Abstract:
A magnetoresistive sensor system ( 400 ) is provided, wherein the system comprises a magnetic field source ( 402 ), a magnetoresistive sensor ( 403 ) having an easy axis, and a differentiation element ( 404 ), wherein the magnetic field source ( 402 ) is adapted to emit an auxiliary magnetic field generated from an oscillating input signal ( 401 ), wherein the auxiliary magnetic field is orthogonal to the easy axis of the magnetoresistive sensor ( 403 ), wherein the magnetoresistive sensor ( 403 .) is adapted to sense a signal associated to a superposition of an external magnetic field and the auxiliary alternating magnetic field, and wherein the differentiation element ( 404 ) is adapted to differentiate the sensed signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a magnetoresistance sensor. 
         [0002]    The invention further relates to a method of operating a magnetoresistance sensor. 
         [0003]    Moreover, the invention relates to a program element. 
         [0004]    Further, the invention relates to a computer-readable medium. 
       BACKGROUND OF THE INVENTION 
       [0005]    Magnetoresistance or magnetoresistive sensors like anisotropic magnetoresistance sensors (AMR) or giant magnetoresistance sensors (GMR) are widely used nowadays. For example, AMR sensors are key-building blocks in many automotive applications, e.g. for antilock braking systems (ABS), engine management, transmission or security systems. Especially in the field of automotive transmission a large air gap capability is required. 
         [0006]    Typically, magnetoresistive sensors for rotational speed measurements are linearized by a Barber pole construction. Since Barber pole sensors possess two stable output characteristics a bias magnet is necessary to prevent flipping between positive and negative magnetization directions. However, the auxiliary field H x  in x-direction, i.e. parallel to the easy axis of the magnetoresistive sensor, provided by the bias magnet reduces the sensitivity as shown in the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         R 
                         sensor 
                       
                     
                     
                        
                       H 
                     
                   
                   ~ 
                   
                     1 
                     
                       
                         H 
                         0 
                       
                       + 
                       
                         H 
                         x 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0007]    A main disadvantage of Barber pole sensors is reduced sensitivity caused by this auxiliary field. Furthermore, for increasing air gaps the output signal of the sensor head decreases which leads to heavy requirements for the signal processing and conditioning units of smart sensor systems. Thus, state of the art AMR speed sensors equipped with standard magnets consisting of iron, ferrite, or AlNiCo-alloy are not able to provide air cap capabilities which are required in many fields of applications such as automotive transmission systems. 
         [0008]    Thus, there may be a need to provide a magnetoresistive sensor having an improved sensitivity. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to provide a magnetoresistive sensor system having an improved sensitivity and a method of operating the same. 
         [0010]    In order to achieve the object defined above, a magnetoresistive sensor system, a method of operating a magnetoresistive sensor system, a program element, and a computer-readable medium according to the independent claims are provided. 
         [0011]    According to an exemplary embodiment a device a magnetoresistive sensor system is provided, wherein the system comprises a magnetic field source, a magnetoresistive sensor having an easy axis, and a differentiation element, wherein the magnetic field source is adapted to emit an auxiliary magnetic field from an oscillating input signal, wherein the auxiliary magnetic field is orthogonal to the easy axis of the magnetoresistive sensor, wherein the magnetoresistive sensor is adapted to sense a signal associated to a superposition of an external magnetic field and the auxiliary alternating magnetic field, and wherein the differentiation element is adapted to differentiate the sensed signal. In particular, the magnetoresistive sensor may be a non-linearized magnetoresistive sensor, e.g. may not have linearized characteristics like a Barber pole sensor as known in the prior art. In particular, the differentiation element may be adapted to perform a differentiation in time of the sensed signal. The auxiliary field may also be called exciting field. 
         [0012]    According to an exemplary embodiment a method of linearizing a sensor transfer function of a magnetoresistive sensor is provided, the method comprises generating an auxiliary magnetic field orthogonal to an easy axis of the magnetoresistive sensor, sensing a signal associated to a superposition of an external magnetic field and the auxiliary alternating magnetic field, and differentiating the sensed signal. In particular, the differentiation may be performed by using a differentiation element. 
         [0013]    According to an exemplary embodiment a program element is provided, which, when being executed by a processor, is adapted to control or carry out a method according to an exemplary embodiment. 
         [0014]    According to an exemplary embodiment a computer-readable medium is provided, in which a computer program is stored which, when being executed by a processor, is adapted to control or carry out a method according to an exemplary embodiment. 
         [0015]    Such a provision of a differentiation of the sensed signal of a magnetoresistive sensor by an additional alternating field perpendicular to the easy axis of the magnetoresistive sensor may improve the sensitivity of the magnetoresistive sensor system. In this case the sensitivity of the magnetoresistive sensor system may only be limited by the accuracy of time measurement. Therefore, it may be possible to provide a larger sensing distance, i.e. it may be possible to implement a larger air gap which may be a key requirement for automotive transmission applications. Such an improved sensitivity of a magnetoresistive sensor may also open up new fields of application. In particular, compared to the known Barber pole sensor the saving of the additional bias magnet may lead to reduced production costs. In general such a magnetoresistive sensor system may be used in rotational speed sensors in the automotive sector. Due to the increased sensitivity it may be possible to reliable measure weak magnetic fields. In particular, a magnetoresistive sensor system according to an exemplary embodiment may provide a phase modulation of the signal to be sensed. 
         [0016]    A gist of an exemplary aspect of the invention may be seen in the fact that the sensed signal is quasi phase modulated by using a differentiation element. Thus, the measurement of weak magnetic fields may be performed by a duty cycle analysis. To provide a signal where the duty cycle corresponds to the magnitude of an external magnetic field to be measured, an alternating excitation of the sensor element as well as an adequate signal pre-processing may be performed. For the excitation an auxiliary or exciting magnetic field is used, which excites the magnetoresistive sensor and is superimposed to the external magnetic field to be measured. Thus, has an orthogonal direction than the known auxiliary magnetic fields used for Barber pole sensors to suppress flipping. The exciting magnetic field has a polarization which is orthogonal to the easy axis of the magnetoresistive sensor. Such an exciting magnetic field may be generated by a coil which is implemented in a chip the magnetoresistive sensor itself is implemented. In particular, the magnetoresistive sensor may be formed by a non-linear element, i.e. a magnetization sensor having a non-linear transfer function, so that the external field may modulate the amplitude of the excitation. Starting from this amplitude modulated excitation signal a new signal may be derived, where the duty cycle represents the magnitude of the external magnetic field. In this case the sensitivity of the magnetoresistive sensor system may only be limited by the accuracy of time measurement. 
         [0017]    Summarizing, the basic embodiment of an exemplary magnetoresistive sensor system may be the provision of a magnetic field source which provides or generates an auxiliary magnetic field which is an alternating field and has a direction which is orthogonal to the easy axis of a magnetoresistive sensor of the magnetoresistive sensor system. The modulated signal may then be processed by a differentiation element in order to achieve a phase modulated sensor signal. 
         [0018]    Next, further exemplary embodiments of the invention will be described. 
         [0019]    In the following, further exemplary embodiments of the magnetoresistive sensor system will be explained. However, these embodiments also apply for the method of linearizing a sensor transfer function of a magnetoresistive sensor, for the program element and for the computer-readable medium. 
         [0020]    According to another exemplary embodiment of the magnetoresistive sensor system the magnetoresistive sensor is an anisotropic magnetoresistive sensor and/or and giant magnetoresistive sensor. 
         [0021]    According to another exemplary embodiment the magnetoresistive sensor system further comprises a mixer adapted to mix the differentiated signal and the oscillating input signal. In particular, the oscillating input signal may also be differentiated before it is fed to the mixer. For the provision of the differentiated oscillating input signal a further differentiation unit or element may be provided. 
         [0022]    According to another exemplary embodiment the magnetoresistive sensor system further comprises a lowpass filter, wherein the lowpass filter is adapted to filter the mixed filtered signal. 
         [0023]    The provision of a lowpass filter may be a suitable measure to suppress and/or eliminate distortions in an output signal of the magnetoresistive sensor system. 
         [0024]    According to another exemplary embodiment of the magnetoresistive sensor system the magnetic field source is a coil. 
         [0025]    In particular, the oscillating signal may be provided by an oscillator providing a driving signal for the coil. Specifically, the oscillating signal may be a sinusoidal signal. 
         [0026]    The exemplary embodiments and aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to these examples of embodiment. In particular, it should be noted that features which are described in connection with one exemplary embodiment or aspect may be combined with other exemplary embodiments and other exemplary aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
           [0028]      FIG. 1  schematically shows a simplified circuit diagram of a current fed anisotropic magnetoresistive (AMR) sensor. 
           [0029]      FIG. 2  schematically shows superposition of a magnetic excitation and an external magnetic field. 
           [0030]      FIG. 3  schematically shows phase modulated sensor signals. 
           [0031]      FIG. 4  shows a schematically block diagram of elements of an analog processing unit. 
           [0032]      FIG. 5  shows a schematically block diagram of a digital signal processing unit. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    The illustration in the drawing is schematically. In different drawings, similar or identical elements are provided with similar or the same reference signs. 
         [0034]    In the following, referring to  FIGS. 1 to 5  some basic principles of a magnetoresistive sensor system according to an exemplary embodiment will be explained. 
         [0035]      FIG. 1  shows a simplified circuit diagram of a current fed anisotropic magnetoresistive sensor (AMR sensor)  100  with sinusoidal excitation. In detail  FIG. 1  shows a magnetic field source  101  comprising a drive coil  102  and a source  103  of an oscillating voltage, e.g. an oscillator. Furthermore,  FIG. 1  shows a magnetoresistive sensor  104  comprising an AMR element  105 . In particular, it should be noted that the easy axis of the AMR element  105  has a direction which is substantially vertical in  FIG. 1 , i.e. in the direction of the voltage U sensor  indicated in  FIG. 1 , while the direction of the magnetic field induced by the drive coil  102  is substantially horizontally in  FIG. 1 , i.e. substantially parallel to the voltage U exc  indicated in  FIG. 1 . In particular, it should be noted that the AMR element  104  is a non-linearized AMR element, i.e. does not provide a linearized output signal as a Barber pole for example. The drive coil  102  generates a sinusoidal magnetic excitation in y-direction, i.e. in a direction orthogonal to the easy axis of the AMR element, commonly referred to as the x-direction. 
         [0036]    For small magnetic fields H y  in y-direction or a negligible field in x-direction (H x →0) AMR sensors without Barber poles can be described by 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     sensor 
                   
                   = 
                   
                     
                       R 
                       0 
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                          
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   
                                     H 
                                     y 
                                   
                                   
                                     H 
                                     0 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein H 0  represents a constant comprising the so-called demagnetizing and anisotropic field. The excitation created by the drive coil is given by 
         [0000]        H   exc ( t )= Ĥ   exc  sin(ω exc   t )  (3) 
         [0000]    wherein H exc  represents the excitation magnetic field induced by the drive coil and Ĥ exc  represents the amplitude of the same. The coil is driven by an oscillation voltage source or oscillatory circuit by a voltage according to the following equation: 
         [0000]        u   exc ( t )= Û   exc  sin(ω exc   t ).  (4) 
         [0037]    In the following the magnetic field which has to be measured is called the external field H ext , which remains constant for an adequate choice of ω exc =2πf exc . So we find the magnetic input signal of the AMR sensor: 
         [0000]        H   y ( t )= H   ext   +Ĥ   exc  sin(ω exc   t )  (5) 
         [0038]      FIG. 2  schematically shows the superposition of the external magnetic field and the excitation magnetic field. In particular,  FIG. 2  shows the resistance R in Ω over the field H y  in A/m as the line  201 . Furthermore, the magnetic fields H ext (t) and H exc (t) are schematically shown as lines  202  and  203 , respectively. As mentioned above as H exc (t) is used a sinusoidal excitation is used. As can be seen in  FIG. 2  the resistance distribution is a symmetric distribution, i.e. the resistance is identical for −H y  and +H y . 
         [0039]    Starting from the equations (1) and (4) the resulting R sensor (t) can be calculated and represents an amplitude modulation of H exc  by H ext . If equation (5) is put into (2) and for R 0 &gt;&gt;ΔR the following equation will be achieved: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       sensor 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       R 
                       0 
                     
                     - 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         
                           
                             R 
                              
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     ext 
                                   
                                   + 
                                   
                                     
                                       
                                         H 
                                         ^ 
                                       
                                       exc 
                                     
                                      
                                     
                                       sin 
                                        
                                       
                                         ( 
                                         
                                           
                                             ω 
                                             exc 
                                           
                                            
                                           t 
                                         
                                         ) 
                                       
                                     
                                   
                                 
                                 
                                   H 
                                   0 
                                 
                               
                               ) 
                             
                           
                           2 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0040]    For I senser =const. the output of the sensor is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       u 
                       sensor 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         R 
                         0 
                       
                        
                       
                         I 
                         sensor 
                       
                     
                     - 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         
                           
                             
                               RI 
                               sensor 
                             
                              
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     ext 
                                   
                                   + 
                                   
                                     
                                       
                                         H 
                                         ^ 
                                       
                                       exc 
                                     
                                      
                                     
                                       sin 
                                        
                                       
                                         ( 
                                         
                                           
                                             ω 
                                             exc 
                                           
                                            
                                           t 
                                         
                                         ) 
                                       
                                     
                                   
                                 
                                 
                                   H 
                                   0 
                                 
                               
                               ) 
                             
                           
                           2 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0041]    From equation (7) follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         u 
                         sensor 
                       
                     
                     
                        
                       t 
                     
                   
                   = 
                   
                     
                       - 
                       2 
                     
                      
                     
                       ω 
                       exc 
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     
                       RI 
                       sensor 
                     
                      
                     
                       
                         
                           H 
                           ^ 
                         
                         exc 
                       
                        
                       
                         ( 
                         
                           
                             
                               H 
                               ext 
                             
                             + 
                             
                               
                                 
                                   H 
                                   ^ 
                                 
                                 exc 
                               
                                
                               
                                 sin 
                                  
                                 
                                   ( 
                                   
                                     
                                       ω 
                                       exc 
                                     
                                      
                                     t 
                                   
                                   ) 
                                 
                               
                             
                           
                           
                             H 
                             0 
                           
                         
                         ) 
                       
                     
                      
                     cos 
                      
                     
                         
                     
                      
                     
                       
                         ( 
                         
                           
                             ω 
                             exc 
                           
                            
                           t 
                         
                         ) 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0042]    The product of (8) and 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         u 
                         exc 
                       
                     
                     
                        
                       t 
                     
                   
                   = 
                   
                     
                       ω 
                       exc 
                     
                      
                     
                       
                         U 
                         ^ 
                       
                       exc 
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         
                           
                             ω 
                             exc 
                           
                            
                           t 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    leads to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           m 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                         = 
                           
                          
                         
                           
                             
                                
                               
                                 u 
                                 sensor 
                               
                             
                             
                                
                               t 
                             
                           
                            
                           
                             
                                
                               
                                 u 
                                 exc 
                               
                             
                             
                                
                               t 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             - 
                             
                               ω 
                               exc 
                               2 
                             
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             RI 
                             sensor 
                           
                            
                           
                             
                               U 
                               ^ 
                             
                             exc 
                           
                            
                           
                             
                               
                                 H 
                                 ^ 
                               
                               exc 
                             
                             ( 
                             
                               
                                 
                                   H 
                                   ext 
                                 
                                 + 
                                 
                                   
                                     
                                       H 
                                       ^ 
                                     
                                     exc 
                                   
                                    
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           ω 
                                           exc 
                                         
                                          
                                         t 
                                       
                                       ) 
                                     
                                   
                                 
                               
                               
                                 H 
                                 0 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                          
                         
                           
                             ( 
                             
                               1 
                               + 
                               
                                 cos 
                                  
                                 
                                   ( 
                                   
                                     2 
                                      
                                     
                                       ω 
                                       exct 
                                     
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0000]    which corresponds to the signal after mixing of the time differentiated sensed signal and the time differentiated excitation signal. 
         [0043]    After lowpass filtering with a cut-off frequency of approximately f exc  a signal u(t) 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       u 
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         - 
                         
                           ω 
                           0 
                           2 
                         
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       
                         RI 
                         sensor 
                       
                        
                       
                         
                           U 
                           ^ 
                         
                         exc 
                       
                        
                       
                         
                           
                             H 
                             ^ 
                           
                           exc 
                         
                          
                         
                           ( 
                           
                             
                               
                                 H 
                                 ext 
                               
                               + 
                               
                                 
                                   
                                     H 
                                     ^ 
                                   
                                   exc 
                                 
                                  
                                 
                                   sin 
                                    
                                   
                                     ( 
                                     
                                       
                                         ω 
                                         exc 
                                       
                                        
                                       t 
                                     
                                     ) 
                                   
                                 
                               
                             
                             
                               H 
                               0 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0000]    can be derived, which is shown in  FIG. 3  and will be described in more detail later on. The signal u(t), which can be derived by a signal processing unit of a magnetoresistive sensor system shown in  FIG. 4  has a zero crossing at 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       t 
                       root 
                     
                     = 
                     
                       
                         1 
                         
                           ω 
                           exc 
                         
                       
                        
                       
                         arcsin 
                          
                         
                           ( 
                           
                             - 
                             
                               
                                 H 
                                 ext 
                               
                               
                                 
                                   H 
                                   ^ 
                                 
                                 exc 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0000]    which defines a change in the duty cycle of the corresponding signal 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       u 
                       * 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             1 
                           
                         
                         
                           
                             
                               - 
                               1 
                             
                           
                         
                       
                        
                       for 
                        
                       
                         
                           
                             
                               
                                 u 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               ≥ 
                               1 
                             
                           
                         
                         
                           
                             
                               
                                 u 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               &lt; 
                               1 
                             
                           
                         
                       
                        
                       
                         
 
                       
                        
                       by 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     t 
                   
                   = 
                   
                     
                        
                       
                         t 
                         root 
                       
                        
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0044]    This means that for a positive external field H ext  u(t) is positive for t=[0,T/2+2Δt] and negative for t=[T/2+2Δt,T] as shown in  FIG. 3 . So it is possible to define two time domains for a positive 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     postive 
                   
                   = 
                   
                     
                       T 
                       2 
                     
                     ± 
                     
                       2 
                        
                       
                           
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       t 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and a negative 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     negative 
                   
                   = 
                   
                     
                       T 
                       2 
                     
                     ∓ 
                     
                         
                     
                      
                     
                       2 
                        
                       
                           
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       t 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0000]    signal u*(t). The difference of (15) and (16) is offset free and corresponds to the magnitude of the external field: 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     H 
                   
                   = 
                   
                     
                       
                         t 
                         postive 
                       
                       - 
                       
                         t 
                         negative 
                       
                     
                     = 
                     
                       
                         4 
                          
                         Δ 
                          
                         
                             
                         
                          
                         t 
                       
                       = 
                       
                         4 
                          
                         
                           
                              
                             
                               arcsin 
                                
                               
                                 ( 
                                 
                                   - 
                                   
                                     
                                       H 
                                       ext 
                                     
                                     
                                       H 
                                       exc 
                                     
                                   
                                 
                                 ) 
                               
                             
                              
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
         [0045]    In order to get the time interval t H  an analog as well as a digital implementation is possible. Since digital signal processing has the advantage of an implicit analog-to-digital conversion, a digital realization is preferred. 
         [0046]    The signals u(t) and u*(t) are depicted in  FIG. 3 . In detail in the upper part of  FIG. 3  the signal u(t) is shown, i.e. the phase modulated mixed signal, which depends on H exc , e.g. is proportional. Beside the sinusoidal signal labelled  301  some intervals are marked in  FIG. 3 . In detail these intervals are the interval of 
         [0000]    
       
         
           
             
               T 
               2 
             
             + 
             
               2 
                
               Δ 
                
               
                   
               
                
               t 
             
           
         
       
     
         [0000]    labelled  302  and which lies between the lines  303  and  304 , and 
         [0000]    
       
         
           
             
               T 
               2 
             
             - 
             
               2 
                
               Δ 
                
               
                   
               
                
               t 
             
           
         
       
     
         [0000]    labelled  305  and which lies between the lines  304  and  306 . A third interval  307  is depicted as well and corresponds to the period T=2π/ω exc  of the signal and which lies between the lines  308  and  309 . Furthermore, the external signal H ext  and which corresponds to a shift of the signal u(t) is depicted in  FIG. 3 . 
         [0047]    In the lower part of  FIG. 3  the corresponding signal u*(t)  310  is depicted, i.e. the signal corresponding to equation (13). 
         [0048]    As already mentioned  FIG. 4  schematically shows a block diagram of a signal processing unit which can be used to derive the signal u(t). In detail  FIG. 4  shows a schematically block diagram of a portion of a magnetoresistive sensor system  400  according to an exemplary embodiment including a processing unit. The magnetoresistive sensor system comprises an oscillator  401  adapted to generate a driving signal, e.g. a sinusoidal voltage signal, which can be fed into a drive coil  402 . The drive coil  402  generates an exciting magnet field H exc  which is superimposed to an external magnet field H ext  to be measured. Furthermore, the magnetoresistive sensor system comprises a magnetoresistive sensor  403 , e.g. an anisotropic magnetoresistive sensor or a giant magnetoresistive sensor, which measures the superimposed magnet field. It should be noted that the magnetoresistive sensor has an easy axis having a direction which is orthogonal to the direction of the exciting magnet field H exc . An output signal of the magnetoresistive sensor  403  is a sensor voltage U sensor  which is fed into a first differentiation unit  404 . The differentiated signal of the magnetoresistive sensor  403  is then fed into a mixer  405  together with the sinusoidal voltage signal of the oscillator  401  which is also differentiated by a second differentiation unit  406 . Afterwards, the mixed signal m(t) is fed into a lowpass filter  407  which generates the output voltage u(t). 
         [0049]      FIG. 5  shows a schematically block diagram of a digital signal processing unit  500 . The digital signal processing unit  500  comprises a comparator  501  into which the output signal u(t) is fed and compared with ground GND. Furthermore, the digital signal processing unit  500  comprises an up/down counter  502  into which an output signal of the comparator  501  is fed and which counts up for a positive u*(t) and down if u*(t) is negative. For a rising edge of U/  D  the counter output is valid and proportional to t H . After that a reset of the counter is needed in order to prepare the next measurement. In particular, it should be mentioned that the sensitivity of the smart magnetoresistive sensor system only depends on the clock frequency of the digital counter. 
         [0050]    Summarizing according to an exemplary aspect of the invention a magnetoresistive sensor system may be provided which is based on a non-linearized magnetoresistive sensor. In order to measure weak magnetic fields an exciting magnet field H exc  is used which is generated by using a sinusoidal signal and has a direction which is orthogonal to the easy axis of the magnetoresistive sensor. A differentiation element is used to process the output signal of the magnetoresistive sensor which is then mixed with the differentiated sinusoidal voltage signal of an oscillator and lowpass filtered in order to achieve a phase modulated output signal u(t). 
         [0051]    It should be noted that the term “comprising” does not exclude other elements or features and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments or aspects may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.