Patent Description:
This invention relates generally to magnetic field sensors and, more particularly, to magnetic field sensors implementing differential chopping.

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, 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 strength or density of a magnetic field.

Some magnetic field sensors employ differential magnetic field sensing whereby more than one magnetic field sensing element is used to generate a differential signal that is relatively immune stray magnetic field effects. For example, the output signals from two spatially separated magnetic field sensing elements can be independently amplified and the amplifier output signals can be combined (e.g., subtracted) such that any effects from stray magnetic fields experienced by both sensing elements tend to cancel in the resulting differential magnetic field signal, whereas the field that is desired to be detected can be readily detectable from the resulting differential magnetic field signal.

It is known that Hall Effect elements exhibit an undesirable DC offset voltage. Techniques have been developed to reduce the DC offset voltage, while still allowing the Hall Effect element to sense a magnetic field. One such technique is commonly referred to as "chopping" or "current spinning" and entails driving a Hall Effect element in two or more different directions and receiving output signals at different output terminals as the Hall Effect element is differently driven. In this way, selected drive and signal contact pairs are interchanged during each phase of the chopping and offset voltages of the different driving arrangements tend to cancel.

Chopping is also a well-known technique applied to amplifiers to reduce an offset component and low frequency noise (i.e., flicker noise) of signals applied to the amplifier. Amplifiers implementing chopping are often referred to as chopper stabilized amplifiers.

According to the present invention, a magnetic field sensor includes a first magnetic field sensing element configured to generate a first magnetic field signal indicative of a magnetic field associated with a target and a second magnetic field sensing element spaced from the first magnetic field sensing element and configured to generate a second magnetic field signal indicative of the magnetic field associated with the target. A switching module is coupled to receive the first and second magnetic field signals and configured to generate a combined signal having alternating portions associated with the first and second magnetic field signals. A polarity module is coupled to receive the combined signal and configured to invert a polarity of portions of the combined signal associated with one of the first magnetic field signal or the second magnetic field signal to generate an inverted polarity signal. The sensor further includes a filter coupled to receive the inverted polarity signal and configured to generate an averaged signal corresponding to an average of the first and second magnetic field signals.

With this arrangement, advantageously the benefits of differential magnetic field sensing are achieved with only a single amplifier as compared to a conventional arrangement in which each sensing element has a dedicated amplifier, thereby reducing area and power requirements associated with differential sensing.

Features may include one or more of the following individually or in combination with other features, as defined in the appended dependent claims. A clock module can be coupled to the switching module and to the polarity module and configured to provide a clock signal to the switching module and the polarity module. In embodiments, the switching module can be a multiplexer configured to selectively couple the first magnetic field signal or the second magnetic field signal to an output of the switching module based the clock signal. The polarity module is configured to invert the polarity of the one of the alternating portions of the combined signal associated with the first magnetic field signal or with the second magnetic field signal based the clock signal.

In embodiments, the sensor includes an amplifier coupled to receive the combined signal and configured to generate an amplified signal. An analog-to-digital converter can be provided with an input coupled to receive the amplified signal (or the combined signal) and an output at which a digital signal is provided for coupling to the polarity module. In embodiments, the polarity module includes an amplifier coupled to receive the combined signal and configured to amplify the combined signal by a gain having a positive polarity during first ones of the alternating portions of the combined signal and by a gain having a negative polarity during second ones of the alternating portions of the combined signal.

Example filters include a cascaded integrator-comb (CIC) decimation filter or a switch capacitor notch filter.

The magnetic field sensor can include a first offset modulator coupled to the first magnetic field sensing element and configured to generate a first modulated magnetic field signal for coupling to the switching module and a second offset modulator coupled to the second magnetic field sensing element and configured to generate a second modulated magnetic field signal for coupling to the switching module, wherein a rate of modulation by the first offset modulator and the second offset modulator is different than a rate of the clock signal.

A signal processor can be coupled to receive the averaged signal and configured to generate a sensor output signal indicative of one or more of a current associated with the target, a speed of motion of the target, a direction of motion of the target, an angle of the target, or a position of the target. In embodiments, the signal processor is configured to generate a sensor output signal indicative of the magnetic field in more than one dimension. The first and second magnetic field sensing elements can include one or more Hall effect elements or magnetoresistance elements.

Also described is a method for sensing a magnetic field associated with a target, including generating a first magnetic field signal with a first magnetic field sensing element, the first magnetic field signal indicative of a magnetic field associated with the target and generating a second magnetic field signal with a second magnetic field sensing element spaced from the first magnetic field sensing element, the second magnetic field signal indicative of the magnetic field associated with the target. The method can further include generating a combined signal having first portions associated with the first magnetic field signal and second portions alternating with the first portions and associated with the second magnetic field signal. A polarity of the first portions or the second portions of the combined signal can be inverted to generate an inverted polarity signal and the inverted polarity signal can be filtered to generate an averaged signal corresponding to an average of the first and second magnetic field signals.

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.

Before describing the present invention, some introductory concepts and terminology are explained. As used herein, the term "magnetic field sensing element" is used to describe a variety of types of electronic elements that can sense a magnetic field. The magnetic field sensing elements can be, but are not limited to, Hall Effect elements, magnetoresistance elements, or magnetotransistors. As is known, there are different types of Hall Effect elements, for example, planar Hall elements, vertical Hall elements, and circular vertical Hall (CVH) 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.

As is known, some of the above-described magnetic field sensing elements tends to have an axis of maximum sensitivity parallel to 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 perpendicular to a substrate that supports the magnetic field sensing element. In particular, most, but not all, types of magnetoresistance elements tend to have axes of maximum sensitivity parallel to the substrate and most, but not all, types of Hall elements tend to have axes of sensitivity perpendicular to a substrate.

As used herein, the term "magnetic field sensor" is used to describe a circuit that includes a magnetic field sensing element. Magnetic field sensors 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, 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 (e.g., a linear magnetic field sensor) that senses a density of a magnetic field. The circuits and techniques described herein apply to any magnetic field sensor capable of detecting a magnetic field.

As used herein, the term "magnetic field signal" is used to describe any signal that results from a magnetic field experienced by a magnetic field sensing element.

Referring to <FIG>, a magnetic field sensor system <NUM> includes a magnetic field sensor <NUM> having a plurality of spaced magnetic field sensing elements <NUM>, <NUM> for sensing a magnetic field affected by a target <NUM> and generating respective magnetic field signals 24a, 26a. A modulator, as may take the form of a switching module <NUM>, is coupled to receive the first and second magnetic field signals 24a, 26a and is configured to generate a combined signal <NUM> having alternating portions associated with the first and second magnetic field signals. An example combined signal is shown in <FIG>.

A polarity module <NUM>, is coupled to receive the combined signal <NUM> (or a processed version of the combined signal as will be explained) and is configured to invert a polarity of portions of the combined signal associated with one of the first magnetic field signal or the second magnetic field signal to generate an inverted polarity signal <NUM>. An example inverted polarity signal is shown in <FIG>.

A filter <NUM> is coupled to receive the inverted polarity signal <NUM> and is configured to generate an averaged signal <NUM> corresponding to an average of the first and second magnetic field signals (which averaged signal is referred to herein alternatively as a differential magnetic field signal). With this arrangement, an average of the first and second magnetic field signals 24a, 26a is provided with fewer components and requiring less power consumption as compared, for example, to a conventional configuration in which two magnetic field signals are independently amplified and then combined to generate a differential magnetic field signal.

Target <NUM> may be a magnetic element such as a ring magnet or gear with teeth and/or valleys. In some embodiments, the target <NUM> is a ferromagnetic element and the magnetic field can be generated by a magnet <NUM> positioned in proximity to the target, such as in a "so-called" back bias configuration.

The example sensor <NUM> includes two Hall effect elements <NUM>, <NUM>, each spaced from the other by a predetermined distance and configured to generate a respective magnetic field signal 24a, 26a indicative of a magnetic field associated with the target. It will be appreciated that other types of magnetic field sensing elements may be used and still benefit from the advantages described herein and also more than two sensing elements can be used.

In embodiments, each of elements <NUM>, <NUM> can include a set of individual elements, such as three elements, selected and/or configured to provide three-dimensional magnetic field sensing. For example, each element <NUM>, <NUM> can include planar and vertical Hall elements arranged to measure magnetic fields in three dimensions. Using this type of configuration in different orientations relative to the target <NUM> allows each set of elements <NUM>, <NUM> to sense a different field attributable to the target and the same stray fields. When the difference is taken, any effects from the stray field are removed.

The magnetic field sensor <NUM> includes a clock module <NUM> coupled to the switching module <NUM> and to the polarity module <NUM> and configured to provide a clock signal <NUM> to both the switching module <NUM> and the polarity module <NUM>. The clock signal <NUM> controls both the rate at which the switching module <NUM> modulates the magnetic field signals 24a, 26a to generate the combined signal <NUM> and also the rate at which the polarity module <NUM> inverts the polarity of the combined signal. In an example embodiment, the clock signal <NUM> has a frequency on the order of <NUM>.

In the example embodiment, the switching module <NUM> is a multiplexer configured to selectively couple the first magnetic field signal 24a or the second magnetic field signal 26a to an output of the switching module to generate the combined signal <NUM> at a rate controlled by the clock signal <NUM>.

The magnetic field sensor <NUM> may include an amplifier <NUM> coupled to receive the combined signal <NUM> and configured to generate an amplified signal <NUM>.

In some embodiments (e.g., <FIG> and <FIG>), the amplifier <NUM> can be eliminated and the polarity module <NUM> may take the form of an amplifier coupled to receive the combined signal and configured to amplify the combined signal by a gain having a positive polarity during first ones of the alternating portions of the combined signal and by a gain having a negative polarity during second ones of the alternating portions of the combined signal in order to thereby generate the inverted polarity signal. In general, the polarity inversion can be implemented in the analog domain or in the digital domain anywhere along the signal path before the filter <NUM>.

An analog-to-digital converter (ADC) <NUM> may be provided to convert the amplified signal <NUM> (or the combined signal <NUM>) into a digital signal <NUM> for coupling to the polarity module <NUM>. In embodiments containing the ADC <NUM>, the polarity module <NUM> can operate simply by inverting the sign of the digital representation of the signal. And, in embodiments not containing the ADC <NUM> in which the polarity module <NUM> thus operates in the analog domain, polarity inversion can be accomplished by reversing inputs to a differential amplifier with a multiplexer for example.

A component of the inverted polarity signal <NUM> corresponding to an average of the magnetic field signals 24a, 26a appears at one frequency; and a difference between the magnetic field signals 24a, 26a appears at lower frequencies and is removed by the filter.

It will be appreciated that in embodiments not including the ADC <NUM>, the filter <NUM> is an analog rather than a digital filter.

A signal processor <NUM> is coupled to receive the averaged signal <NUM> and is configured to generate a sensor output signal <NUM> indicative of one or more of a current associated with the target <NUM>, a speed of motion of the target, a direction of motion of the target, an angle of the target, or a position of the target.

The magnetic field sensor <NUM> may be provided in the form of an integrated circuit in some embodiments.

Referring also to <FIG> and <FIG>, a magnetic field sensor <NUM> includes magnetic field sensing elements <NUM>, <NUM>', switching module <NUM> (as may include sub-modules <NUM>, <NUM>), amplifier <NUM>, ADC <NUM>, and filter <NUM>. Here each magnetic field sensing element <NUM>, <NUM>' is chopped to generate respective magnetic field signals 250a, 250b as will be described. Switching modules <NUM>, <NUM>, each receive a respective magnetic field signal 250a, 250b and function to generate a differential combined signal <NUM> containing alternating portions associated with the first and second magnetic field signals 250a, 250b.

A clock generator <NUM> generates a clock signal <NUM> coupled to the switching module <NUM> to control operation of the switching module to generate the combined signal <NUM>.

Amplifier <NUM> amplifies the combined signal <NUM> and in the embodiment of <FIG> and <FIG> may additionally function to invert a polarity of the combined signal. More particularly, amplifier <NUM> is coupled to receive the combined signal <NUM> and configured to amplify the combined signal by a first gain having a positive polarity during first ones of the alternating portions of the combined signal <NUM> and by a second gain having a negative polarity during second ones of the alternating portions of the combined signal <NUM> in order to thereby generate an inverted polarity signal <NUM>. Such polarity inversion can be accomplished by selectively coupling and then reversing the coupling of input signal lines of amplifier <NUM> to the amplifier inputs. Optional ADC <NUM> can be provided to convert the amplified signal <NUM> into a digital signal <NUM> for coupling to filter <NUM>.

Filter <NUM> (like filter <NUM> of <FIG>) is responsive to the inverted polarity signal <NUM> (or digital version thereof <NUM>) and is configured to generate an averaged signal <NUM> corresponding to an average of the first and second magnetic field signals 250a, 250b (which averaged signal <NUM> is referred to herein alternatively as a differential magnetic field signal).

Although not shown in <FIG> and <FIG>, the magnetic field sensor <NUM> can further include a signal processor that can be the same as or similar to processor <NUM> of <FIG> and thus, that can respond to the averaged signal <NUM> to generate a sensor output signal as may be indicative of one or more of a current associated with the target, a speed of motion of the target, a direction of motion of the target, an angle of the target, or a position of the target.

Magnetic field sensor <NUM> differs from sensor <NUM> (<FIG>) in that, in addition to including differential chopping generally as described above in connection with <FIG>, sensor <NUM> also includes offset chopping circuitry <NUM>, <NUM>'. Offset chopping circuitry <NUM>, <NUM>' operates to reduce offset associated with the Hall plates <NUM>, <NUM>' as described further in <CIT> entitled "Chopped Hall Effect Sensor".

Offset chopping circuitry <NUM> includes a Hall effect element <NUM> having typically four contacts and a modulation circuit <NUM> to periodically connect the supply voltage and inputs of switching module <NUM> to one pair of contacts or the other. Similarly, offset chopping circuitry <NUM>' includes a Hall effect element <NUM>' having typically four contacts and a modulation circuit <NUM>' to periodically connect the supply voltage and inputs of switching module <NUM> to one pair of contacts or the other. Use of such switched Hall plates provides a way to discriminate the Hall offset voltage from the magnetically induced signal from the field desired to be detected.

More particularly, the first offset modulator <NUM> coupled to the first magnetic field sensing element <NUM> is configured to generate first modulated magnetic field signal 250a for coupling to the switching module <NUM> and the second offset modulator <NUM>' coupled to the second magnetic field sensing element <NUM>' is configured to generate second modulated magnetic field signal 250b for coupling to the switching module <NUM>. For simplicity, the offset modulation circuitry <NUM>, <NUM>' is described in connection with example circuitry <NUM>. It will be appreciated however that circuitry <NUM>' operates in the same manner as circuitry <NUM>.

Example Hall plate <NUM> includes four equally spaced contacts 252a, 252b, 252c, and 252d, each coupled to a first terminal of a respective switch 256a, 256b, 256c, and 256d, as shown. A second terminal of switches 256b and 256c are coupled to provide the positive node of the switched Hall output signal 250a and the second terminal of switches 256a and 256d are coupled to provide the negative node of the switched Hall output signal 250a.

Switches 260a, 260b, 260c, and 260d are arranged to selectively couple the Hall contacts 252a, 252b, 252c, 252d to the supply voltage Vs and ground. More particularly, switches 256b, 256d, 260a, and 260c are controlled by a two state clock signal CLK and switches 256a, 256c, 260b, and 260d are controlled by a complementary clock signal CLK/, as shown. The Hall offset voltage can be represented as a voltage source <NUM> between the Hall element <NUM> and the modulation circuit <NUM>.

In operation, during a first phase of the CLK signal, current flows from terminal 252a to 252c and the switched Hall output signal 250a is equal to VH + Vop, where Vop is the Hall plate offset voltage or Hall offset signal component <NUM> and VH is the magnetic signal component. During the alternate phase of the CLK signal, current flows from terminal 252b to 252d and the switched Hall output signal 250a is equal to VH - Vop. Thus, the modulation circuit <NUM> modulates the Hall offset signal component Vop <NUM> and the magnetic signal component VH remains substantially invariant. The Hall offset signal component thus modulated can be demodulated by the amplifier <NUM> and removed by l filter <NUM>.

The rate of modulation by the first offset modulator <NUM> and the second offset modulator <NUM>' is greater than a rate of the clock signal <NUM>. In an example embodiment, the offset chopping frequency (i.e., the rate of the clock signal CLK) is on the order of <NUM> and the frequency of the clock signal <NUM> is on the order <NUM>. More generally, the rate of modulation by the first and second offset modulators <NUM>, <NUM>' is different than (and in some embodiments may be a multiple of) the rate of switching by switching module <NUM>.

Referring to <FIG>, waveforms associated with the differential chopping operation of <FIG> are shown in which the x-axis represents arbitrary units of time and the y-axis represents field strength in Gauss. <FIG> show respective example magnetic field signal waveforms <NUM>, <NUM>. Magnetic field signal <NUM> of <FIG> can correspond to magnetic field signal 24a as sensed by sensing element <NUM> for example and magnetic field signal <NUM> of <FIG> can correspond to magnetic field signal 26a as sensed by sensing element <NUM>, for example. Because sensing elements <NUM>, <NUM> are at different respective positions relative to the target <NUM>, the sensed field strengths G1, G2 are different.

Referring to <FIG>, a waveform <NUM> shows an example combined signal <NUM> that can be the same as or similar to combined signal <NUM> provided at the output of multiplexer <NUM> of <FIG>. Thus, first portions 324a of combined signal <NUM> correspond to the first magnetic field signal <NUM> of <FIG> (and thus have a field strength of G1) and alternating second portions 324b of combined signal <NUM> correspond to the second magnetic field signal <NUM> of <FIG> (and thus have a field strength of G2).

Referring to <FIG>, a waveform <NUM> shows an example inverted polarity signal <NUM> that can be the same as or similar to inverted polarity signal <NUM> provided at the output of polarity module <NUM> of <FIG>. Thus, in inverted polarity signal <NUM>, portions of the combined signal <NUM> associated with one of the first magnetic field signal or the second magnetic field signal are inverted. Stated differently, either portions 324a or portions 324b of combined signal <NUM> are inverted to generate inverted polarity signal <NUM>. Here, portions 324b of combined signal <NUM> (which portions are associated with the second magnetic field signal <NUM>) are inverted. The resulting inverted polarity signal <NUM> thus has first portions 334a associated with the first magnetic field signal <NUM> and having a field strength G1 and second, alternating portions 334b associated with the second magnetic field signal <NUM> but with an inverted polarity and thus, having a field strength -G2, as shown.

A first component of the inverted polarity signal <NUM> corresponds to a difference between the first and second magnetic field signals (e.g., signals 24a and 26a) and appears at a relatively low frequency. A second component of the inverted polarity signal <NUM> corresponds to an average of the first and second magnetic field signals. The output of the filter <NUM> corresponds to the averaged component of the first and second magnetic field signals and the effects of stray fields are eliminated by filtering the difference component.

Referring to <FIG>, a method <NUM> for sensing a magnetic field associated with a target begins at block <NUM> following which a first magnetic field signal indicative of a magnetic field associated with the target is generated with a first magnetic field sensing element or elements at block <NUM>. At block <NUM>, a second magnetic field signal indicative of the magnetic field associated with the target is generated by a second magnetic field sensing element or elements spaced from the first element(s). For example, the first magnetic field signal can be generated by first sensing element <NUM> and second magnetic field signal can be generated by second sensing element <NUM> of <FIG>.

At block <NUM>, a combined signal having first portions associated with the first magnetic field signal and second portions alternating with the first portions and associated with the second magnetic field signal is generated. In the embodiment of <FIG> for example, the combined signal <NUM> is generated by multiplexer <NUM>.

A polarity of the first portions or the second portions of the combined signal is inverted at block <NUM> to generate an inverted polarity signal. For example, polarity module <NUM> of <FIG> can generate inverted polarity signal <NUM>.

At block <NUM>, the inverted polarity signal is filtered (e.g., by filter <NUM> of <FIG>) to generate a filtered signal (e.g., signal <NUM> of <FIG>). As described, the filtered signal is representative of an average of the first and second magnetic field signals and is substantially immune to the effects of stray fields common to the sensing elements.

Having described preferred embodiments, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. 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.

Claim 1:
A magnetic field sensor comprising:
a first magnetic field sensing element (<NUM>) configured to generate a first magnetic field signal (24a) indicative of a magnetic field associated with a target;
a second magnetic field sensing element (<NUM>) spaced from the first magnetic field sensing element (<NUM>) and configured to generate a second magnetic field signal (26a) indicative of the magnetic field associated with the target;
a switching module (<NUM>) coupled to receive the first and second magnetic field signals (24a, 26a) and configured to generate a combined signal (<NUM>) having alternating portions associated with the first and second magnetic field signals (24a, 26a);
a polarity module (<NUM>) coupled to receive the combined signal (<NUM>) and configured to invert a polarity of portions of the combined signal (<NUM>) associated with one of the first magnetic field signal (24a) or the second magnetic field signal (26a) to generate an inverted polarity signal (<NUM>); and
a filter (<NUM>) coupled to receive the inverted polarity signal (<NUM>) and configured to generate an averaged signal (<NUM>) corresponding to an average of the first and second magnetic field signals (24a, 26a).