Patent Publication Number: US-9897464-B2

Title: Magnetic field sensor to detect a magnitude of a magnetic field in any direction

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application of U.S. patent application Ser. No. 14/830,098, filed Aug. 19, 2015, entitled “MAGNETIC FIELD SENSOR TO DETECT A MAGNITUDE OF A MAGNETIC FIELD IN ANY DIRECTION,” which is a continuation-in-part of U.S. patent application Ser. No. 14/277,218, filed May 14, 2014, entitled “MAGNETIC FIELD SENSOR FOR DETECTING A MAGNETIC FIELD IN ANY DIRECTION,” which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/827,280 filed May 24, 2013. The applications cited in this paragraph are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     As is known, there are a variety of types of magnetic field sensing elements, including, but not limited to, Hall Effect elements, magnetoresistance elements, and magnetotransistors. As is also known, there are different types of Hall Effect elements, for example, planar Hall elements, vertical Hall elements, and circular Hall elements. As is also known, there are different types of magnetoresistance elements, for example, anisotropic magnetoresistance (AMR) elements, giant magnetoresistance (GMR) elements, tunneling magnetoresistance (TMR) elements, Indium antimonide (InSb) elements, and magnetic tunnel junction (MTJ) elements. 
     Hall Effect elements generate an output voltage proportional to a magnetic field. In contrast, magnetoresistance elements change resistance in proportion to a magnetic field. In a circuit, an electrical current can be directed through the magnetoresistance element, thereby generating a voltage output signal proportional to the magnetic field. 
     Magnetic field sensors, which use magnetic field sensing elements, are used in a variety of applications, including, but not limited to, a current sensor that senses a magnetic field generated by a current carried by a current-carrying conductor, a magnetic switch (also referred to herein as a proximity detector) that senses the proximity of a ferromagnetic or magnetic object, a rotation detector that senses passing ferromagnetic articles, for example, gear teeth, and a magnetic field sensor that senses a magnetic field density of a magnetic field. Magnetic switches are used as examples herein. However, the circuits and techniques described herein apply also to any magnetic field sensor. 
     Conventional magnetic switches can sense a magnetic field above a threshold level in one dimension, i.e., along a line. Some conventional magnetic switches can sense a magnetic field above a threshold in two dimensions, i.e., in a plane. 
     SUMMARY 
     In one aspect, a magnetic field sensor includes first and second magnetic field sensing elements having respective first and second maximum response axes. The first and second maximum response axes point along respective first and second different coordinate axes. In response to a magnetic field, the first and second magnetic field sensing elements are operable to generate first and second magnetic field signals. The magnetic field sensor also includes an electronic circuit coupled to receive the first and the second magnetic field signals. The electronic circuit is configured to determine a magnitude of a vector sum of the first and the second magnetic field signals and provide one or more signals in response to the magnitude of the vector sum determined. 
     In another aspect, a magnetic field sensor includes first, second, and third magnetic field sensing elements having respective first, second and third maximum response axes. The first, second and third maximum response axes point along respective first, second, and third different coordinate axes. In response to a magnetic field, the first, second, and third magnetic field sensing elements are operable to generate first, second, and third magnetic field signals. The magnetic field sensor also includes an electronic circuit coupled to receive the first, the second and the third magnetic field signals. The electronic circuit is configured to determine a magnitude of a vector sum of the first, the second and the third magnetic field signals and provide one or more signals in response to the magnitude of the vector sum determined. 
     A method includes receiving a first magnetic field signal from a first magnetic field sensing element and receiving a second magnetic field signal from a second magnetic field sensing element. The first and second magnetic field sensing elements have respective first and second maximum response axes. The first second and second maximum response axes point along respective first and second different coordinate axes. In response to a magnetic field, the first and second magnetic field sensing elements are operable to generate the first and the second magnetic field signals. The method also includes determining a magnitude of a vector sum of the first and the second magnetic field signals and providing one or more signals in response to the magnitude of the vector sum determined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawings, in which: 
         FIG. 1  is a pictorial showing an integrated circuit having a magnetic field sensor therein disposed upon a substrate, and showing three coordinate axes; 
         FIG. 2 , which includes  FIGS. 2A, 2B, and 2C  in combination, is a block diagram showing an exemplary magnetic field sensor, in the form of a magnetic switch, which can be used as the magnetic field sensor of  FIG. 1 , which has a planar Hall element and two vertical Hall elements, and which has a so-called omni comparator; 
         FIG. 3  is a graph showing three exemplary clock signals that can be used within the magnetic field sensor of  FIG. 2 ; 
         FIG. 4  is a block diagram of an exemplary omni comparator that can be used as the omni comparator of  FIG. 2 ; 
         FIG. 5  is a block diagram of an exemplary transfer function that describes function of the omni comparator of  FIG. 4 ; 
         FIG. 6  is a pictorial showing three-dimensional operating point thresholds (BOP) and three-dimensional release point thresholds (BRP); 
         FIG. 7  is a block diagram showing a wired or gate structure that can be used in conjunction with the magnetic field sensor of  FIG. 2 ; 
         FIG. 8 , which includes  FIGS. 8A, 8B, and 8C  in combination, is a block diagram of another exemplary magnetic field sensor, in the form of a magnetic switch, which can be used as the magnetic field sensor of  FIG. 1 , and which has a planar Hall element and two magnetoresistance element circuits; 
         FIG. 9  is a block diagram of an exemplary magnetoresistance element circuit that can be used as the magnetoresistance element circuits of  FIG. 8 ; 
         FIG. 10  is a block diagram of an alternate embodiment of power and clocking portions that can be used in the magnetic field sensors of  FIGS. 2 and 8 ; 
         FIG. 11  is a block diagram of another alternate embodiment of power and clocking portions that can be used in the magnetic field sensors of  FIGS. 2 and 8 ; 
         FIG. 12  is a block diagram of another example of a magnetic field sensor, in the form of a magnetic switch, which can be used as the magnetic field sensor of  FIG. 1 , which has a planar Hall element and two vertical Hall elements; 
         FIG. 13  is a flowchart of an example of a process to determine if a magnitude of a magnetic field is greater than a threshold voltage; and 
         FIG. 14  is a simplified block diagram of an example of a computing device on which any portion of the process of  FIG. 13  may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “magnetic field sensing element” is used to describe a variety of electronic elements that can sense a magnetic field. The magnetic field sensing element can be, but is not limited to, a Hall Effect element, a magnetoresistance element, or a magnetotransistor. As is known, there are different types of Hall Effect elements, for example, a planar Hall element, a vertical Hall element, and a Circular Vertical Hall (CVH) element. As is also known, there are different types of magnetoresistance elements, for example, a semiconductor magnetoresistance element such as Indium Antimonide (InSb), a giant magnetoresistance (GMR) element, an anisotropic magnetoresistance element (AMR), a tunneling magnetoresistance (TMR) element, and a magnetic tunnel junction (MTJ). The magnetic field sensing element may be a single element or, alternatively, may include two or more magnetic field sensing elements arranged in various configurations, e.g., a half bridge or full (Wheatstone) bridge. Depending on the device type and other application requirements, the magnetic field sensing element may be a device made of a type IV semiconductor material such as Silicon (Si) or Germanium (Ge), or a type II/I-V semiconductor material like Gallium-Arsenide (GaAs) or an Indium compound, e.g., Indium-Antimonide (InSb). 
     As is known, some of the above-described magnetic field sensing elements tend to have an axis of maximum sensitivity in the plane of a substrate that supports the magnetic field sensing element, and others of the above-described magnetic field sensing elements tend to have an axis of maximum sensitivity normal to a substrate that supports the magnetic field sensing element. In particular, planar Hall elements tend to have axes of sensitivity normal to a substrate, while metal based or metallic magnetoresistance elements (e.g., GMR, TMR, AMR) and vertical Hall elements tend to have axes of sensitivity in the plane of the substrate. 
     As used herein, the term “magnetic field sensor” is used to describe a circuit that uses a magnetic field sensing element, generally in combination with other circuits. 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, and a magnetic field sensor that senses a magnetic field density of a magnetic field. 
     Referring to  FIG. 1 , an exemplary integrated circuit  100  includes a housing  102 , for example, a plastic housing, a plurality of leads, of which a lead  104  is one example, and an integrated circuit substrate  106 , for example, a semiconductor substrate upon which a magnetic field sensor can be disposed. 
     An x, y, z Cartesian coordinate system is shown and will be referenced in figures that follow. 
     Referring now to  FIG. 2 , an exemplary magnetic field sensor  200 , in the form of magnetic switch, includes a planar (or horizontal) Hall element  202 , which has a maximum response axis directed out of the page. The magnetic field sensor  200  also includes a first vertical Hall element  204  with a maximum response axis directed up and down on the page when the page is held in landscape mode. The magnetic field sensor  200  also includes a second vertical Hall element  206  with a maximum response axis directed right to left on the page when the page is held in landscape mode. 
     It is intended that the maximum response axis of the planar Hall element  202  points in the direction of the z-axis of  FIG. 1 . It is intended that the maximum response axis of the first vertical Hall element  204  points in the direction of the x-axis of  FIG. 1 . It is further intended that the maximum response axis of the second vertical Hall element  206  points in the direction of the y-axis of  FIG. 1 . 
     While orthogonal Cartesian coordinates are shown and described herein, it should be appreciated that orientations of the maximum response axes of the planar Hall element  202 , the first vertical Hall element  204 , and the second vertical Hall element  206  need not point in orthogonal directions. Orthogonal directions are merely used as an example herein. 
     It is known that Hall elements tend to generate an output voltage signal that has both a magnetically responsive signal portion and an unwanted DC offset signal portion. Current spinning (also referred to as chopping) is a known technique used to reduce the offset signal portion. Chopping can be applied to both planar Hall elements and vertical Hall elements. With chopping, selected drive and signal contact pairs are interchanged during each phase of the chopping. 
     Chopping tends to result in a frequency domain separation of the magnetically responsive signal portion of an output signal from a Hall element with respect to the offset signal portion of the output signal from the Hall element. In so-called “signal modulation,” the magnetically responsive signal portion is shifted to a higher frequency and the offset signal portion remains at baseband. In so-called “offset modulation,” the offset signal portion is shifted to a higher frequency and the magnetically responsive signal portion remains at baseband. For a planar Hall element, these two types of chopping are described, for example, in U.S. patent application Ser. No. 13/095,371, filed Apr. 27, 2011, entitled “Circuits and Methods for Self-Calibrating or Self-Testing a Magnetic Field Sensor.” For a vertical Hall element, chopping is described in U.S. patent application Ser. No. 13/766,341, filed Feb. 13, 2013, entitled “MAGNETIC FIELD SENSOR AND RELATED TECHNIQUES THAT PROVIDE VARYING CURRENT SPINNING PHASE SEQUENCES OF A MAGNETIC FIELD SENSING ELEMENT.” Both of these applications are assigned to the assignee of the present application and both are incorporated by reference herein in their entirety. 
     The signal modulation type chopping is described in figures herein. However, in other embodiments, the offset modulation type of chopping can be used. 
     Furthermore, magnetic field sensor are shown herein that employ chopping arrangements, in other embodiments, no chopping is used, 
     In accordance with the above-described chopping, power chopping switches  208  apply chopped drive signals  208   b  to the planar Hall element  202 , chopped drive signals  208   a  to the first vertical Hall element  204 , and chopped drive signals  208   c  to the second vertical Hall element  206 . The chopped drive signals change phases at a rate determined by a clock signal with a frequency, Fchop. The power chopping switches  208  also receive the sample clock, Sclk, signal. It will become apparent from discussion below in conjunction with  FIG. 3  that the power chopping switches can decode the sample clock, Sclk, and can apply the chopped drive signals  208   a ,  208   b ,  208   c  sequentially and one at a time so that only one of the three Hall elements is operational at a time. 
     Also in accordance with the above-described chopping, signal chopping switches  214  select signal contacts of the planar Hall element  202 , signal chopping switches  210  select signal contacts of the first vertical Hall element  204 , and signal chopping switches  212  select signal contacts of the second vertical Hall element  212 . As described in further detail below, the chopping, and operation of the planar Hall element  202 , the first vertical Hall element  204 , and the second vertical Hall element  206  occur from time to time in accordance with the sample clock, Sclk, received by the power chopping switches  208  and by the various signal chopping switches  210 ,  212 ,  214 . 
     The signal chopping switches  210 ,  212 ,  214  are also coupled to receive the chopping frequency clock, Fchop, described more fully below. 
     A time division multiplex module  220  is coupled to receive three different differential signals  210   a ,  210   b , and  212   a ,  212   b , and  214   a ,  214   b  from the signal chopping switches  210 ,  212 ,  214 . The time division multiplex module  220  is also coupled to receive the sample clock, Sclk. It will be appreciated that the three differential signals  210   a ,  210   b , and  212   a ,  212   b , and  214   a ,  214   b  are chopped signals, for which the magnetically responsive signal portion can be shifted to a higher frequency in accordance with the chopping frequency, Fchop. The unwanted offset signal portion remains at baseband within the three differential signals  210   a ,  210   b , and  212   a ,  212   b , and  214   a ,  214   b.    
     While differential signals are described above and below, it will be appreciated that, in other embodiments, similar circuits can be designed that use single ended signals. 
     The time division multiplex module  220  is configured to sequentially select from among the three different differential signals  210   a ,  210   b , and  212   a ,  212   b , and  214   a ,  214   b  and to provide a differential sequential signal at an output therefrom, which is representative of sequential ones of the three differential signals  210   a ,  210   b , and  212   a ,  212   b , and  214   a ,  214   b  received by the time division multiplex module  220 . 
     An amplifier  222  is coupled to receive the differential sequential signal  220   a ,  220   b  from the time division multiplex module  220  and is configured to generate a differential amplified signal  222   a ,  222   b.    
     A modulator  226  is coupled to receive the differential amplified signal  222   a ,  222   b  and to generate a differential modulated signal  224   a ,  224   b . The modulator  226  is operable to do another frequency conversion, i.e., to shift a frequency of the magnetically responsive signal portion back to baseband, and to shift the offset signal portion up to higher frequency in accordance with the chopping frequency, Fchop. It should be appreciated that, the modulator  226  also operates to shift an unwanted offset generated by the amplifier  222  up to the higher frequency. Thus, the differential modulated signal  226   a ,  226   b  generated by the modulator  226  has unwanted offset signal portions shifted to a higher frequency and the magnetically responsive signal portion, which is desired, is at baseband. 
     The differential modulated signal  226   a ,  226   b  generated by the modulator  226  can be received by a filter, here, a switched capacitor filter  228 , which is an analog sampled filter. In some embodiments the switched capacitor filter  228  is a switched capacitor notch filter, which has a transfer function with a first notch at the chopping frequency, Fchop. The filter  228  essentially removes the unwanted offset signal portion that occurs in the differential modulated signal  226   a ,  226   b  at the frequency, Fchop. 
     An exemplary switched capacitor notch filter is described in U.S. Pat. No. 7,990,209, issued Aug. 2, 2011, entitled “SWITCHED CAPACITOR NOTCH FILTER,” which is assigned to the assignee of the present invention and incorporated by reference herein in its entirety. 
     The switched capacitor filter  228  is configured to generate a filtered signal  228   a , which is received by a comparator  230 , referred to herein as an omni comparator for reasons that will be apparent from discussion below. The omni comparator  230  is described more fully below in conjunction with  FIG. 4 . 
     From discussion above, in accordance with a sampling clock signal, Sclk, received by the time division multiplex module  220 , it will be apparent that the filtered signal  228   a  provided by the switched capacitor filter  228  is, at some sequential times, representative of a signal generated by the planar Hall element  202 , at some other sequential times representative of a signal generated by the first vertical Hall element  204 , and at some other sequential times representative of a signal generated by the second vertical Hall element  206 . 
     The omni comparator  230  is coupled to receive threshold signals  240   a ,  240   b  from a digital to analog converter  240 . In some embodiments, the threshold signals  240   a ,  204   b  can be the same for each one of the sequential signals described above and provided by the switched capacitor filter  228 , or different thresholds can be provided for each one of the sequential signals described above and provided by the switched capacitor filter  228 . 
     The comparator  230  is configured to generate a comparison signal received by an inverter  231 , which generates an inverted comparison signal  231   a  received by registers  232 . 
     The registers  232  are also coupled to receive the sample clock signal, Sclk. The registers are operable, by way of decoding the sample clock signal, Sclk, to sequentially store comparison values (e.g., zero or one) corresponding to the comparisons of the sequential signals  228   a  provided by the switch capacitor filter  228  with appropriate thresholds  240   a ,  240   b . Thus, in some embodiments, a comparison value can be stored in a register  232   a  that is representative of a sensed magnetic field in an x direction being above a threshold signal, another comparison value can be stored in a register  232   b  that is representative of the sensed magnetic field in a y direction being above a threshold signal, and another comparison value can be stored in a register  232   c  that is representative of the sensed magnetic field in a z direction being above a threshold signal. As described above the threshold signals  240   a ,  240   b  can be the same or they can be different for each one of the Hall elements. 
     As is described in conjunction with  FIG. 4  below, the omni comparator  230  uses the threshold signals  240   a ,  240   b  to result in two bidirectional operating point thresholds and two bidirectional release point thresholds. Thus, a first state of the comparison values (e.g., one or high state) can be representative of a sensed magnetic field being greater than a corresponding operating point threshold in one of two parallel directions (e.g., along the x, y, or z axes of  FIG. 1 ) represented by the sample signal  228   a . A second different state of the comparison values (e.g., zero or low state) can be representative of the sensed magnetic field being below a corresponding release point threshold in one of the two parallel directions. The thresholds are further described below in conjunction with  FIG. 6 . 
     The registers  232  provide output values  232   aa ,  232   ba ,  232   ca . A logic gate  234  is coupled to receive the output values  232   aa ,  232   ba ,  232   ca . If any one of the output values  232   aa ,  232   ba ,  232   ca  is indicative of a magnetic field being above an associated operating point threshold in a direction of a corresponding coordinate axis, an output signal  234   a  changes state. 
     A select output gate  236  can be coupled to receive the output values  232   aa ,  232   ba ,  232   ca  and also coupled to receive the output signal  234   a . By way of a select control signal provided from outside of the magnetic field sensor  200  by a user, the select output gate  236  can provide as an output signal either the output signal  234   a , all of the output values  232   aa ,  232   ba ,  232   ca , or any one or more of the output values  232   aa ,  232   ba ,  232   ca.    
     The magnetic field sensor can include one or more of a sensitivity adjust memory  216 , a detection threshold memory  238 , and an offset adjust memory  242 , each of which can be programmed with values by a user via a programs signal from outside of the magnetic field sensor  200 . The memories can be non-volatile memories. 
     The sensitivity adjust memory  216  can provide sequential sensitivity values  216   a  that can take on three different values determined in accordance with the sample clock signal, Sclk. Thus, the sensitivity adjust memory can provide a sensitivity value  216   a  that is appropriate for which Hall element is presently powered up in a sequential fashion. A digital-to-analog converter  218  can be coupled to receive the sequential sensitivity values  216   a  and can provide sequential sensitivity signal  218   a.    
     The power chopping switches  208  can be coupled to receive a signal from a current source  219  as a drive signal. The drive signal can be adjusted to three different values depending upon the three different values of the sequential sensitivity signal  218   a . In this way, the planar Hall element  202 , the first vertical Hall element  204 , and the second vertical Hall element  206  can each be driven with different amounts of drive signal to achieve either different sensitivities to a magnetic field, or preferably, the same sensitivities to the magnetic field. 
     In an alternate embodiment, the sensitivities of the three Hall elements are instead adjusted by way of sequential sensitivity values  216   b  coupled to a digital-to-analog converter  224 , which sequentially adjusts a gain of the amplifier  222 . 
     The detection threshold memory  238  can be used to store three thresholds (e.g., three symmetrical sets of two thresholds) that can be used to compare with each one of the three sequential signals within the output signal  228   a  from the switched capacitor filter  228 . The three stored threshold can be the same or they can be different. Function of the magnetic field sensor  200  when the thresholds are the same and when the thresholds are different are described below in conjunction with  FIG. 6 . 
     The offset adjust memory  242  can be used to store three offset correction values that can be sequentially applied to the omni comparator  230  (or, in other embodiments, to the amplifier  222 ) in accordance with the three sequential signals within the output signal  228   a  from the switched capacitor filter  228 . It will be recognized that, while chopping is described in conjunction with the magnetic field sensor  200 , still some residual DC offset may exist and the offset correction values applied through a digital-to-analog converter  244  to the omni comparator  230  can be used compensate for the residual offsets. 
     The magnetic field sensor  200  can include a micropower regulator  248  coupled to receive the magnetic field sensor power supply voltage, Vcc, and configured to generate a first regulated voltage, Vreg 1 , which can continuously power an oscillator  250 , a power clock generator  252 . The micropower regulator  248 , the oscillator  250 , the power clock generator  252 , the output registers  232 , the logic gate  234 , and the select output gate  236  can remain powered up by Vreg 1  at all times during operation of the magnetic field sensor  200 . The oscillator  250  can generate a continuous clock signal  250   a , and the power clock generator  252  can generate a continuous power clock signal, Pclk. The various clock signals are described more fully below in conjunction with  FIG. 3 . 
     The power clock generator is configured to generate a power clock signal, Pclk received by a second voltage regulator  254  configured to generate a second regulated voltage, Vreg 2 , which turns on and off in accordance with states of the power clock signal, Pclk. The second regulator voltage, Vreg 2 , is used to power all portions of the magnetic field sensor except for the oscillator  250 , the power clock generator  252 , the output registers  232 , the logic gate  234  and the select output gate  236 . Thus, in operation, substantial portions of the magnetic field sensor  200  power on and off (or to a low power state) at a cycle rate and a duty cycle determined by the power clock signal, Pclk. Essentially, the magnetic field sensor powers up from time to time, senses a magnetic field in the environment, determines if the magnetic field is above operating point thresholds stores such information into the registers  232  and makes available an indication of same at all times. As a result, micropower operation is achieved. 
     The magnetic field sensor  200  can also include a sample clock module  256  coupled to receive the clock signal  250   a  and the chopping clock module  258  also coupled to receive the clock signal  250   a . The sample clock module  256  and the chopping clock module  258  can also be coupled to receive the power clock signal, Pclk. The sample clock module  256  is configured to generate the sample clock, Sclk. The chopping clock module  258  is configured to generate the chopping clock with a frequency, Fchop. 
     Referring now to  FIG. 3 , a signal  300  is representative of the power clock signal, Pclk, of  FIG. 2 . The power clock signal  300  can have sequential high states  302   a ,  302   b  with time durations of Thigh, between which are low states  304   a ,  304   b  with time durations of Tlow. From discussion above in conjunction with  FIG. 2 , it will be appreciated that the majority of the circuits within the magnetic field sensor  200  are powered on during high states of the power clock signal, Pclk, and powered off during the low states of the power clock signal, Pclk. However, it should be recognized that the shorter time duration states can instead be low states and the longer time duration state can instead be a high state without departing from the invention. 
     In order to achieve micropower operation, in some embodiments, Thigh/(Thigh+Tlow) is less than or equal to 0.001, i.e., the power on duty cycle is less than 0.1 percent. However, in other embodiments, the duty cycle can be in a range of ten percent to 0.001 percent, or any duty cycle less than about ten percent. 
     In some embodiments, a sample time period, Thigh+Tlow, is about fifty milliseconds. Thus, the three Hall elements of  FIG. 2  must be sampled in a time period of about fifty microseconds. However, other sample time periods are also possible in a range of about ten seconds to about ten milliseconds. 
     The above sample time period of fifty milliseconds is selected in accordance with a bandwidth of a sensed magnetic field. By Nyquist, 1/(sample time period) must be greater than two times the bandwidth of the signal to be sampled. Thus, if the sample time period is fifty milliseconds, the maximum bandwidth of the signal to be sample is ten Hz. 
     While the above represents a narrow detected bandwidth, the bandwidth of the electronic circuits of the magnetic fields sensor  200  of  FIG. 2  must be substantially greater than ten Hz in order to pass the short time duration samples of the three Hall Effect elements. It is known that wider bandwidth results in larger amounts of thermal noise. The thermal noise manifests itself as an apparent shift in the trip points at the omni comparator  230 . Thus, in some embodiments, the threshold values stored in the detection threshold memory  238  are adjusted to account for the thermal noise resulting from the wide bandwidth. 
     A signal  320  is representative of the sample clock signal, Sclk, of  FIG. 2 . Within each high state of the signal  300 , the signal  320  has three high states and two low states  322   a ,  322   b . Each high state of the signal  320  corresponds to one sample of one of the Hall elements in the magnetic field sensor  200  of  FIG. 2 . Thus, during a first high state, one of the three Hall elements is sampled and sent through the electronic channel of the magnetic field sensor  200 , during a second high state another one of the three Hall elements is sampled and sent through the electronic channel, and during a third high state another one of the three Hall elements is sampled and sent through the electronic channel. 
     It should be appreciated that the various modules of the magnetic field sensor  200  of  FIG. 2  that receive the sample clock signal, Sclk, must decode the sample clock signal. However, in other embodiments, the sample clock signal, Sclk, can be provided as three separate signals, each having one of the high states in sequence. 
     A signal  340  is representative of the chopping clock signal with the frequency, Fchop of  FIG. 2 . For four state chopping, there are four or more clock pulses in the signal  340  for each one of the high states  322   a  of the sample clock signal  320 . At other times, the chopping clock signal is inactive, or low. 
     It will be appreciated that all of the sampling of the Hall elements of  FIG. 2 , and all of the chopping of the Hall elements, occurs during the high or active state of the power clock signal, Pclk,  300 . At other times, most of the magnetic field sensor  200  is powered off. 
     Referring now to  FIGS. 4 and 5  together, a comparator circuit  435  can be the same as or similar to the omni comparator  230  of  FIG. 2 . The comparator circuit  435  has a plurality of terminals,  435   a - 435   e , and includes first and second and comparators  436 ,  438 . The comparator  436  has a first terminal  436   a  coupled to a first reference voltage V TH  at terminal  435   a , a second input terminal  436   b  coupled to an input voltage V IN  at terminal  435   b  and an output terminal  436   c  coupled to comparator circuit output terminal  435   d  where an output voltage V OUT  is provided. A reference voltage, V REF  is coupled to terminal  435   e  and provides a reference voltage to comparators  436 ,  438 . 
     Reference is made below to threshold voltages V TH+ , V TH− , V TL+ , V TL− . It will be recognized that the threshold voltages V TH+  and V TL+  are representative of the above-described operating point thresholds (i.e., threshold signals  240   a ,  240   b  of  FIG. 2 ) and the threshold voltages V TH−  and V TL−  are representative of the above-described release point thresholds, which are a result of hysteresis within the comparator circuit  435 . 
     The comparator  438  includes a first input terminal  438   a  coupled at input port  435   b  to the input voltage V IN  and a second input terminal,  438   b , coupled to a threshold voltage V TL  at terminal  435   c . An output terminal  438   c  of comparator  438  is coupled to provide the output voltage V OUT  at the output terminal  435   d.    
     In this particular embodiment, comparators  436 ,  438  are provided having a means for including hysteresis such that the reference or threshold voltages V TH , V TL  can be represented as V TH+  and V TH−  and V TL+  and V TL− , respectively. The values V TH+ , V TH− , V TL+ , V TL−  represent the comparator switch points depending upon the value of the output voltage V OUT . In operation, and as seen in  FIG. 5 , once the output voltage V OUT  switches (e.g. from a high level to a low level), then the switch point changes from V TH+  to V TH− . Likewise, once the output voltage V OUT  switches from a low level to a high level, then the switch point changes from V TH−  to V TH+ . 
     As can be seen in  FIG. 5 , the same holds true as the input voltage V IN  assumes negative voltages (i.e., voltage values on the left hand side of the Vout-axis in  FIG. 5 ). That is, once the output voltage V OUT  switches then the switch point changes from −V TL+  to −V TL−  and vice-versa depending upon whether the output is switching from low to high or from high to low. 
     If the output voltage V OUT  is high and the input voltage V IN  has a value greater than or equal to zero, when the input voltage V IN  meets or exceeds the voltage V TH+ , the output voltage switches from a value of V HIGH  to V LOW  and the switch point changes from V TH+  to V TH− . Thus the value of the output voltage V OUT  will not switch from V LOW  to V HIGH  until the input voltage V IN  reaches the value V TH− . 
     It should be appreciated that, in other embodiments and applications, it may be preferable to utilize comparators which do not have hysteresis and thus switching occurs at a single voltage level, e.g., V TH+  and −V TL+ , i.e., only operating point thresholds are used. 
     With reference to only one of the Hall elements of  FIG. 2 , and to one corresponding directional axis of  FIG. 1 , in operation, and with reference now to  FIG. 5 , the input voltage V IN  is generated in response to a magnetic field being provided to and removed from a magnetic field sensing device which senses the magnetic field and provides a corresponding signal in response thereto. 
     Assuming the input voltage V IN  is at or near zero volts (i.e. V IN =0 volts), the output voltage V OUT  is at a first predetermined voltage level V HIGH . In response to a magnetic field, the Hall element (e.g.,  202 ,  204 ,  206  of  FIG. 1 ) that causes the input voltage provides either a positive or a negative input voltage V IN . If the input voltage provided by the Hall element moves in a positive direction from zero volts toward the threshold voltage, V TH+ , when the threshold voltage meets and/or exceeds the threshold voltage level V TH+ , then the output voltage V OUT  changes from the predetermined signal level, V HIGH  to a second predetermined voltage level V LOW . When the input voltage moves past the threshold voltage V TH−  in a negative-going direction, the output voltage changes from V LOW  back to V HIGH . 
     Likewise, as the input voltage moves in a negative direction from zero volts and reaches and/or exceeds the threshold voltage −V TL+ , the output voltage V OUT  changes from the first value V HIGH  to the second value V LOW . Similarly, as the input voltage V IN  moves from −V TL+  and reaches and/or exceeds the voltage level −V TL− , the voltage level then changes from the output voltage level V LOW  to V HIGH . 
     While the graph of  FIG. 5  is representative of a particular polarity of output signal from the comparator circuit  435 , it should be recognized that a similar circuit can generate the opposite polarity. 
     Referring now to  FIG. 6 , a graph has x, y, and z axes representative of the same axes in  FIG. 1 . The axes are each indicative of magnetic field strengths. An outer box is representative of the operating point thresholds (BOP) in three dimensions corresponding to the three dimensions of the Hall elements  202 ,  204 ,  206  of  FIG. 2 . An inner box is representative of the release point thresholds (BRP) in three dimensions corresponding to the three dimensions of the Hall elements. 
     In operation of the magnetic field sensor  200  of  FIG. 2 , it should be understood that a magnetic field experienced by the magnetic field sensor  200  having any of its Cartesian coordinates on the reference axes described in  FIG. 1  is greater in absolute value than the operating point thresholds, i.e., outside of the outer box, results in a change of state of the output signal  234   a  of the magnetic field sensor  200  to a first state indicative of a detection of the magnetic field. It should also be recognized that a magnetic field experienced by the magnetic field sensor  200  having any of its Cartesian coordinates on the reference axes described in  FIG. 1  lower in absolute value than the release point thresholds, i.e., inside of the inner box, results in a change of state of the output signal  234   a  of the magnetic field sensor  200  to a second different state indicative of a lack of detection of the magnetic field. 
     In view of the above, the magnetic field sensor  200  operates as a three-dimensional switch operable to detect a magnetic field that can be pointing in any direction with a magnitude that is beyond the outer box. 
     While square boxes are shown, any one of more dimensions of the two boxes can be reshaped (i.e., to a shape other than a cube) by changing threshold values stored in the detection threshold memory  238  of  FIG. 2 . 
     Referring now to  FIG. 7 , for embodiments in which the output signals  232   aa ,  232   ba ,  232   ca  are provided as output signals by way of the selection output module  236  of  FIG. 2 , a wired OR function can be brought provided by three FETS, or otherwise, by three transistors, each coupled to a pull-up resistor terminating in a user selectable power supply voltage, Vdd. 
     An output signal ORout from the wired OR circuit can behave very much like the output signal  234   a  of  FIG. 2 , but for which high states are determined by the power supply voltage, Vdd. 
     Referring now to  FIG. 8 , in which like elements of  FIG. 2  are shown having like reference designations, an alternate exemplary magnetic field sensor includes a planar Hall element  802 . However, in place of the first vertical Hall element  204  and the second vertical Hall element  206 , the magnetic field sensor  800  includes a first magnetoresistance circuit  804  and a second magnetoresistance circuit  808 . As described above, magnetoresistance elements and circuits tend to have a maximum response axis parallel to a substrate on which they are constructed, in a sense similar to vertical Hall elements. 
     Magnetoresistance elements and magnetoresistance circuits are not chopped, and thus, the chopping is applied only to the planar Hall element  802 . 
     In order to adjust sensitivities, the magnetic field sensor  800  can include digital-to-analog converters  820 ,  824  coupled to current sources  822 ,  826 , respectively. The current sources  822 ,  826  are coupled to drive the magnetoresistance circuits  804 ,  806 , respectively. 
     The digital-to-analog converters  820 ,  824  can be coupled to a sensitivity adjust memory  818 . The sensitivity just memory  818  can be the same as or similar to the sensitivity adjust memory  216  of  FIG. 2 . Values stored in the sensitivity adjust memory  818  can adjust sensitivities of the planar Hall element  802 , the first magnetoresistance circuit  804 , and the second magnetoresistance circuit  808  by way of magnitudes of drive signals provided by the current sources  814 ,  822 ,  826 , respectively. 
     Amplifiers  834 ,  830 ,  832  are coupled to the planar Hall element  802 , the magnetoresistance circuit  804 , and the magnetoresistance circuit  808 , respectively. In an alternate embodiment, the sensitivity adjust memory  818  can provide sensitivity adjust values to a digital-to-analog converter  828 , which can sequentially adjust gains of the amplifiers  834 ,  830 ,  832 , resulting in a sensitivity adjustment of the magnetic field sensing elements. 
     A chopping modulator  836  can be the same as or similar to the chopping modulator  226  of  FIG. 2 . A switched capacitor notch filter  838  can be the same as or similar to the switched capacitor notch filter  228  of  FIG. 2 . 
     A time division multiplexing module  840  can be coupled to receive signals from the amplifiers  830 ,  832  and from the switched capacitor notch filter  838 . In operation, by way of a sample clock signal, Sclk, which can be the same as or similar to the sample clock signals of  FIGS. 2 and 3 , the time division multiplexing module  840  can sequentially select from among the input signals to provide a sequential output signal to the omni comparator  230 , which can be the same as or similar to the omni comparator  230  of  FIG. 2  and the comparator circuit  435  of  FIG. 4 . 
     Other portions of the magnetic field sensor  800  can be the same as or similar to portions of the magnetic field sensor  200  of  FIG. 2 . However, a chopping clock signal generated by a chopping clock module  868  at the frequency Fchop can be different than the signal  340  of  FIG. 3 . In particular, for four phase chopping, the signal  340  includes a minimum of twelve chopping pulses, for a minimum of four pulses for each one of three Hall elements in the magnetic field sensor  200  of  FIG. 2 . In contrast, the magnetic field sensor  800  has only one Hall element, and therefore, a minimum of four chopping clock pulses are needed for four phase chopping. 
     While four phase chopping as described herein, it will be recognized that chopping can use more than four phases or fewer than four phases, in which case the chopping clocks can have more than the number of pulses shown or fewer than the number of pulses shown, accordingly. 
     Referring now to  FIG. 9 , an exemplary magnetoresistance circuit  900  is configured in a bridge arrangement having two magnetoresistance elements  902 ,  904 , and two static resistors  906 ,  908 . The magnetoresistance circuit  900  can be the same as or similar to each one of the magnetoresistance circuits  804 ,  808  of  FIG. 8 . To achieve the two magnetoresistance circuits  804 ,  806 , which have maximum response axes pointed along different coordinate axes, the magnetoresistance circuit  900  is merely fabricated a two instances on the substrate with the magnetoresistance elements  902 ,  904  parallel to each other but at different angles on the substrate. 
     While one particular form of magnetoresistance circuit  900  is shown, there are many forms of magnetoresistance circuits, in the form of the magnetoresistance circuit can depend on the type of magnetoresistance elements used. 
     Referring now to  FIG. 10 , in which like elements of  FIGS. 2 and 8  are shown having like reference designations, an alternate exemplary power and clocking circuit  1000  can be used in place of the power and clocking circuit of  FIGS. 2 and 8 . 
     The power and clocking circuit can include a ramp generator  1002  that provides a ramp signal  1002   a  to a sample and hold module  1004 . The sample and hold module  1004  provides a sample and hold signal  1004   a , which is a sample and held version of the ramp signal  1002   a , to a voltage controlled oscillator (VCO) chopping clock module  1006 . The voltage controlled chopping clock module  1006  is configured to generate a chopping clock that has a variable frequency Fchopalt 1 . 
     In operation, upon each on state (e.g., high state) of the Pclk signal (e.g., a signal  300  of  FIG. 3 ), the sample and hold module  1004  holds a new value of the ramp signal  1002   a  generated by the ramp generator. Thus, the sampling held signal  1004   a  can be a step signal that steps upward (and/or downward) causing frequency Fchopalt 1  of the chopping signal to step upward (and/or downward) in frequency during each on state of the Pclk signal. 
     The upward (and/or downward) steps in frequency can be equal steps or unequal steps. 
     The varying frequency Fchopalt 1  has advantages in rejecting possible noise signals that may occur in the magnetic field sensors  200 ,  800  of  FIGS. 2 and 8 . 
     Benefits of having a chopping frequency that changes are described, for example, in U.S. patent application Ser. No. 12/845,115, file Jul. 28, 2010, and entitled “MAGNETIC FIELD SENSOR WITH IMPROVED DIFFERENTIATION BETWEEN A SENSED MAGNETIC FIELD AND A NOISE SIGNAL,” which is assigned to the assignee of the present invention and which is incorporated by reference herein in its entirety. 
     Referring now to  FIG. 11 , in which like elements of  FIGS. 2 and 8  are shown having like reference designations, another alternate exemplary power and clocking circuit  1100  can be used in place of the power and clocking circuit of  FIGS. 2 and 8 . 
     The power and clocking circuit  1100  can include an analog-to-digital converter  1102  coupled to receive the signal  228   a  from the switched capacitor filter  228  of  FIG. 2 , or alternatively, from the time division multiplex module  840  of  FIG. 8 . 
     The analog-to-digital converter  1102  is configured to generate a digital signal  1102   a  representative of amplitudes of magnetic field signals generated by the three magnetic field sensing elements of  FIGS. 2 and 8 , i.e., representative of magnetic fields in the three coordinate axes. 
     The power clocking circuit  1100  can also include registers  1104  coupled to receive and store the digital signal  1102   a  from a plurality of samples of the signals from the magnetic field sensing elements, i.e., associated with a plurality of the Pclk high states (see, e.g., a signal  300  of  FIG. 3 ). 
     A digital signal processor  1106  can be coupled to receive the plurality of values  1104   a  from the registers  1104 . The digital signal processor can include registers  1107  that can provide a control signal  1106   a  that can speed up or slow down an oscillator  1108 . 
     The registers  1104 , the registers  1107 , and the oscillator  1108  can remain continually powered on by way of the voltage Vreg 1 . 
     In operation, the digital signal processor can determine how fast the magnetic field experienced by the magnetic field sensing elements of  FIGS. 2 and 8  is changing. According to the rate of change of magnetic field sensed by the digital signal processor, the digital signal processor can cause the oscillator  1108  to generate a clock signal  1108   a  that can speed up or slow down depending on the sensed rate of change of the magnetic field. Thus, for a faster rate of change of the sensed magnetic field, the oscillator  1108  can cause the PCLK signal, the Sclk signal, and/or the chopping clock signal, with a frequency Fchopalt 2 , to run faster. Thus, for a fast rate of change of the sensed magnetic field, sampling of the magnetic field sensing elements of  FIGS. 2 and 8  can achieve a faster sampling rate. Conversely, for a slow rate of change of the sensed magnetic field, sampling of the magnetic field sensing elements of  FIGS. 2 and 8  can achieve a slower sampling rate, therefore conserving power. 
     It should be appreciated that the techniques shown and described above in conjunction with  FIGS. 10 and 11  can be used separately, or, in alternate embodiments, the two techniques can be used together. 
     While  FIGS. 1 to 11 , focus on changes in a magnetic field in one direction in either in the x-, y- or z-axis, measurements may be taken in a combination of two or more axes. For example, a true three-dimensional magnetic vector amplitude sensor is further described herein that measures magnetic field amplitude in more than one axis. The goal of this sensor is to sense the absolute amplitude (called herein “magnitude”) of a magnetic vector in space regardless of the polarity or orientation of the magnetic field. In one example, this type sensor increases sensitivity in detecting magnetic tampering in applications such as smart meters, automated teller machines (ATMs), gambling/gaming devices, electronic locks, and so forth. In applications such as tamper detection in smart meters, the apparent sensitivity is reduced when the applied field is off axis, (i.e., not aligned in either in the x-, y-, or z-axis). The techniques further described herein allow the sensor to sense the true vector amplitude of an external magnetic field. In one example, the techniques described herein determine a vector sum of the three components (or some representation/approximation) and outputs a linear representation of the applied field amplitude and/or applies a threshold and delivers a digital indication of the applied field amplitude. In another example, the output is a signal (e.g., a warning signal) indicating that the magnitude of the magnetic field detected exceeds a threshold value. 
     Referring to  FIG. 12 , a sensor  1200  includes some of the features of  FIG. 2 , except the sensor  1200  includes a multiplexor  1252  (e.g., 3:1 mux), which includes the functionality of a TDM (e.g., TDM  220 ) and a chopper. In particular, the multiplexor  1252  receives the three sets of differential signals from the respective Hall elements  202 ,  204 ,  206  (e.g., receiving differential signals  210   a ,  210   b , the differential signals  212   a ,  212   b  and the differential signals  214   a ,  214   b ). The differential signals  210   a ,  210   b  in the x-axis are converted to a digital signal by the ADC (analog-to-digital converter)  1254 , differential signals  212   a ,  212   b  in the y-axis are converted to a digital signal by the ADC  1254  and differential signals  214   a ,  214   b  in the z-axis are converted to a digital signal by the ADC  1254 . 
     The DSP (digital signal processor)  1256  provides binary signals  1270  and  1280 . In one example, the binary signal  1270  is a warning signal that indicates (e.g., a “1”) if the magnitude of the vector sum (√{square root over (B x   2 +B y   2 +B z   2 )}) is greater than a threshold hold value corresponding to a particular magnetic field magnitude. In one example, the threshold value may be adjustable (e.g., by a user). 
     In one example, the binary signals  1280  may be transmitted using an n bit (n&gt;0) signal that indicates the value of the vector sum (√{square root over (B x   2 +B y   2 +B z   2 )}) in binary form. 
     The DSP  1256  may also output a signal to a DAC (digital-to-analog converter)  1258  and the DAC  1258  outputs a signal  1290 , which is an analog signal indicating the value of the vector sum (√{square root over (B x   2 +B y   2 +B z   2 )}). 
     In some examples, instead of taking the sum of the squares and the square root of the sum, the DSP  1256  converts the magnetic signals in the x, y and z directions to digital values and compares them to a table which indicates if the x, y, and z coordinates relate to a point outside of a sphere. In some examples, where only two coordinate axes are used, the DSP converts the magnetic signals to digital values and compares them to a table which indicates if the coordinates relate to a point outside of a circle. In further examples, the lookup table methodology is not limited to “circles” or “spheres” but can be used to implement any arbitrary or predetermined response desired. In other examples, a threshold may vary as a function of the phase of the resultant vector and therefore may describe any shape in 2D or 3D space. 
     Referring to  FIG. 13 , a process  1300  is an example of a process to determine if a magnitude of a magnetic field is greater than a threshold voltage. Process  1300  receives a first signal corresponding to magnetic field in x-direction ( 1302 ). For example, the vertical Hall element  202  generates a first magnetic field signal (e.g., differential signals  210   a ,  210   b ) that is received by the processor  1202  after chopping. 
     Process  1300  receives a second signal corresponding to magnetic field in y-direction ( 1306 ). For example, the vertical Hall element  204  generates a second magnetic field signal (e.g., differential signals  212   a ,  212   b ) that is received by the processor  1202  after chopping. 
     Process  1300  receives a third signal corresponding to magnetic field in z-direction ( 1310 ). For example, the planar Hall element  202  generates a second magnetic field signal (e.g., differential signals  214   a ,  214   b ) that is received by the processor  1202 . 
     Process  1300  generates a magnitude signal corresponding to the vector sum ( 1314 ). For example, the processor  1202  generates a magnitude signal  1210  corresponding to the vector sum of the first, second and third magnetic field signals. 
     Process  1300  sends a warning signal if the magnitude signal is above a threshold ( 1316 ). For example, the magnitude signal  1210  is sent to a comparator  1220  to compare the signal to a signal  1216  corresponding to a magnetic field threshold value. If the magnitude signal  1210  exceeds the signal  1216  a warning signal  1230  is generated. 
     Referring to  FIG. 14 , in one example, a computing device  1400  includes a processor  1402 , a volatile memory  1404  and a non-volatile memory  1406  (e.g., hard disk). The non-volatile memory  1406  stores computer instructions  1412 , an operating system  1416  and data  1418 . In one example, the computer instructions  1412  are executed by the processor  1402  out of volatile memory  1404  to perform all or part of the processes described herein (e.g., process  1300 ). 
     The processes described herein (e.g., process  1300 ) are not limited to use with the hardware and software of  FIG. 14 ; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. 
     The processes described herein are not limited to the specific examples described. For example, one of ordinary skill in the art would recognize that the techniques described herein may be used using only two of the three Cartesian axes. In other examples, the process  1300  is not limited to the specific processing order of  FIG. 13 . Rather, any of the processing blocks of  FIG. 13  is combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
     The processing blocks (for example, in the process  1300 ) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate. 
     All references cited herein are hereby incorporated herein by reference in their entirety. 
     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.