Patent Publication Number: US-2023134025-A1

Title: Angle sensor with a single die using a single target

Description:
BACKGROUND 
     Magnetic field sensors are used in a variety of applications, including, but not limited to, an angle sensor that senses an angle of a direction of a magnetic field; a current sensor that senses a magnetic field generated by a current carried by a current-carrying conductor; a magnetic switch that senses the proximity of a ferromagnetic object; a rotation detector that senses passing ferromagnetic articles, for example, magnetic domains of a ring magnet or a ferromagnetic target (e.g., gear teeth) where the magnetic field sensor is used in combination with a back-biased or other magnet; a magnetic field sensor that senses a magnetic field density of a magnetic field, a linear sensor that senses a position of a ferromagnetic target; and so forth. 
     SUMMARY 
     In one aspect, an angle sensor includes magnetic-field sensing elements that include a first pair of magnetic-field sensing elements, a second pair of magnetic-field sensing elements; a third pair of magnetic-field sensing elements; and a fourth pair of magnetic-field sensing elements; and processing circuitry configured to determine an angle of a rotating ring magnetic having a plurality of North-South pole pairs each having a unique period length. The processing circuitry includes a first bridge formed from the first and second pairs of magnetic-field sensing elements and a second bridge formed from the third and fourth pairs of magnetic-field sensing elements. The angle includes a value from 0° to 360°. The first, second, third and fourth pairs of magnetic-field sensing elements are each disposed on a first axis. The first, second, third and fourth pairs of magnetic-field sensing elements each have a sensitivity in a first direction along the first axis. The angle sensor is formed on a single die. The angle sensor is an off-axis angle sensor or a side-shaft angle sensor. 
     In another aspect, an angle sensor system includes a ring magnet having a plurality of North-South pole pairs each having a unique period length and an angle sensor. The angle sensor includes magnetic-field sensing elements that includes a first pair of magnetic-field sensing elements, a second pair of magnetic-field sensing elements, a third pair of magnetic-field sensing elements, a fourth pair of magnetic-field sensing elements; and processing circuitry configured to determine an angle of the ring magnet. The processing circuitry includes a first bridge formed from the first and second pairs of magnetic-field sensing elements and a second bridge formed from the third and fourth pairs of magnetic-field sensing elements. The angle includes a value from 0° to 360°. The first, second, third and fourth pairs of magnetic-field sensing elements are each disposed on a first axis. The first, second, third and fourth pairs of magnetic-field sensing elements each have a sensitivity in a first direction along the first axis. The angle sensor is formed on a single die. The angle sensor is an off-axis angle sensor or a side-shaft angle sensor. 
     In a further aspect, a ring magnet includes a plurality of North-South pole pairs and each of the North-South pole pairs have a unique period length. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing features may be more fully understood from the following description of the drawings. The drawings aid in explaining and understanding the disclosed technology. Since it is often impractical or impossible to illustrate and describe every possible embodiment, the provided figures depict one or more illustrative embodiments. Accordingly, the figures are not intended to limit the scope of the broad concepts, systems and techniques described herein. Like numbers in the figures denote like elements. 
         FIG.  1    is a diagram of an angle sensor system; 
         FIG.  2 A  is a diagram of an example of a ring magnet of the angle sensor system of  FIG.  1   ; 
         FIG.  2 B  is graph of an example of a distribution of poles versus polar angle of the ring magnet of  FIG.  2 A ; 
         FIG.  3    is a graph of an example of pitch angle versus pitch index for the ring magnet of  FIG.  2 A ; 
         FIG.  4    is a graph of a magnetic-field map depicting an example of cycloid distortion; 
         FIG.  5    is a graph of a magnetic-field map depicting an example of a topological defect; 
         FIGS.  6 A and  6 B  are examples of off-axis angle sensor placements with respect to a ring magnet where pole magnetizations are along an axis of rotation of the ring magnet; 
         FIGS.  7 A and  7 B  are examples of side-shaft angle sensor placements with respect to a ring magnet where pole magnetizations are radial; 
         FIG.  8    is a diagram of an example of an angle sensor with four pairs of magnetic-field sensing elements; 
         FIGS.  9 A and  9 B  are diagrams of examples of a left bridge and a right bridge respectively using the four pairs of magnetic-field sensing elements of  FIG.  8   ; 
         FIGS.  10 A and  10 B  are a block diagram of an example of processing circuitry; 
         FIG.  11    is a block diagram of another example of the processing circuitry; 
         FIG.  12    is a graph of an example of a first signal resulting from adding output signals from the left and right bridges of  FIGS.  9 A and  9 B  and an example of a second signal resulting from subtracting the output of the right bridge from the left bridge of  FIGS.  9 A and  9 B ; 
         FIG.  13    is a graph representing amplitudes of the first signal of  FIG.  12    and representing amplitudes of the second signal of  FIG.  12   ; 
         FIG.  14    is a graph of representing the difference of the third and fourth signals of  FIG.  13   . 
         FIG.  15    is a diagram of another example of an angle sensor with four additional pairs of magnetic sensing elements from the off-axis sensor in  FIG.  8   ; 
         FIGS.  16 A and  16 B  are diagrams of examples of two additional bridges using the four additional pairs of magnetic-field sensing elements of  FIG.  15   ; 
         FIGS.  17 A and  17 B  are a block diagram of a further example of the processing circuitry; and 
         FIG.  18    is a block diagram of a still further example of processing circuitry. 
     
    
    
     DETAIL DESCRIPTION 
     Described herein are techniques to fabricate an angle sensor with a single die that can determine an angle using a single target. In one example, the single target is a ring magnet having North-South pole pairs each having a unique period length. In one example, the angle sensor is an off-axis angle sensor. In another example, the angle sensor is a side-shaft angle sensor. 
     Referring to  FIG.  1   , an example of an angle sensor is an angle system  100 . The angle system  100  includes a ring magnetic  102  and an angle sensor  106 . In one example, the ring magnet  102  is a ring magnet with a plurality North-South pole pairs. In one example, the angle sensor  106  is an off-axis angle sensor. In another example, the angle sensor  106  is a side-shaft angle sensor. 
     The angle sensor  106  includes magnetic-field sensing elements  112  and processing circuitry  124 . The magnetic-field sensing elements  112  detects changes in a magnetic field caused by the rotating ring magnet  102  and provides signals indicative of the changes in the magnetic-field to the processing circuitry  124 . Based on the signals from the magnetic-field sensing elements  112 , the processing circuitry  124  determines an angle of the ring magnet  102  and provides the angle of the ring magnet  102  in an output signal  150 . 
     In one example, the magnetic-field sensing elements  112  may include vertical Hall elements, horizontal Hall elements and/or magnetoresistance elements. Magnetoresistance elements may include a giant magnetoresistance element (GMR) and/or a tunneling magnetoresistance element (TMR). 
     Referring to  FIGS.  2 A and  2 B , an example of the ring magnet  102  is a ring magnet  102 ′. The ring magnet  102 ′ includes thirty-nine North-South pole pairs (e.g., North-South pole pair  202   a,  North-South pole pair  202   b ). Each North-South pole pair has a unique period length. That is, no two North-South pole pairs have the same period length. For example, the North-South pole pair  202   b  has a smaller period length than the North-South pole pair  202   a.  A graph  250  depicts a distribution of poles versus the polar angle of the ring magnet  102 . 
     In other examples, the period length may increase linearly with absolute angle. In other examples, the period length may alternate in the upper and lower part of the target so that successive periods do not present large local gradients in period length. In further examples, the period length may be randomly distributed (with or without period length gradient capping). In still further examples, the period length may be distributed so that the smallest periods are placed at target phase requiring the highest resolution and the largest periods are placed where the lowest resolution is required. 
     Referring to  FIG.  3   , a graph  300  depicts a pitch (period length) versus pitch index of the ring magnet  102 ′ ( FIG.  2 A ). A pitch index corresponds to a unique North-South pole pair. Thus, each of the thirty-nine points in the graph  300  has unique period length. That is, no pitch index has the same pitch (period length). Thus, by knowing the pitch, the pitch index may be identified and therefore a location on the ring magnet (e.g., the ring magnet  102  ( FIG.  1   )) may be identified. 
     Referring to  FIGS.  4  and  5   , a size of a ring magnet, the number of North-South pole pairs, and the period length of each North-South pole pairs may each be selected to avoid cycloid distortion and topological defects. For example, a graph  400  depicts cycloid distortion  402  and a graph  500  depicts topological defect  502 , where an air gap in each graph is the distance of ring magnet (e.g., the ring magnet  102  ( FIG.  1   )) from a sensor (e.g., the off-axis sensor  106  ( FIG.  1   )). 
     In one example, a pitch distribution may be selected to control topological effects. For example, a pitch distribution may be selected by alternating in the upper and lower half of the ring magnet to provide lower topological defects. For example, in  FIG.  3   , the maximum pitch is in the center and then decreasing pitches are distributed alternatively left and right. In another example, the change in pitch length decreases from the maximum pitch at a constant rate. 
     A maximum pitch (PitchMax) is the largest pitch length in a ring magnet. A minimum pitch (PitchMin) is the smallest pitch in a ring magnet. A value corresponding to (PitchMax−PitchMin)/(PitchMax+PitchMin) may be selected to be less than 50%. Otherwise, strong cycloid distortions are created (e.g., 10° over the air gap (i.e., the gradient of the field angle versus the air gap)). 
     In one example, the average pitch (i.e., average pitch length) is selected so that half a period corresponds to a bridge spacing (e.g., distance between magnetic-field sensing element  802   a  and magnetic-field sensing element  804   a ) within +/−10%. 
     Referring to  FIGS.  6 A and  6 B , an example of the ring magnet  102  is a ring magnet  102 ″. Magnetization of North and South poles are along the z-axis of rotation of the ring magnet  102 ″. For example, an arrow  602  shows a magnetization direction. 
     An example of the angle sensor  106  is the off-axis angle sensor  106 ′. In  FIG.  6 A , the off-axis angle sensor  106 ′ is placed on top of the ring magnet  102 ″. If the off-axis angle sensor  106 ′ is parallel to the tangent plane to the target (i.e., the plane xz), then either vertical Hall plates or in-plane magnetoresistance elements may be used for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )). In one particular example, tunneling magnetoresistance element (TMR) with vortex topology are used for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )). 
     Another example of the angle sensor  106  is the off-axis angle sensor  106 ″. In  FIG.  6 B , the off-axis angle sensor  106 ′ is placed on top of the ring magnet  102 ″. If the off-axis angle sensor  106 ″ is placed in the plane parallel to the face of the target (i.e., plane xy), then a planar Hall plate and vertical Hall may be used to respectively sense the magnetic field in the x-direction (Hx) and the magnetic field in the z-direction (Hz). In another example, an in-plane magnetoresistance element and a perpendicular magnetic anisotropy (PMA) magnetoresistance element may be used respectively for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )) to sense the magnetic field in the x-direction (Hx) and the magnetic field in the z-direction (Hz). 
     Referring to  FIGS.  7 A and  7 B , an example of the ring magnet  102  is a ring magnet  102 ″′. Magnetization of North and South poles are radial. For example, an arrow  702  shows a magnetization direction. 
     An example of the angle sensor  106  is the side-shaft angle sensor  106 ″′. In  FIG.  7 A , the side-shaft angle sensor  106 ″′ is placed beside the ring magnet  102 ″′. If the side-shaft angle sensor  106 ′″ is parallel to the tangent plane to the ring magnet  102 ′″ (i.e., plane xz), then a planar Hall plate and a vertical Hall plate may be used for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )) to respectively sense the magnetic field in the x-direction (Hx) and the magnetic field in the y-direction (Hy). In another example, an in-plane magnetoresistance element and a PMA device may be used for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )) to respectively sense the magnetic field in the x-direction (Hx) and the magnetic field in the y-direction (Hy). 
     An example of the sensor  106  is the side-shaft angle sensor  106 ″″. In  FIG.  7 B , the side-shaft angle sensor  106 ″″ is placed beside the ring magnet  102 ″′. If the sensor plane is in the plane parallel to the face of the ring magnet  102 ″′ (i.e., plane xy), then either vertical Hall plates or in-plane magnetoresistance element may be used. In one particular example, tunneling magnetoresistance elements (TMR) are used for the magnetic-field sensing elements (e.g., magnetic-field sensing elements  112  ( FIG.  1   )). 
     Referring to  FIG.  8   , an example of the angle sensor  106  ( FIG.  1   ) is an angle sensor  800 . The angle sensor  800  includes a magnetic-field sensing elements  112 ′, which is an example of the magnetic-field sensing elements  112  ( FIG.  1   ). The magnetic-field sensing elements  112 ′ include a first set of magnetic-field sensing elements  802   a,    802   b  in a first location; a second pair of magnetic-field sensing elements  804   a,    804   b  in a second location; a third pair of magnetic-field sensing elements  806   a,    806   b  in a third location; and a fourth pair of magnetic-field sensing elements  808   a,    808   b  located in a fourth location. The first, second, third and fourth pairs of magnetic-field sensing elements  802   a,    802   b,    804   a,    804   b,    806   a,    806   b,    808   a,    808   b  are placed on an axis A. 
     The magnetic-field sensing element  802   a  has a reference direction  812   a  and magnetic-field sensing element  802   b  has a reference direction  812   b.  The magnetic-field sensing element  804   a  has a reference direction  814   a  and the magnetic-field sensing element  804   b  has a reference direction  814   b.  The magnetic-field sensing element  806   a  has a reference direction  816   a  and magnetic-field sensing element  806   b  has a reference direction  816   b.  The magnetic-field sensing element  808   a  has a reference direction  818   a  and the magnetic-field sensing element  808   b  has a reference direction  818   b.    
     The reference directions  812   a,    812   b,    814   a,    814   b,    816   a,    816   b,    818   a,    818   b  are the same and zero degrees with respect to the axis A. The reference direction is the direction that the magnetic-field sensing element is the most sensitive to changes in a magnetic field. 
     Referring to  FIGS.  9 A and  9 B , a left bridge  902  (e.g., a differential bridge) is formed by having the magnetic-field sensing elements  802   a,    804   a  on one leg of the left bridge and the magnetic-field sensing elements  802   b,    804   b  on the other leg of the left bridge. A right bridge  904  (e.g., a differential bridge) is formed by having the magnetic-field sensing elements  806   a,    808   a  on one leg of the right bridge and the magnetic-field sensing elements  806   b,    808   b  on the other leg of the right bridge. 
     In one example, the bridges  902 ,  904  are gradiometers that reject a stray magnetic field along the reference axis (axis A). In this example, the magnetic field sensing elements  812   a,    812   b,    814   a,    814   b,    816   a,    816   b,    818   a,    818   b  may be a TMR (e.g., a vortex TMR or a PMA TMR). 
     In other examples, where a stray magnetic field is not significant, then the magnetic field sensing elements  812   a,    812   b,    814   a,    814   b,    816   a,    816   b,    818   a,    818   b  may be any GMR/TMR implemented without PMA or vortex topology. 
     Referring to  FIGS.  10 A and  10 B , an example of the processing circuitry  124  ( FIG.  1   ) is processing circuitry  124 ′. The output of the bridge  902  is received by a subtractor  1004   a  where the output of the bridge  902  is reduced by offset trims  1002 . An output of the subtractor  1004   a  is received by multiplicator  1014   a  and multiplied by gain trims  1012 . An output of the multiplicator  1014   a  is converted to a digital signal by an analog-to-digital converter (ADC)  1016   a  to produce an offset gains trims digital signal  1018   a.    
     The output of the bridge  904  is received by a subtractor  1004   b  where the output of the bridge  904  is reduced by the offset trims  1002 . An output of the subtractor  1004   b  is received by multiplicator  1014   b  and multiplied by the gain trims  1012 . An output of the multiplicator  1014   b  is converted to a digital signal by an analog-to-digital converter (ADC)  1016   b  to produce an offset gains trims digital signal  1018   b.    
     The offset gain trims digital signal  1018   a  is added to the offset gain trims digital signal  1018   b  by the adder  1022   a  to form a first signal  1024   a.  The offset gain trims digital signal  1018   b  is subtracted from the offset gain trims digital signal  1018   a  by the subtractor  1022   b  to form a second signal  1024   a.    
     An amplitude circuit  1032  outputs amplitudes of the first signal  1024   a  to a divider  1040  and an amplitude circuit  1034  outputs amplitudes of the second signal  1024   b  to the divider  1040 . The divider  1040  divides the amplitudes from the first signal  1024   a  by the amplitudes of the second signal  1024   b  to form a period index  1050 . In other examples, the divider  1040  may be replaced by a subtractor. 
     An arctangent circuit  1036  divides the second signal  1024   a  from the first signal  1024   a  and performs an arctangent function to determine a local angle signal  1055 . The local angle is defined to be the angle within the North-South pole period corresponding to the period index. In one example, the local angle θ is: 
     
       
         
           
             
               θ 
               = 
               
                 arctan 
                 ⁡ 
                 ( 
                 
                   
                     
                       L 
                       - 
                       R 
                     
                     
                       L 
                       + 
                       R 
                     
                   
                   × 
                   
                     
                       Amplitude 
                       ⁢ 
                           
                       
                         ( 
                         
                           L 
                           + 
                           R 
                         
                         ) 
                       
                     
                     
                       Amplitude 
                       ⁢ 
                           
                       
                         ( 
                         
                           L 
                           - 
                           R 
                         
                         ) 
                       
                     
                   
                 
                 ) 
               
             
             , 
           
         
       
     
     where L is the output of the left bridge  904 , and R is the output of the right bridge  902 . 
     The period index  1050  is corrected by the harmonic correction circuit  1062  based on the local angle  1055  and the local angle  1055  is corrected by the harmonic correction circuit  1064  based on the period index  1050 . The absolute angle conversion circuit  1068  receives the harmonic corrected signals from the harmonic correction circuits  1062 ,  1064  to form an absolute angle  1080  which is converted to a sensor output protocol signal  150 . 
     In one example, the absolute angle conversion circuit  1068  includes a register (not shown) that stores a position of each North-South pole pair and their length represented by the terms PolePos and PolLength, respectively, which are vectors. A term PeriodIndex corresponds to the period index  1050 . The output of the absolute angle conversion circuit  1068  is: 
       output=PolPos[PeriodIndex]+LocalAnge*PoleLength[PeriodIndex]/360. 
     Referring to  FIG.  11   , another example of the processing circuitry  124  ( FIG.  1   ) is the processing circuitry  124 ″. The processing circuitry  124 ″ is the same as the processing circuitry  124 ′ except the adder  1022   a;  the subtractor  1022   b;  the amplitude circuits  1032 ,  1034 ; the divider  1040 ; the arctangent circuit  1036 , the harmonic correction circuits  1062 ,  1064  and the absolute angle conversion circuit  1068  are replaced by a neural network circuit  1075 . 
     In one example, the neural network circuit  1075  is a network of elementary units. Each unit determines a linear combination of all its inputs and a bias term, and then processes the result through an activation function that is a nonlinear function (except on the output units where it may be linear). The units are organized in layers and each layer takes as inputs the outputs of the previous layers. The first layer takes as an input an input layer, which includes parameters fed to the neural network circuit  1075 . The number of units in each layer may be different. In one example, the neural network circuit  1075  is a multilayer perceptron (MLP). 
     Referring to  FIG.  12   , a graph  1200  includes a plot  1202 , which is the second signal  1024   b  ( FIG.  10 A ) or the difference of an output from the offset gain trims  1018   a  ( FIG.  10 A ) less an output signal from the output gain trims  1018   b  ( FIG.  10 A ). The graph  1200  also includes a plot  1204 , which is the first signal  1024   a  ( FIG.  10 A ) or the summation of the output from the offset gain trims  1018   a  ( FIG.  10 A ) and the output signal from the offset gain trims  1018   b  ( FIG.  10 A ). 
     Referring to  FIG.  13   , a graph  1300  includes a plot  1302 , which includes amplitudes of the plot  1202  ( FIG.  12   ). The graph  1300  also includes a plot  1304 , which includes amplitudes of the plot  1204 . 
     Referring to  FIG.  14   , a graph  1400  includes a set of points (e.g., a point  1402 ), which is the difference of amplitudes between the plot  1304  ( FIG.  13   ) and the plot  1302  ( FIG.  13   ). A set of points  1404  to the left of the graph  1402  are not used since these points represent a beginning of the first period when tracking has not yet been initialized. Therefore, not using the set of points  1404 , the remaining points are similar to the points in the graph  300  ( FIG.  3   ). Thus, using the outputs of the right and left bridges  902 ,  904 , the points in  FIG.  14    may be used to identify the North-South pole pair on the ring magnet. In other examples, the graph  1400  may be replaced with a graph that takes the ratios of the amplitudes of the plots  1302 ,  1304  ( FIG.  13   ). 
     Referring to  FIG.  15   , an example of the angle sensor  102  ( FIG.  1   ) is an angle sensor  1500 . The angle sensor  1500  includes a magnetic-field sensing elements  112 ″. The magnetic-field sensing elements  112 ″ is the same as magnetic-field sensing elements  112 ′ except the magnetic-field sensing elements  112 ″ includes additional magnetic-field sensing elements. In particular, the magnetic-field sensing elements  112 ″ further includes include a fifth pair of magnetic-field sensing elements  1502   a,    1502   b  in a fifth location; a sixth pair of magnetic-field sensing elements  1504   a,    1504   b  in a sixth location; a seventh pair of magnetic-field sensing elements  1506   a,    1506   b  in a seventh location; and an eighth pair of magnetic-field sensing elements  1508   a,    1508   b  located in an eighth location. The fifth, sixth, seventh and eighth pairs of magnetic-field sensing elements  1502   a,    1502   b,    1504   a,    1504   b,    1506   a,    1506   b,    1508   a,    1508   b  are placed on an axis B parallel to the axis A. 
     The magnetic-field sensing element  1502   a  has a reference direction  1512   a  and magnetic-field sensing element  1502   b  has a reference direction  1512   b.  The magnetic-field sensing element  1504   a  has a reference direction  1514   a  and the magnetic-field sensing element  1504   b  has a reference direction  1514   b.  The magnetic-field sensing element  1506   a  has a reference direction  1516   a  and magnetic-field sensing element  1506   b  has a reference direction  1516   b.  The magnetic-field sensing element  1508   a  has a reference direction  1518   a  and the magnetic-field sensing element  1508   b  has a reference direction  1518   b.  The reference directions  1512   a,    1512   b,    1514   a,    1514   b,    1516   a,    1516   b,    1518   a,    1518   b  are the same and orthogonal to the axis A and to the axis B. 
     Referring to  FIGS.  16 A and  16 B , with the additional magnetic-field sensing elements  1512   a,    1512   b,    1514   a,    1514   b,    1516   a,    1516   b,    1518   a,    1518   b,  additional bridges are formed. For example, a left bridge  1602  (e.g., a differential bridge) is formed by having the magnetic-field sensing elements  1502   a,    1504   a  on one leg of the left bridge  1602  and the magnetic-field sensing elements  1502   b,    1504   b  on the other leg of the left bridge  1602 . A right bridge  1604  (e.g., a differential bridge) is formed by having the magnetic-field sensing elements  1506   a,    1508   a  on one leg of the right bridge  1604  and the magnetic-field sensing elements  1506   b,    1508   b  on the other leg of the right bridge  1604 . 
     In one example, the bridges  1602 ,  1604  are gradiometers that reject a stray magnetic field along the reference direction. In this example, the magnetic field sensing elements  1502   a - 1502   d  may be a TMR (e.g., a vortex TMR or a PMA TMR). 
     In other examples, where a stray magnetic field is not significant, then the magnetic field sensing elements  1502   a - 1502   d  may be any GMR/TMR implemented without PMA or vortex topology. 
     Referring to  FIGS.  17 A and  17 B , another example of the processing circuitry  124  is processing circuitry  124 ″′. Processing circuitry  124 ″′ is the same as processing circuitry  124 ′ except the processing circuitry  124 ″′ includes, for example, additional components such as a subtractor  1004   c,  a subtractor  1004   d,  a multiplicator  1014   c,  a multiplicator  1014   d,  an ADC  1016   c,  an ADC  1016   d,  an adder  1022   c,  and an adder  1022   d  to process outputs of the bridges  1602 ,  1604 . 
     The output of the bridge  1602  is received by the subtractor  1004   c  where the output of the bridge  1602  is reduced by the offset trims  1002 . An output of the subtractor  1004   c  is received by the multiplicator  1014   c  and multiplied by gain trims  1012 . An output of the multiplicator  1014   c  is converted to a digital signal by the ADC  1016   c  to produce an offset gains trims digital signal  1018   c.    
     The output of the bridge  1604  is received by the subtractor  1004   d  where the output of the bridge  1604  is reduced by the offset trims  1002 . An output of the subtractor  1004   d  is received by multiplicator  1014   d  and multiplied by the gain trims  1012 . An output of the multiplicator  1014   d  is converted to a digital signal by the ADC  1016   d  to produce an offset gains trims digital signal  1018   d.    
     The offset gain trims digital signal  1018   c  is added to the offset gain trims digital signal  1018   d  by the adder  1022   c  to form a third signal  1024   c.  The offset gain trims digital signal  1018   d  is subtracted from the offset gain trims digital signal  1018   c  by the subtractor  1022   d  to form a fourth signal  1024   d.    
     A normalization circuit  1732  normalizes the first signal  1024   a  and the third signal  1024   c  to produce an output signal that squares each signal  1024   a,    1024   c,  determines a sum of the two square terms and determines the absolute value of the square root of the sum. 
     A normalization circuit  1734  normalizes the second signal  1024   b  and the fourth signal  1024   d  to produce an output signal that squares each signal  1024   b,    1024   d,  determines the sum of the two square terms and determines an absolute value of the square root of the sum. 
     The divider  1040  divides the output of the normalization circuit  1732  by the output of the normalization circuit  1734  to produce the period index  1050 . 
     In some examples, the first signal  1024   a  is equal to the sum of the output of the left bridge  902  and the right bridge  904  times an ellipticity correction error E CF . The second signal  1024   b  is the difference of the outputs of the left and right bridges  902 ,  904  times E CF . In one example, the ellipticity correction error ECF is added by the offset gain trims  1018   a,    1018   b.    
     Referring to  FIG.  18   , another example of the processing circuitry  124  ( FIG.  1   ) is the processing circuitry  124 ″″. The processing circuitry  124 ″″ is the same as the processing circuitry  124 ′ except the adders  1022   a,    1022   c;  the subtractors  1022   b,    1022   d;  the normalization circuits  1732 ,  1734 ; the divider  1040 ; the arctangent circuit  1036 ; the harmonic correction circuit  1062 ,  1064 ; and the absolute angle conversion circuit  1068  are replaced by the neural network circuit  1076 . In one example, the neural network circuit  1076  is an MLP. 
     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.