Abstract:
A system including a magnet, a first magneto-resistive sensing element, and a second magneto-resistive sensing element. The magnet is configured to provide a magnetic field. The first magneto-resistive sensing element is situated in the magnetic field and the second magneto-resistive sensing element is situated in the magnetic field. The second magneto-resistive sensing element is laterally a greater distance from vertical magnetic field lines of the magnetic field than the first magneto-resistive sensing element.

Description:
BACKGROUND 
       [0001]    Typically, magneto-resistive (XMR) sensors include a supporting magnet and one or more XMR sensor elements for measuring a magnetic field. The supporting magnet and the XMR sensor elements are in a fixed position relative to each other. The XMR sensor elements do not usually operate in their saturation range and the supporting magnet provides a back bias magnetic field that is superimposed on the XMR sensor elements. The supporting magnet acts as a source of the magnetic field and for some types of XMR sensor elements, such as anisotropic magneto-resistive (AMR) sensor elements, the magnetic field stabilizes the transfer characteristic of the XMR sensor elements. As the position of a detected object changes relative to the source of the magnetic field, the magnetic field produces a proportional voltage signal in the XMR sensor elements. Suitable XMR sensor elements include AMR sensor elements, giant magneto-resistive (GMR) sensor elements, tunneling magneto-resistive (TMR) sensor elements, and colossal magneto-resistive (CMR) sensor elements. XMR sensors can be used as proximity sensors, motion sensors, position sensors, or speed sensors. 
         [0002]    Often, in speed sensors, a permanent magnet is attached to a magnetic field sensor that includes multiple XMR sensor elements. The magnetic field sensor is placed in front of a toothed magnetically permeable wheel or disk. As the disk rotates, the teeth pass in front of the magnetic field sensor and generate small field variations in the magnetic field. These small field variations are detected by the XMR sensor elements and include information about rotational speed and angular position of the rotating disk. 
         [0003]    However, diverging magnetic field lines of the back bias magnetic field may provide components that affect the XMR sensor elements. Even if the toothed wheel or gear wheel is symmetrically aligned with the XMR sensor elements, where a tooth center or gap center is directly between the XMR sensor elements and in the middle of the back bias magnetic field, the diverging magnetic field lines may drive the XMR sensor elements into saturation and render the XMR sensor elements useless for detecting variations in the magnetic field. 
         [0004]    For these and other reasons there is a need for the present invention. 
       SUMMARY 
       [0005]    One embodiment described in the disclosure provides a system including a magnet, a first magneto-resistive sensing element, and a second magneto-resistive sensing element. The magnet is configured to provide a magnetic field. The first magneto-resistive sensing element is situated in the magnetic field and the second magneto-resistive sensing element is situated in the magnetic field. The second magneto-resistive sensing element is laterally a greater distance from vertical magnetic field lines of the magnetic field than the first magneto-resistive sensing element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
           [0007]      FIG. 1  is a diagram illustrating one embodiment of an XMR speed sensor. 
           [0008]      FIG. 2  is a diagram illustrating one embodiment of a permanent magnet and magnetic field lines that flow through a magnetic field sensor. 
           [0009]      FIG. 3  is a diagram illustrating one embodiment of a permanent magnet having a vertical symmetry plane that is laterally offset from each of the XMR sensor elements. 
           [0010]      FIG. 4  is a diagram illustrating the resistance of an XMR sensor element versus the x-component of the magnetic field through the XMR sensor element. 
           [0011]      FIG. 5  is a diagram illustrating one embodiment of a sensor bridge. 
           [0012]      FIG. 6  is a diagram illustrating one embodiment of a sensor bridge that includes tap lines between XMR sensor elements. 
           [0013]      FIG. 7  is a diagram illustrating another embodiment of the sensor bridge of  FIG. 6 . 
           [0014]      FIG. 8  is a diagram illustrating one embodiment of a sensor bridge that has balanced resistor divide networks. 
           [0015]      FIG. 9  is a diagram illustrating one embodiment of an XMR speed sensor that includes direction detection. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
         [0017]    It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise. 
         [0018]      FIG. 1  is a diagram illustrating one embodiment of an XMR speed sensor  20 . A permanent magnet  22  is situated next to a magnetic field sensor  24  that is in spaced apart relation to a toothed magnetically permeable wheel or disk  26 . Permanent magnet  22  and magnetic field sensor  24  are held in a fixed position relative to each other. In one embodiment, magnetic field sensor  24  and toothed magnetically permeable disk  26  are held in a fixed position relative to each other. In other embodiments, permanent magnet  22  and magnetic field sensor  24  can be used in another suitable sensor, such as a proximity sensor, a motion sensor, or a position sensor. 
         [0019]    Magnetic field sensor  24  includes a sensor circuit  28  that includes a first XMR sensor element  30  and a second XMR sensor element  32 . Permanent magnet  22  and first and second XMR sensor elements  30  and  32  are held in a fixed position relative to each other. In one embodiment, sensor circuit  28  is an integrated circuit chip. In one embodiment, each of the first and second XMR sensor elements  30  and  32  is an AMR sensor element. In one embodiment, each of the first and second XMR sensor elements  30  and  32  is a GMR sensor element. In one embodiment, each of the first and second XMR sensor elements  30  and  32  is a TMR sensor element. In one embodiment, each of the first and second XMR sensor elements  30  and  32  is a CMR sensor element. 
         [0020]    Permanent magnet  22  provides a back bias magnetic field  34  that is superimposed on first and second XMR sensor elements  30  and  32 . Magnetic field  34  provides diverging magnetic field lines that flow through first XMR sensor element  30 . The diverging magnetic field lines through first XMR sensor element  30  have non-zero x-direction and y-direction components. Magnetic field  34  provides magnetic field lines that flow through second XMR sensor element  32  in the y-direction. The magnetic field lines that flow through second XMR sensor element  32  have an x-direction component magnitude that is less than the magnitude of the x-direction component through first XMR sensor element  30 . In one embodiment, the magnetic field lines that flow through second XMR sensor element  32  have an x-direction component that is substantially equal to zero. In one embodiment, the magnetic field lines that flow through second XMR sensor element  32  have an x-direction component that is non-zero, but the magnitude is less than the magnitude of the x-component through first XMR sensor element  30 . In one embodiment, first XMR sensor element  30  is saturated and second XMR sensor element  32  operates in an unsaturated or active region of the sensor element. In other embodiments, each of the first and second XMR sensor elements  30  and  32  operate in the unsaturated or active region of the sensor element. 
         [0021]    Toothed magnetically permeable disk  26  includes teeth  36  and gaps  38 . Disk  26  rotates in a clockwise direction or a counter-clockwise direction. 
         [0022]    In operation, as disk  26  rotates the teeth  36  and gaps  38  pass through magnetic field  34  and create magnetic field variations in magnetic field  34 . The magnetic field variations include x-direction components that are detected via second XMR sensor element  32 . These magnetic field variations include information about rotational speed and angular position of rotating disk  26 . 
         [0023]    Speed sensor  20  includes permanent magnet  22  positioned to provide magnetic field lines that flow through second XMR sensor element  32  and have an x-direction component magnitude that is less than the magnitude of the x-direction component through first XMR sensor element  30 . Thus, second XMR sensor element  32  is unsaturated and biased to detect variations in magnetic field  34 . Also, the first and second XMR sensor elements  30  and  32  can be put in a sensor bridge to detect variations in magnetic field  34 . 
         [0024]      FIG. 2  is a diagram illustrating one embodiment of permanent magnet  22  and magnetic field lines that flow through magnetic field sensor  24 . Permanent magnet  22  and magnetic field sensor  24  are spaced apart and in a fixed position relative to each other. Magnetic field sensor  24  includes sensor circuit  28  that includes first XMR sensor element  30  and second XMR sensor element  32 . Permanent magnet  22  and first and second XMR sensor elements  30  and  32  are also spaced apart and in a fixed position relative to each other. 
         [0025]    Permanent magnet  22  provides back bias magnetic field  34  that is superimposed on first and second XMR sensor elements  30  and  32 . Magnetic field  34  provides substantially non-diverging magnetic field lines, such as magnetic field line  50 , that flow through second XMR sensor element  32 . Magnetic field  34  also provides diverging magnetic field lines, such as diverging magnetic field line  52  that flows through first XMR sensor element  30  and diverging magnetic field line  54 . 
         [0026]    The magnetic field through first XMR sensor element  30  is represented via magnetic field vector  56 , which includes a negative non-zero x-component  58  and a negative non-zero y-component  60 . The diverging magnetic field lines, such as diverging magnetic field line  52 , include negative non-zero x-component  58  that negatively saturates first XMR sensor element  30 . The negatively saturated first XMR sensor element  30  provides a minimum resistance value. 
         [0027]    The magnetic field through second XMR sensor element  32  is represented via magnetic field vector  62 , which includes an x-component that is substantially zero and a negative non-zero y-component. In one embodiment, second XMR sensor element  32  is an AMR sensor element and the non-zero y-component of magnetic field vector  62  biases second XMR sensor element  32 . 
         [0028]    Thus, first XMR sensor element  30  is saturated and second XMR sensor element  32  operates in the unsaturated region of the sensor element. In other embodiments, permanent magnet  22  can be turned around to provide a magnetic field having magnetic field lines that flow in the opposite direction, such that first XMR sensor element  30  is positively saturated to provide a maximum resistance value. 
         [0029]    Due to material asymmetries in permanent magnet  22  and/or due to position tolerances of permanent magnet  22  relative to first and second XMR sensor elements  30  and  32 , the x-component of the magnetic field through second XMR sensor element  32  may not be zero. However, it is possible to hold the x-component of the magnetic field through second XMR sensor element  32  within the unsaturated, dynamic range of second XMR sensor element  32 . 
         [0030]    In operation, second XMR sensor element  32  responds to magnetic field variations generated via rotating disk  26  and provides substantially sinusoidal resistance variations in the resistance of second XMR sensor element  32  in response to the magnetic field variations. First XMR sensor element  30  is saturated and remains saturated as disk  26  rotates. 
         [0031]    The x-component of the magnetic field Bx that flows through first XMR sensor element  30  is Bx(xL) in Equation I and the x-component of the magnetic field Bx that flows through second XMR sensor element  32  is Bx(xR) in Equation II. 
         [0000]        Bx ( xL )= BxL+B 0*sin(2 πfpt+dφ )  Equation I 
         [0000]        Bx ( xR )= BxR+B 0*sin(2π fpt−dφ )  Equation II 
         [0000]    Where: BxL is a magnetic offset at first XMR sensor element  30 ; BxR is a magnetic offset at second XMR sensor element  32 ; B 0  is an amplitude dependent on the air gap between magnetic field sensor  24  and disk  26 ; f is the rotational frequency of disk  26  in cycles per second; p is the number of teeth on disk  26 ; t is the time; and dphi is a phase shift that is dependent on the distance between first and second XMR sensor elements  30  and  32  and dependent on the widths of the teeth  36  and the widths of the gaps  38  between the teeth  36 . 
         [0032]    In one embodiment, BxR is equal to 0 and BxL is equal to 40 milli-Tesla (mT), where the value of a magnetic field that saturates a sensor element Bsat is 10 mT. Thus, first XMR sensor element  30  is saturated. Also, B 0  is about 1 mT, such that first XMR sensor element  30  remains saturated and second XMR sensor element  32  operates in the unsaturated region and responds to magnetic field variations. 
         [0033]      FIG. 3  is a diagram illustrating one embodiment of permanent magnet  22  having a vertical symmetry plane at  64  that is laterally offset from each of the XMR sensor elements  30  and  32 . Also, the vertical symmetry plane at  64  is laterally offset from a centerline  70  that is centered between first and second XMR sensor elements  30  and  32 . First and second XMR sensor elements  30  and  32  are differently saturated, such that one of the first and second XMR sensor elements  30  and  32  is closer to saturation than the other one. 
         [0034]    Permanent magnet  22  and magnetic field sensor  24  are spaced apart and in a fixed position relative to each other. Magnetic field sensor  24  includes sensor circuit  28  that includes first XMR sensor element  30  and second XMR sensor element  32 . Permanent magnet  22  and first and second XMR sensor elements  30  and  32  are also spaced apart and in a fixed position relative to each other. 
         [0035]    Permanent magnet  22  provides magnetic field  34  that is superimposed on first and second XMR sensor elements  30  and  32 . Magnetic field  34  provides substantially non-diverging magnetic field lines, such as magnetic field line  64 , in the vertical symmetry plane at  64 . Magnetic field  34  provides diverging magnetic field lines, such as diverging magnetic field line  66  that flows through second XMR sensor element  32  and diverging magnetic field line  68  that flows through first XMR sensor element  30 . 
         [0036]    The magnitude of the x-component of magnetic field line  66  through second XMR sensor element  32  is less than the magnitude of the x-component of magnetic field line  68  through first XMR sensor element  30 . Thus, first and second XMR sensor elements  30  and  32  are differently saturated, where first XMR sensor element  30  is closer to saturation than second XMR sensor element  32 . 
         [0037]    In one aspect, differently saturated and closer to saturation are defined by the vertical symmetry plane at  64  being laterally offset from centerline  70  by more than ⅛ th  of the spacing between first and second XMR sensor elements  30  and  32 . 
         [0038]    In one aspect, differently saturated and closer to saturation are defined by the ratio of the magnitudes of the x-components of the magnetic fields through first and second XMR sensor elements  30  and  32 , where the magnitude of the maximum magnetic field Bmax divided by the magnitude of the minimum magnetic field Bmin is greater than 2 and the magnitudes of the magnetic fields Bmax and Bmin are the magnitudes of the time averages of the magnetic fields through XMR sensor elements  30  and  32 . In one embodiment, the time average of the magnetic field in operation is negative Bsat for first XMR sensor element  30  and zero for second XMR sensor element  32 , as shown in  FIG. 2 . In one embodiment, the time average of the magnetic field in operation is (0.9*−Bsat) for first XMR sensor element  30  and (0.1*Bsat) for second XMR sensor element  32 . In operation, second XMR sensor element  32  responds to magnetic field variations generated via rotating disk  26  and provides substantially sinusoidal resistance variations in the resistance of second XMR sensor element  32  in response to the magnetic field variations. In some embodiments, first XMR sensor element  30  also responds to magnetic field variations generated via rotating disk  26  and provides substantially sinusoidal resistance variations in the resistance of first XMR sensor element  30  in response to the magnetic field variations. In other embodiments, first XMR sensor element  30  is saturated and remains saturated as disk  26  rotates. 
         [0039]      FIG. 4  is a diagram illustrating the resistance of an XMR sensor element R(Bx) at  80  versus the x-component of the magnetic field through the XMR sensor element Bx at  82 . The XMR sensor element is similar to each of the first and second XMR sensor elements  30  and  32 . 
         [0040]    At  84 , the x-component of the magnetic field Bx is equal to zero and the resistance of the XMR sensor element R( 0 ) is equal to a resistance that is half way between a minimum resistance of Rmin at  86  and a maximum resistance of Rmax at  88 . 
         [0041]    As the x-component of the magnetic field Bx becomes negative, the resistance of the XMR sensor element R(Bx) decreases. At  90 , the x-component of the magnetic field Bx equals negative Bsat and the resistance of the XMR sensor element R(−Bsat) is substantially equal to Rmin at  86 . As the x-component of the magnetic field Bx becomes positive, the resistance of the XMR sensor element R(Bx) increases. At  92 , the x-component of the magnetic field Bx equals positive Bsat and the resistance of the XMR sensor element R(+Bsat) is substantially equal to Rmax at  88 . 
         [0042]    In one embodiment, first XMR sensor element  30  (shown in  FIG. 2 ) is negatively saturated via the negative x-component  58  of magnetic field vector  56 . This negative x-component  58  is less than negative Bsat at  90  and first XMR sensor element  30  provides the minimum resistance value Rmin at  86 . Second XMR sensor element  32  is unsaturated and receives the sinusoidal x-component Bx(xR) of Equation II, which is between negative Bsat at  90  and positive Bsat at  92 , to provide a resistance between Rmin at  86  and Rmax at  88 . Second XMR sensor element  32  provides sinusoidal resistance changes in response to the magnetic field variations caused via rotating disk  26 . 
         [0043]      FIG. 5  is a diagram illustrating one embodiment of a sensor bridge  100  that includes XMR sensor elements  102 ,  104 ,  106 , and  108  for detecting magnetic field variations. Sensor bridge  100  is a sensor circuit in a magnetic field sensor that is in an XMR speed sensor, such as sensor circuit  28  in magnetic field sensor  24  that is in XMR speed sensor  20  of  FIG. 1 . Each of the XMR sensor elements  102 ,  104 ,  106 , and  108  is similar to one of the XMR sensor elements  30  and  32 . In other embodiments, sensor bridge  100  can be used in another suitable sensor, such as a proximity sensor, a motion sensor, or a position sensor. 
         [0044]    Sensor bridge  100  has the topology of a Wheatstone bridge and includes first XMR sensor element  102 , second XMR sensor element  104 , third XMR sensor element  106 , and fourth XMR sensor element  108 . One side of first XMR sensor element  102  is electrically coupled to one side of second XMR sensor element  104  and to power supply voltage Vsup at  110 . The other side of first XMR sensor element  102  is electrically coupled to one side of fourth XMR sensor element  108  via first bridge line  112 . The other side of second XMR sensor element  104  is electrically coupled to one side of third XMR sensor element  106  via second bridge line  114 . The other side of third XMR sensor element  106  is electrically coupled to the other side of fourth XMR sensor element  108  and to a reference, such as ground, at  116 . First XMR sensor element  102  and fourth XMR sensor element  108  are part of one resistor divide network, and second XMR sensor element  104  and third XMR sensor element  106  are part of another resistor divide network. Sensor bridge  100  provides output voltage Vout at  118 , which is the voltage difference between voltages on the first and second bridge lines  112  and  114 . 
         [0045]    Sensor bridge  100  is held in a spaced apart and fixed position relative to a permanent magnet, such as permanent magnet  22 , and relative to a toothed magnetically permeable disk, such as disk  26 . The magnetic field through each of the first and third XMR sensor elements  102  and  106  has a negative non-zero x-component and a non-zero y-component. The negative non-zero x-component negatively saturates first XMR sensor element  102  and third XMR sensor element  106 . As a result, first XMR sensor element  102  provides a resistance RI that is the minimum resistance Rmin and third XMR sensor element  106  provides a resistance R 3  that is the minimum resistance Rmin, as shown in Equation III. 
         [0000]      R1=R3=Rmin  Equation III 
         [0046]    The back bias magnetic field through each of the second and fourth XMR sensor elements  104  and  108  has an x-component that is substantially zero and a non-zero y-component. Second XMR sensor element  104  and fourth XMR sensor element  108  are unsaturated and operate in a dynamic region of the sensor element. Second XMR sensor element  104  provides resistance R 2  and fourth XMR sensor element  108  provides resistance R 4  in Equation IV. 
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         [0047]    In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations. Sensor bridge  100  detects the magnetic field variations and provides an output voltage Vout at  118  that varies according to Equation V. 
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                                       R 
                                       min 
                                     
                                   
                                 
                               
                                
                               
                                 
                                   B 
                                   0 
                                 
                                 
                                   B 
                                   sat 
                                 
                               
                                
                               
                                 sin 
                                  
                                 
                                   ( 
                                   
                                     
                                       2 
                                        
                                       π 
                                        
                                       
                                           
                                       
                                        
                                       ftp 
                                     
                                     - 
                                     
                                       d 
                                        
                                       
                                           
                                       
                                        
                                       ϕ 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   V 
                 
               
             
           
         
       
     
         [0048]    The hub or stroke H of the XMR sensor elements  102 ,  104 ,  106 , and  108  is given in Equation VI and substituted into Equation V to provide the output voltage Vout at  118  in Equation VII. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                      
                     
                         
                     
                      
                     
                       max 
                       / 
                       R 
                     
                      
                     
                         
                     
                      
                     min 
                   
                   = 
                   
                     1 
                     + 
                     H 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   VI 
                 
               
             
             
               
                 
                   
                     V 
                     out 
                   
                   ≅ 
                   
                     
                       V 
                       sup 
                     
                      
                     
                       H 
                       
                         4 
                         + 
                         H 
                       
                     
                      
                     
                       ( 
                       
                         
                           - 
                           1 
                         
                         + 
                         
                           
                             4 
                             
                               4 
                               + 
                               H 
                             
                           
                            
                           
                             
                               B 
                               0 
                             
                             
                               B 
                               sat 
                             
                           
                            
                           
                             sin 
                              
                             
                               ( 
                               
                                 
                                   2 
                                    
                                   π 
                                    
                                   
                                       
                                   
                                    
                                   ftp 
                                 
                                 - 
                                 
                                   d 
                                    
                                   
                                       
                                   
                                    
                                   ϕ 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   VII 
                 
               
             
           
         
       
     
         [0049]    A speed sensor including sensor bridge  100  can use a permanent magnet that does not include gaps or magnetic field focusing layers. The amplitude of the output voltage Vout at  118  is about ½ the amplitude of an output voltage provided via a sensor bridge that has unsaturated sensor elements on both sides of the bridge. In one embodiment, a signal conditioning circuit includes an amplification stage that amplifies output voltage Vout at  118  and reduces the signal-to-noise ratio. In one embodiment, a stronger permanent magnet is used to increase the amplitude of output voltage Vout at  118 . 
         [0050]    Also, the output voltage Vout at  118  includes the offset voltage Voffset in Equation VIII. 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     offset 
                   
                   = 
                   
                     
                       V 
                       sup 
                     
                      
                     
                       
                         - 
                         H 
                       
                       
                         ( 
                         
                           4 
                           + 
                           H 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   VIII 
                 
               
             
           
         
       
     
         [0000]    In one embodiment, the supply voltage Vsup=1.5 volts and H=0.1, such that Voffset is equal to −37 milli-volts. 
         [0051]    The offset voltage Voffset can be reduced or eliminated via the design of the sensor bridge. In one embodiment, the offset voltage Voffset is reduced or eliminated via increasing the resistance of the XMR sensor elements that are in negative saturation. The increase should be a factor of about (1+H/2) with respect to the unsaturated XMR sensor elements in the sensor bridge. In one embodiment, the offset voltage Voffset is reduced or eliminated via decreasing the resistance of the unsaturated XMR sensor elements with respect to the XMR sensor elements in negative saturation. In one embodiment, the offset voltage Voffset is reduced or eliminated via decreasing the resistance of the XMR sensor elements in positive saturation. The decrease should be a factor of about (1-H/2) with respect to the unsaturated XMR sensor elements in the sensor bridge. In one embodiment, the offset voltage Voffset is reduced or eliminated via increasing the resistance of the unsaturated XMR sensor elements with respect to the XMR sensor elements in positive saturation. 
         [0052]    Resistances can be increased and decreased via selectively shunting in and out XMR sensor elements in series with other XMR sensor elements. In one embodiment, XMR sensor elements are shunted in and out via fusible links. In one embodiment, XMR sensor elements are shunted in and out via metal oxide semiconductor (MOS) switches. 
         [0053]    In other embodiments, the sensor that includes sensor bridge  100  can have a testmode that outputs the offset voltage Voffset. The permanent magnet is positioned in relation to the magnetic field sensor that includes the sensor bridge to reduce or eliminate the offset voltage Voffset and to maintain unsaturated XMR sensor elements on at least one side of the sensor bridge. This can be done without the magnetically permeable disk in place or with the disk in place and in a symmetric position relative to the XMR sensor elements. Also, this can be done with a uniformly rotating disk, where the voltage offset Voffset is the time average over an integer multiple of periods. 
         [0054]      FIG. 6  is a diagram illustrating one embodiment of a sensor bridge  200 . Sensor bridge  200  includes taps or tap lines between XMR sensor elements, such that power is supplied to one of the tap lines and a reference is coupled to another one of the tap lines to increase and decrease resistances in sensor bridge  200  and reduce or eliminate the offset voltage Voffset. Sensor bridge  200  is a sensor circuit in a magnetic field sensor that is in an XMR speed sensor, such as sensor circuit  28  in magnetic field sensor  24  that is in XMR speed sensor  20  of  FIG. 1 . In other embodiments, sensor bridge  200  can be used in another suitable sensor, such as a proximity sensor, a motion sensor, or a position sensor. 
         [0055]    Sensor bridge  200  has the topology of a Wheatstone bridge and includes first XMR sensor element  202 , second XMR sensor element  204 , third XMR sensor element  206 , and fourth XMR sensor element  208 . XMR sensor elements  202 ,  204 ,  206 , and  208  are similar to XMR sensor elements  102 ,  104 ,  106 , and  108  (shown in  FIG. 5 ). Sensor bridge  200  also includes fifth XMR sensor element  210 , sixth XMR sensor element  212 , seventh XMR sensor element  214 , and eighth XMR sensor element  216 . XMR sensor elements  210 ,  212 ,  214 , and  216  have smaller resistances than XMR sensor elements  202 ,  204 ,  206 , and  208  and are switched in and out of the resistor divide networks of sensor bridge  200  to increase and decrease resistances. Each of the XMR sensor elements  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 , and  216  is similar to one of the XMR sensor elements  30  and  32 . In one embodiment, each of the XMR sensor elements  202 ,  204 ,  206 , and  208  has a nominal resistance of (Rmax+Rmin)/2 in a magnetic field having an x-component that is substantially zero. In one embodiment, each of the XMR sensor elements  210 ,  212 ,  214 , and  216  has a nominal resistance of ((Rmax+Rmin)/2)*(H/4) in a magnetic field having an x-component that is substantially zero. 
         [0056]    One side of fifth XMR sensor element  210  is electrically coupled to one side of sixth XMR sensor element  212  via first tap line  218 . The other side of fifth XMR sensor element  210  is electrically coupled to one side of first XMR sensor element  202  via second tap line  220 , and the other side of sixth XMR sensor element  212  is electrically coupled to one side of second XMR sensor element  204  via third tap line  222 . Power supply Vsup at  224  is electrically coupled to a fourth tap line  226  that can be electrically coupled to one of first tap line  218 , second tap line  220 , or third tap line  222 . 
         [0057]    The other side of first XMR sensor element  202  is electrically coupled to one side of fourth XMR sensor element  208  via first bridge line  228 , and the other side of second XMR sensor element  204  is electrically coupled to one side of third XMR sensor element  206  via second bridge line  230 . Sensor bridge  200  provides output voltage Vout at  232 , which is the voltage difference between voltages on the first and second bridge lines  228  and  230 . 
         [0058]    One side of seventh XMR sensor element  214  is electrically coupled to one side of eighth XMR sensor element  216  via fifth tap line  234 . The other side of seventh XMR sensor element  214  is electrically coupled to the other side of third XMR sensor element  206  via sixth tap line  236 , and the other side of eighth XMR sensor element  216  is electrically coupled to the other side of fourth XMR sensor element  208  via seventh tap line  238 . A reference, such as ground, at  240  is electrically coupled to an eighth tap line  242  that can be electrically coupled to one of fifth tap line  234 , sixth tap line  236 , or seventh tap line  238 . 
         [0059]    In this example, power supply Vsup at  224  is electrically coupled to third tap line  222  via fourth tap line  226  and reference  240  is electrically coupled to seventh tap line  238  via eighth tap line  242 . The first resistor divide network includes sixth XMR sensor element  212 , fifth XMR sensor element  210 , first XMR sensor element  202 , and fourth XMR sensor element  208 . The second resistor divide network includes second XMR sensor element  204 , third XMR sensor element  206 , seventh XMR sensor element  214 , and eighth XMR sensor element  216 . 
         [0060]    Sensor bridge  200  is held in a spaced apart and fixed position relative to a permanent magnet, such as permanent magnet  22 , and relative to a toothed magnetically permeable disk, such as disk  26 . 
         [0061]    In one embodiment, the magnetic field through each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  has a negative non-zero x-component and a non-zero y-component. The negative non-zero x-component negatively saturates the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214 . As a result, each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  provides the minimum resistance Rmin. The magnetic field through each of the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  has an x-component that is substantially zero and a non-zero y-component. As a result, second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  are unsaturated and operate in their dynamic region. 
         [0062]    With power supply Vsup at  224  electrically coupled to third tap line  222  and reference  240  electrically coupled to seventh tap line  238 , the offset voltage Voffset is reduced or eliminated via increasing the resistance in the portions of the resistor divide networks in negative saturation. The resistance of fifth XMR sensor element  210  is added to the resistance of first XMR sensor element  202  and the resistance of seventh XMR sensor element  214  is added to the resistance of third XMR sensor element  206 . Also, the offset voltage Voffset is reduced or eliminated via decreasing the resistance of the unsaturated portions of the resistor divide networks. The resistance of the sixth XMR sensor element  212  is not added to the resistance of second XMR sensor element  204  and the resistance of eighth XMR sensor element  216  is not added to the resistance of fourth XMR sensor element  208 . In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations. Sensor bridge  200  detects the magnetic field variations and provides output voltage Vout at  232 , which is the voltage difference between voltages on the first and second bridge lines  228  and  230 . 
         [0063]    In one embodiment, the magnetic field through each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  has an x-component that is substantially zero and a non-zero y-component. As a result, each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  are unsaturated and operate in their dynamic region. The magnetic field through each of the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  has a positive non-zero x-component and a non-zero y-component. The positive non-zero x-component positively saturates the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216 . As a result, second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  provide the maximum resistance Rmax. 
         [0064]    With power supply Vsup at  224  electrically coupled to third tap line  222  and reference  240  electrically coupled to seventh tap line  238 , the offset voltage Voffset is reduced or eliminated via decreasing the resistance in the portions of the resistor divide networks in positive saturation. The resistance of the sixth XMR sensor element  212  is not added to the resistance of second XMR sensor element  204  and the resistance of eighth XMR sensor element  216  is not added to the resistance of fourth XMR sensor element  208 . Also, the offset voltage Voffset is reduced or eliminated via increasing the resistance of the unsaturated portions of the resistor divide networks. The resistance of fifth XMR sensor element  210  is added to the resistance of first XMR sensor element  202  and the resistance of seventh XMR sensor element  214  is added to the resistance of third XMR sensor element  206 . In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations detected via sensor bridge  200 , which provides output voltage Vout at  232 . 
         [0065]      FIG. 7  is a diagram illustrating one embodiment of sensor bridge  200 , wherein power supply Vsup at  224  is electrically coupled to second tap line  220  via fourth tap line  226 . Reference  240  is electrically coupled to sixth tap line  236  via eighth tap line  242 . The first resistor divide network includes first XMR sensor element  202 , fourth XMR sensor element  208 , eighth XMR sensor element  216 , and seventh XMR sensor element  214 . The second resistor divide network includes fifth XMR sensor element  210 , sixth XMR sensor element  212 , second XMR sensor element  204 , and third XMR sensor element  206 . 
         [0066]    Sensor bridge  200  is held in a spaced apart and fixed position relative to a permanent magnet, such as permanent magnet  22 , and relative to a toothed magnetically permeable disk, such as disk  26 . 
         [0067]    In one embodiment, the magnetic field through each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  has a positive non-zero x-component and a non-zero y-component. The positive non-zero x-component positively saturates the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214 . As a result, each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  provides the maximum resistance Rmax. The magnetic field through each of the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  has an x-component that is substantially zero and a non-zero y-component. As a result, second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  are unsaturated and operate in their dynamic region. 
         [0068]    With power supply Vsup at  224  electrically coupled to second tap line  220  and reference  240  electrically coupled to sixth tap line  236 , the offset voltage Voffset is reduced or eliminated via decreasing the resistance in the portions of the resistor divide networks in positive saturation. The resistance of the fifth XMR sensor element  210  is not added to the resistance of first XMR sensor element  202  and the resistance of seventh XMR sensor element  214  is not added to the resistance of third XMR sensor element  206 . Also, the offset voltage Voffset is reduced or eliminated via increasing the resistance of the unsaturated portions of the resistor divide networks. The resistance of sixth XMR sensor element  212  is added to the resistance of second XMR sensor element  204  and the resistance of eighth XMR sensor element  216  is added to the resistance of fourth XMR sensor element  208 . In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations detected via sensor bridge  200 , which provides output voltage Vout at  232 . 
         [0069]    In one embodiment, the magnetic field through each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  has an x-component that is substantially zero and a non-zero y-component. As a result, each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  are unsaturated and operate in their dynamic region. The magnetic field through each of the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  has a negative non-zero x-component and a non-zero y-component. The negative non-zero x-component negatively saturates the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216 . As a result, second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  provide the minimum resistance Rmin. 
         [0070]    With power supply Vsup at  224  electrically coupled to second tap line  220  and reference  240  electrically coupled to sixth tap line  236 , the offset voltage Voffset is reduced or eliminated via increasing the resistance in the portions of the resistor divide networks in negative saturation. The resistance of sixth XMR sensor element  212  is added to the resistance of second XMR sensor element  204  and the resistance of eighth XMR sensor element  216  is added to the resistance of fourth XMR sensor element  208 . Also, the offset voltage Voffset is reduced or eliminated via decreasing the resistance of the unsaturated portions of the resistor divide networks. The resistance of the fifth XMR sensor element  210  is not added to the resistance of first XMR sensor element  202  and the resistance of seventh XMR sensor element  214  is not added to the resistance of third XMR sensor element  206 . In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations. Sensor bridge  200  detects the magnetic field variations and provides output voltage Vout at  232 , which is the voltage difference between voltages on the first and second bridge lines  228  and  230 . 
         [0071]      FIG. 8  is a diagram illustrating one embodiment of sensor bridge  200 , wherein power supply Vsup at  224  is electrically coupled to first tap line  218  via fourth tap line  226 , and reference  240  is electrically coupled to fifth tap line  234  via eighth tap line  242 . The first and second resistor divide networks are balanced, where the first resistor divide network includes fifth XMR sensor element  210 , first XMR sensor element  202 , fourth XMR sensor element  208 , and eighth XMR sensor element  216 , and the second resistor divide network includes sixth XMR sensor element  212 , second XMR sensor element  204 , third XMR sensor element  206 , and seventh XMR sensor element  214 . 
         [0072]    Sensor bridge  200  is held in a spaced apart and fixed position relative to a permanent magnet, such as permanent magnet  22 , and relative to a toothed magnetically permeable disk, such as disk  26 . The magnetic field through each of the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  has an x-component that is substantially zero and a non-zero y-component. Also, the magnetic field through each of the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  has an x-component that is substantially zero and a non-zero y-component. As a result, the first, third, fifth, and seventh XMR sensor elements  202 ,  206 ,  210 , and  214  are unsaturated and operate in their dynamic region and the second, fourth, sixth, and eighth XMR sensor elements  204 ,  208 ,  212 , and  216  are unsaturated and operate in their dynamic region. 
         [0073]    With power supply Vsup at  224  electrically coupled to first tap line  218  and reference  240  electrically coupled to fifth tap line  234 , the offset voltage Voffset is reduced or eliminated via the balanced resistor divide networks. In operation, the toothed magnetically permeable disk rotates and generates magnetic field variations detected via sensor bridge  200 , which provides output voltage Vout at  232 . 
         [0074]      FIG. 9  is a diagram illustrating one embodiment of an XMR speed sensor  300  that includes direction detection. Permanent magnet  302  is situated next to magnetic field sensor  304  that is in spaced apart relation to toothed magnetically permeable disk  306 . Permanent magnet  302  and magnetic field sensor  304  are held in a fixed position relative to each other. In one embodiment, magnetic field sensor  304  and toothed magnetically permeable disk  306  are held in a fixed position relative to each other. 
         [0075]    Magnetic field sensor  304  includes a sensor circuit  308  that includes a first XMR sensor element  310 , a second XMR sensor element  312 , and a third XMR sensor element  314 . Third XMR sensor element  314  is halfway between first XMR sensor element  310  and second XMR sensor element  312 . Permanent magnet  302  and XMR sensor elements  310 ,  312 , and  314  are held in a fixed position relative to each other. In one embodiment, sensor circuit  308  is an integrated circuit chip. In one embodiment, each of the XMR sensor elements  310 ,  312 , and  314  is an AMR sensor element. In one embodiment, each of the XMR sensor elements  310 ,  312 , and  314  is a GMR sensor element. In one embodiment, each of the XMR sensor elements  310 ,  312 , and  314  is a TMR sensor element. In one embodiment, each of the XMR sensor elements  310 ,  312 , and  314  is a CMR sensor element. 
         [0076]    Permanent magnet  302  provides back bias magnetic field  316  that is superimposed on XMR sensor elements  310 ,  312 , and  314 . Permanent magnet  302  is centered halfway between second XMR sensor element  312  and third XMR sensor element  314 . Magnetic field  316  provides diverging magnetic field lines that flow through first XMR sensor element  310 . The diverging magnetic field lines through first XMR sensor element  310  have a negative non-zero x-direction component and a non-zero y-direction component. Magnetic field  316  provides magnetic field lines that flow through second XMR sensor element  312  and third XMR sensor element  314  in the y-direction. The magnetic field lines that flow through second XMR sensor element  312  and third XMR sensor element  314  have a small non-zero x-direction component that does not saturate second XMR sensor element  312  and third XMR sensor element  314 . As a result, first XMR sensor element  30  is saturated and second and third XMR sensor elements  312  and  314  operate in unsaturated regions. In other embodiments, each of the XMR sensor elements  310 ,  312 , and  314  operate in an unsaturated region. 
         [0077]    Toothed magnetically permeable disk  306  includes teeth  318  and gaps  320 . Disk  306  rotates in a clockwise direction or a counter-clockwise direction. 
         [0078]    In operation, as disk  306  rotates the teeth  318  and gaps  320  pass through magnetic field  316  and create magnetic field variations in magnetic field  316 . The magnetic field variations include x-direction components that are detected via second XMR sensor element  312  and third XMR sensor element  314 . These magnetic field variations include information about rotational speed and angular position of rotating disk  306 . In addition, magnetic field sensor  304  detects the direction of rotation of disk  306  via XMR sensor elements  310 ,  312 , and  314 . 
         [0079]    Speed sensor  300  includes permanent magnet  302  positioned to provide magnetic field lines that flow through second XMR sensor element  312  and third XMR sensor element  314  and that have non-zero x-direction components that are small enough to not saturate second XMR sensor element  312  and third XMR sensor element  314 . Thus, second XMR sensor element  312  and third XMR sensor element  314  are unsaturated and biased to detect variations in magnetic field  316 . 
         [0080]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.