of claims 1 to 8 characterised in that the means for processing (T) the signal derived from the device (D) for measuring a magnetic field (B) include: - A first Sample and Hold circuit intended to record the signal (M1) measured by the measuring device (D) at the first operating point (201); - A second Sample and Hold circuit intended to record the signal (M2) measured by the measuring device (D) at the second operating point (202); - A digital (DSP) or analogue (1301) acquisition system for the linear combination of the signals derived from the first and second Sample and Hold circuits.

10. Method (1) for suppressing low frequency noise associated with the measurement of a magnetic field (B) by a measuring device (D) including at least one magnetoresistive sensor (C), said method including the following steps: - Identifying (ID) a first (201) and a second (202) operating point of the magnetoresistive sensor, the magnetoresistive sensor having a first sensitivity (S1) at the first operating point (201) and a second sensitivity (Ssat) at the second operating point (202), the sensitivity (S1) at the first operating point being high and the sensitivity (Ssat) at the second operating point being low or zero; Said method being further characterized by the following steps: - Modulating (MOD) the sensitivity of the magnetoresistive sensor (C) by switching the magnetoresistive sensor (C) from the first operating point (201) having the first sensitivity (S1) to the second operating point (202) having the second sensitivity (Ssat) and from the second operating point (202) to the first operating point (201); - During the modulation (MOD), measuring (MES) a first response (M1) of the measuring device (D) in the presence of the magnetic field (B) at the first operating point (S1) and a second response (M2) of the measuring device (D) in the presence of the magnetic field (B) at the second operating point (Ssat); - Calculating a linear combination (LIN) of the first response (M1) and the second response (M2) of the measuring system (D).