JP2010107460A - Magnetic sensor control circuit and magnetometric field measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain both high-resolution and high-accuracy measurement and extension of dynamic range in a magnetic field device. <P>SOLUTION: Within the measurement range of magnetic field that is set in the linear area of input/output characteristic of a magnetometric sensor 8, an ADC comprised of a ΔΣ modulator 22 and a digital filter 24 converts the output of the magneto sensor 8 into magnetic field measurement data D<SB>M</SB>with high resolution. A bias magnetic field is applied to the magnetometric sensor 8 through a coil 10 so as to shift the measurement range of magnetic field. Driving current given to the coil 10 is discretely changed by current DAC 28. The moving step of magnetic field measurement range caused by a change in bias magnetic field is set larger than the resolution of magnetic field strength by the ADC. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic sensor control circuit that controls a magnetic sensor unit including a magnetic sensor, and a magnetic field measurement apparatus using the magnetic sensor control circuit.

  When performing azimuth measurement electronically, a magnetic sensor that detects an external magnetic field such as geomagnetism is used. A technique is known in which an alternating magnetic field is applied to a magnetic sensor and a voltage output from the magnetic sensor when the alternating magnetic field is applied when the orientation is obtained using the magnetic sensor.

  In this technique, a magnetic sensor including a magnetoresistive element whose internal resistance changes according to an external magnetic field is used. The magnetic sensor is composed of a bridge circuit including a magnetoresistive element, and the polarity of the output signal of the bridge circuit is switched according to the polarity of the voltage applied to the bridge circuit.

Such a magnetic sensor is mounted on, for example, a mobile phone as an electronic azimuth meter. Here, a magnetic field other than the geomagnetism generated from an electronic component mounted on the mobile phone, for example, a speaker or the like, becomes an offset magnetic field with respect to the geomagnetism and prevents the detection of the geomagnetism. In order to remove the influence of such an offset magnetic field, the magnetic sensor unit is provided with a coil for applying a bias magnetic field to the magnetic sensor, and the current flowing through the coil is adjusted to cancel the offset magnetic field.
JP 2007-271599 A

  The magnetic field to be detected can be a weak magnetic field such as geomagnetism, while the offset magnetic field can have a relatively large intensity. The output voltage of the magnetic sensor exhibits a linear characteristic in a range where the applied magnetic field is relatively small, but the linearity decreases as the applied magnetic field increases. Therefore, when the offset magnetic field is large, there is a problem that the linearity of the magnetic field measurement value is lowered. This problem can be solved if the offset magnetic field can be suitably canceled by the bias magnetic field as described above.

  Here, a circuit that generates a current using a DA converter (DAC: Digital to Analog Converter) is used to adjust the current flowing through the coil that generates the bias magnetic field and to maintain the adjusted current. . However, the number of DAC bits is limited to a relatively small value because of the demand for circuit size reduction and cost reduction. For this reason, if the offset magnetic field is to be canceled with high accuracy, the DAC needs to reduce the amount of change in the output current per step of the input data. As a result, the adjustable range becomes small and it cannot cope with a large offset magnetic field. There's a problem.

  Further, the current generated using the DAC includes flicker noise (1 / f noise) caused by a transistor such as a MOSFET constituting the DAC. Furthermore, offset noise may occur in the output signal from the magnetic sensor in the magnetic sensor or signal processing circuit. There is a problem that these noises cause a reduction in resolution.

  The present invention has been made to solve the above-described problems, and enables high resolution and dynamic range expansion, and eliminates not only the offset magnetic field but also the effect of offset noise caused by flicker noise and the like. It is an object of the present invention to provide a magnetic sensor control circuit and a magnetic field measuring apparatus that enable highly accurate magnetic field measurement.

  The magnetic sensor control circuit according to the present invention has a linear region that generates a magnetic field measurement signal that changes linearly according to the strength of the external magnetic field in input / output characteristics, and the polarity of the magnetic field measurement signal depends on the polarity of the applied voltage. A magnetic sensor comprising: a switching magnetic sensor; and a bias magnetic field generating means for generating a bias magnetic field having a strength corresponding to a drive signal at a measurement position of the magnetic sensor and shifting the position of the linear region on the strength axis of the external magnetic field. A circuit used in a sensor unit, which generates the drive signal to be supplied to the bias magnetic field generating means. By discretely setting the intensity of the drive signal, on the intensity axis of the external magnetic field. Magnetic field measurement range included in the linear region at the selected setting position by selecting any of a plurality of setting positions shifted by a predetermined setting step. A measurement range setting circuit to be arranged and the magnetic field measurement signal obtained in the magnetic field measurement range are converted into digital data, and magnetic field measurement data expressing the magnetic field strength in the magnetic field measurement range with higher resolution than the setting step is generated. An AD converter (ADC: Analog to Digital Converter), a control unit for switching the polarity of the applied voltage, the first magnetic field measurement data obtained by setting the applied voltage to the first polarity, and the applied voltage Data synthesizing means for subtracting the second magnetic field measurement data obtained by setting the second polarity to obtain the measurement value of the external magnetic field.

  The magnetic field measuring apparatus according to the present invention includes the magnetic sensor control circuit and the magnetic sensor unit.

  According to the present invention, the measurement range setting circuit is configured using, for example, a DAC, and the measurement range setting circuit generates a drive signal to the bias magnetic field generation unit with discrete intensity. Thereby, the bias magnetic field also has a discrete strength. The step for setting the intensity of the bias magnetic field is set larger than the required resolution for magnetic field measurement. As a result, the linear region of the magnetic sensor can be moved greatly with a relatively small number of setting steps and can be arranged in a relatively wide magnetic field strength range, and the dynamic range can be expanded.

  On the other hand, an ADC that converts a magnetic field measurement signal into digital data need not be able to quantize the entire magnetic field strength range in which the linear region can be set, and the magnetic field in the linear region at a certain position set by the measurement range setting circuit. It suffices if the measurement range can be quantized. That is, the ADC can quantize the magnetic field strength in a finer step than the bias magnetic field setting step even with a relatively small number of bits, and can obtain high-resolution magnetic field measurement data.

  Further, by subtracting the first magnetic field measurement data and the second magnetic field measurement data obtained by inverting the applied voltage of the magnetic sensor, the influence of offset noise caused by circuits such as the magnetic sensor and the DAC is removed. Therefore, highly accurate magnetic field measurement is possible.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

  FIG. 1 is a schematic circuit configuration diagram of a magnetic measurement device 2 according to the embodiment. The magnetic measuring device 2 includes a magnetic sensor unit 4 and a magnetic sensor control circuit 6.

  The magnetic sensor unit 4 has three magnetic sensors 8 (magnetic sensors 8x, 8y, 8z) corresponding to the three axes of the orthogonal coordinate system (XYZ coordinate system). The magnetic sensors 8x, 8y, and 8z detect components of the external magnetic field in the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively.

Each magnetic sensor 8 is a bridge circuit composed of resistance elements Ra, Rb, Rc, Rd. The resistors Ra and Rd are connected in series between the terminals V _IN1 and V _IN2 , and the resistors Rb and Rc are also connected in series between the terminals V _IN1 and V _IN2 . The connection point between the resistors Ra and Rd is the output terminal _VOUT1 , and the connection point between the resistors Rb and Rc is the output terminal _VOUT2 . Of the resistance elements Ra, Rb, Rc, and Rd, the elements that exhibit magnetoresistance change are Ra and Rc. On the other hand, Rb and Rd are fixed resistors. When a voltage is applied to the pair of input terminals V _IN1 and V _IN2 of this bridge circuit, a voltage divided by the resistors Ra and Rd is obtained at the output terminal VOUT1, and a voltage _divided by the resistors Rb and Rc is output. Obtained at the terminal _VOUT2 . Since the resistances of the resistors Ra and Rc are changed by an external magnetic field, voltage signals corresponding to the external magnetic field are output from the output terminals V _OUT1 and V _{OUT2 in a} differential format. Even if Rb and Rd are magnetoresistive elements and Ra and Rc are fixed resistance elements, they operate similarly because they are differential outputs of the bridge circuit.

  As the magnetoresistive element, an element exhibiting a symmetric change with respect to the magnetic field is used. As such a magnetoresistive element, an MR (Magneto Resistance) element, a GIG (Granular In Gap) element, and the like are known. In the magnetic measuring device 2, for example, a GMR (Giant Magneto Resistance) element which is a kind of MR element is used for the resistances Ra and Rc.

  The magnetic sensor 8 is provided with a coil 10. The coil 10 is bias magnetic field generating means, and can apply a bias magnetic field having a strength corresponding to the supplied current to the resistors Ra and Rc and change the resistances.

The switches SW _X1 , SW _Y1 , and SW _Z1 are provided for applying a voltage to one of the terminals V _IN1 and V _IN2 of the three magnetic sensors 8.

The positive voltage _VCC is applied to one of the terminals V _IN1 and V _IN2 of the magnetic sensor 8, and the ground potential GND is applied to the other. The polarity of the applied voltage can be switched by the switches SW _{S1 to} SW _S4 of the polarity inverting unit 12.

The signal lines from the output terminals V _OUT1 and V _{OUT2 of} the magnetic sensor 8x to the magnetic sensor control circuit 6 are provided with switches SW _X2 and SW _X3 , and similarly to the output terminals V _OUT1 and V _OUT2 of the magnetic sensor 8y. Are provided with switches SW _Y2 and SW _Y3 , and the magnetic sensor 8z is provided with switches SW _Z2 and SW _Z3 . By switching the switches provided at these output terminals, the output signal of any one of the three magnetic sensors 8 can be input to the magnetic sensor control circuit 6.

Further, switches SW _X4 , SW _Y4 , and SW _Z4 are provided in order to selectively supply current to the coils 10 provided along with the magnetic sensors 8.

  The magnetic sensor control circuit 6 includes a differential preamplifier 20, a ΔΣ modulator 22, a digital filter 24, an output interface 26, a current DAC 28, and a control unit 30.

The differential preamplifier 20 amplifies the differential magnetic field measurement signal S _M output from the output terminals V _OUT1 and V _OUT2 of the magnetic sensor 8.

The ΔΣ modulator 22 and the digital filter 24 constitutes a ΔΣ type ADC, to produce a converts the magnetic field measurement signal S _M, which is an analog signal into digital data field measurement data D _M. Field measurement signal S _M, which is amplified by the differential preamplifier 20 is inputted to the ΔΣ modulator 22. The ΔΣ modulator 22 has a 1-bit AD conversion function, and samples the magnetic field measurement signal S _M faster than the sample rate (sampling frequency) of the magnetic field measurement data D _M. As described above, the output quantized roughly with a small number of bits contains a lot of quantization noise. This quantization noise is shifted to a high frequency region by a noise shaping circuit inside the ΔΣ modulator 22.

The output of the ΔΣ modulator 22 is input to the digital filter 24. The digital filter 24 is a low-pass filter (LPF), which attenuates the quantization noise transferred to the high frequency region by the noise shaping circuit, while reducing the low frequency region where the signal component representing the magnetic field measurement data exists. Let it pass. The digital filter 24 performs filtering and downsampling on the high-speed 1-bit data from the ΔΣ modulator 22 to obtain multi-bit data at a relatively low speed. This is output as the magnetic field measured data D _M.

Field measurement data D _M generated by the digital filter 24 is transmitted to the microcomputer from the magnetic sensor control circuit 6 including a semiconductor integrated circuit or the like via a bus or the like. The output interface 26 converts X, Y, the magnetic field measured data D _M of Z axial directions into a format suitable for the data transmission, and sends it to the bus.

Current DAC28 generates and supplies driving current _{I B} to the coil 10 is a bias magnetic field generating means. Current DAC28 is input the desired bias magnetic field data D _B from the control unit 30, the data is converted into a drive current I _B which is a driving signal of the analog.

The control unit 30 switches the switches SW _{X1 to} SW _X3 , SW _{Y1 to} SW _Y3 , SW _{Z1 to} SW _Z3 , applies a voltage among the magnetic sensors 8 x, 8 y, and 8 z, and outputs the output to the differential preamplifier 20. The ones to be taken out are selected in order, and the switches SW _X4 , SW _Y4 , and SW _Z4 are switched corresponding to the selected magnetic sensor 8, and the coil 10 provided in the magnetic sensor 8 is selectively supplied from the current DAC 28. to supply the driving current _{I B.} Further, the control unit 30 performs an operation of inverting the polarity of the applied voltage by controlling the polarity inverting unit 12 in one magnetic field measurement by the selected magnetic sensor 8. The control unit 30 controls the operation of the digital filter 24 in conjunction with the switching of the magnetic sensor 8 and the switching of the polarity of the applied voltage. Further, the control unit 30 controls the operation of the output interface 26 in response to switching of the magnetic sensor 8 or the like.

FIG. 2 is a timing chart relating to the processing of the magnetic sensor 8 and its output signal. The waveform shown as the sensor select indicates whether the magnetic field measurement is performed in the X, Y, or Z axis direction. For example, a magnetic sensor 8x in the X-axis is optionally sensor select the X-axis is the H (High) level period 40x, is applied a voltage and the bias magnetic field, the magnetic field measurement signal S _M is inputted to the differential preamplifier 20 The Y-axis and Z-axis sensor select also shows the same thing. The sensor select for the X axis, the Y axis, and the Z axis is basically set in order of the same length.

  The sensor polarity indicates the state of the polarity reversing unit 12. For example, in the period 42 of L (Low) level, the applied voltage to the magnetic sensor 8 is positive, while the period 44 of H level is applied. It is assumed that the voltage is negative polarity that is reversed from the positive polarity. In one magnetic field measurement period of each magnetic sensor 8, the positive polarity period 42 and the negative polarity period 44 are set at least once, basically by the same length. In the operation shown in FIG. 2, the sensor select periods 40x, 40y, and 40z of each axis are divided into two equal parts, and the first half is set to positive polarity and the second half is set to negative polarity.

ΔΣ modulator 22, a pulse density modulation by sampling the magnetic field measurement signal S _M output from the magnetic sensor 8, which is selected (Pulse Density Modulation: PDM) pulse train to generate. The digital filter 24 performs band limitation on the high frequency region of the output spectrum of the ΔΣ modulator 22 by, for example, moving average processing the pulse train output from the ΔΣ modulator 22. In the moving average process, the number of pulses input to the digital filter 24 is accumulated over a predetermined time width using an adder. Thus, the digital filter 24 has an adder, and the digital filter 24 uses the adder to set the first magnetic field measurement data obtained by setting the applied voltage to the positive polarity and the negative polarity. a second magnetic field measurement data obtained by subtracting performing data synthesis processing for obtaining the magnetic field measured data D _M and.

The control unit 30 sets periods 46 and 48 for operating the digital filter 24 within each of the positive period 42 and the negative period 44. The period 46 and the period 48 are set to the same length. In addition, the control unit 30 sets the operation of the adder to the input data in the digital filter 24 as an addition operation in the positive period 42 and sets the operation of the adder as a subtraction operation in the negative period 44. That is, the adder sequentially adds the input data to the current value in period 46, while subtracting the input data from the current value sequentially in period 48. The addition operation may be performed separately in the periods 46 and 48, and the addition result in the period 48 may be subtracted from the addition result in the period 46 after the period 48 ends. By performing the data synthesis, as described below, to reduce the offset noise of the magnetic field measurement signal S _M resulting from such a flicker noise contained in the output current of the current DAC 28, improve the accuracy of the magnetic field measured data be able to.

When the period 48 ends, the digital filter 24 completes the data synthesis process, and one magnetic field measurement data _DM is obtained. Control unit 30, the resulting magnetic field measured data D _M, the period 50 before starting the next field measurement is read from the counter to hold the addition result, performing a data transfer operation by the output interface 26. Further, when the data transfer operation of the magnetic field measured data D _M, the control unit 30 in the subsequent period 52 resets the counter, ready for the start of the next field measurement.

  FIG. 3 is a timing chart showing another example regarding the processing of the magnetic sensor 8 and its output signal. 3 differs from FIG. 2 in that the number of inversions of the sensor polarity in one magnetic field measurement period (sensor select periods 40x, 40y, and 40z) of each magnetic sensor 8 and the digital filter 24 corresponding thereto. It is in the number of switching. That is, in the operation shown in FIG. 2, the positive polarity period 42 and the negative polarity period 44 are set once in one sensor selection period 40, and the operation of the digital filter 24 is performed in conjunction with the two periods. Divided into periods 46 and 48, addition and subtraction in the digital filter 24 are performed once. On the other hand, in the operation shown in FIG. 3, the positive polarity period 42 and the negative polarity period 44 are set twice in one sensor selection period 40, and the operation of the digital filter 24 is interlocked with this. Are divided into four periods, and addition and subtraction in the digital filter 24 are performed twice. Thus, by increasing the number of times of switching in one sensor select period 40, the characteristics of the digital filter 24 can be changed. For example, lower frequency noise can be reduced.

Next, the offset noise reduction by the data synthesis process will be described. Figure 4 is a schematic graph showing a characteristic of a magnetic field measurement signal S _M to an external magnetic field B. 4, the horizontal axis indicates an external magnetic field B, the vertical axis represents the magnetic field measurement signal S _M. The offset noise of the magnetic field measurement signal S _M of the magnetic sensor 8 representing the size s _N. The magnetic sensor 8 characteristic curve 60 is for the case where the applied voltage is positive, and the characteristic curve 62 is for the case where the applied voltage is negative. If the external magnetic field B is 0, S _M = 0 should be originally obtained regardless of whether the applied voltage is positive or negative, and the characteristic curve 60 and the characteristic curve 62 are the origins of the B−S _M coordinates, that is, (B, S _M ) = (0, 0), and the characteristic curve 60 and the characteristic curve 62 should be vertically symmetrical with respect to S _M = 0. Then, the characteristic curve 60 and characteristic curve 62 intersects with (0,0), each characteristic curve 60, 62 should be symmetrical around the intersection thereof _{P C,} respectively. However, if the offset noise _{s N} is not 0, the intersection _{P C} is shifted to the (0, _{s N)} as shown in FIG.

The value of the magnetic field measurement signal S _M of the positive polarity voltage is applied at an external magnetic field strength B s _+, the value of the magnetic field measurement signal S _M of the negative polarity voltage application s _- to. If the component corresponding to the intensity of the external magnetic field in s ₊ is s _O , s ₊ in the state where the offset noise s _N is not 0 is expressed by the following equation.
s ₊ = s _O + s _N (1)

On the other hand, at the same external magnetic field strength B, s ₋ is expressed by the following equation.
s ₋ = −s _O + s _N (2)

s ₊ corresponds to the addition result of the output of the ΔΣ modulator 22 in the positive operation period 46, while s ₋ corresponds to the addition result in the negative operation period 48. When the expression (2) is subtracted from the expression (1), the offset noise s _N is canceled and the external magnetic field strength s _O can be obtained as shown in the following expression.
s ₊ −s ₋ = 2s _O (3)

  The above-described data synthesizing process in which the adder of the digital filter 24 is the addition operation in the period 46 and the subtraction operation in the period 48 corresponds to the calculation to obtain the expression (3) from the expressions (1) and (2). Based on this principle, the offset noise is reduced by the digital filter 24.

  As the digital filter 24, for example, an FIR (Finite Impulse Response) filter can be used.

Input-output characteristic of the magnetic sensor 8, as shown in the characteristic curve 60, 62 in FIG. 4, a magnetic field measurement signal S _M is linearly depending on the change in the intensity of the external magnetic field B around the intersection thereof P _C, i.e. It has a linear region 64 that varies linearly. On the other hand, the intersection point P increases away when a magnetic field measurement signal from the _C S _M Linearity saturated disappears. The magnetometric apparatus 2, of the input-output characteristics of the magnetic sensor 8, it is utilized easy linear region to determine the external magnetic field intensity B corresponding to the magnetic field measured data D _M.

The magnetic measuring device 2 uses the linear region of the input / output characteristics of the magnetic sensor 8 and adjusts the bias magnetic field generated by the coil 10 to expand the measurable range of the external magnetic field. Figure 5 is a schematic diagram showing input and output characteristics of the magnetic measuring device 2, the horizontal axis external magnetic field strength B, the vertical axis represents the magnetic field measured data D _M. The external magnetic field strength B does not include a component due to a bias magnetic field. Here, for convenience of explanation, the current DAC28 shows an example that can be varied has a resolution of 2 bits, the driving current I _B in four stages. Four stages in response to the drive current _{I B,} shows four characteristic curves 70-1～70-4 in FIG. The change indicated by each characteristic curve 70 is similar to the characteristic curve 60. Incidentally, when I _B = 0 and the bias magnetic field intensity is 0, the input / output characteristics of the magnetic measuring device 2 follow a characteristic curve 70-3 passing through the origin of the B-D _M coordinate.

  A magnetic field measurement range 72 is set in the linear region of each characteristic curve 70, and the dynamic range of the ADC including the ΔΣ modulator 22 and the digital filter 24 is set corresponding to the magnetic field measurement range 72. For example, the ADC quantizes each magnetic field measurement range 72 with a high resolution such as 12 bits or 14 bits.

As described above, the offset noise due to flicker noise or the like of the current DAC28 is removed, in each characteristic curve 70, a point corresponding to the center of the characteristic curve 60 shown at the intersection P _C in FIG. 4, D _It is located at _M = 0. That is, the intersection points P _{B1 to} P _B4 between the characteristic curves 70-1 to 70-4 and the B axis are the centers of the characteristic curves 70.

Here, by reducing the gain of the differential preamplifier 20 and ADC, for example, the slope of the linear region of the characteristic curve 70 is reduced, it is possible to increase the magnetic field measurement range 72, a constant value the drive current I _B Magnetic field measurement in a large intensity range is possible even when fixed to. However, this step of the external magnetic field intensity B corresponding to one step of the D _M is large, the measurement resolution of the external magnetic field is reduced.

Therefore, in the magnetic measuring apparatus 2, as shown in FIG. 5, each characteristic curve 70 is set to have a large inclination in the magnetic field measurement range 72 so that the external magnetic field can be measured with high resolution, while the current DAC 28 is set. by changing the driving current I _B by the intensity of the bias magnetic field discretely varied, as a plurality of characteristic curves 70 misaligned in the B-axis directions are obtained, a magnetic field measured in those magnetic field measurement range 72 The dynamic range is expanded as possible. Incidentally, the purpose of expanding the dynamic range, step change of the drive current I _B, the setting step of the intensity of the bias magnetic field, one step of the required resolution epsilon, i.e. the magnetic field measured data D _M generated by the ADC of the magnetic field measurement Is set to be larger than the step of the external magnetic field intensity B corresponding to.

Moving steps of the magnetic field measurement range 72 corresponding to the drive current I _B may not necessarily be one that continuously arranged magnetic field measurement range 72 on the B-axis, but generally the magnetic field measured by a continuous range It would be desirable to be able to do it. In that case, for example, as shown in FIG. 5, as the magnetic field measurement range 72 adjacent characteristic curve 70 has an overlap region with each other, the change in I _B per step to adjust the gain of the current DAC28 The amount can be set.

For example, the control unit 30, when the magnetic field measurement data D _M of the ADC generates becomes a value corresponding to the upper limit or lower limit of the magnetic field measurement range, change the objective bias magnetic field data D _B to be inputted to the current DAC 28, Switch to the adjacent magnetic field measurement range 72. In this case, for example, the magnetic field measurement data _DM and information indicating the set position of the magnetic field measurement range 72, for example, target bias magnetic field data are output from the output interface 26.

  According to the present invention, the magnetic measuring device 2 can obtain a large dynamic range as described above, but this does not require that the dynamic range of the measurement target magnetic field be large. The magnetic measurement apparatus 2 of the present invention is also suitable when the dynamic range of the measurement target magnetic field is small.

For example, when measuring a weak magnetic field such as geomagnetism, an offset magnetic field may be superimposed on the external magnetic field in addition to the measurement target component. If the offset magnetic field is large, the external magnetic field strength B exceeds the magnetic field measurement range 72, and fluctuations in the measurement target component cannot be measured. In such a case, in such a state that incorporates a magnetic measurement device 2 on the mounting device, measured in advance an offset magnetic field, the desired bias magnetic field data D _B defined on the basis of the offset magnetic field, for example, the control unit The data is stored in a data holding means such as a register 32 in the 30.

Current DAC28 generates a drive current _{I B} based on the object bias magnetic field data _{D B.} The purpose bias magnetic field data D _B, of the magnetic field measurement range 72 of a plurality of setting positions, a value corresponding to a intended to encompass a variation range of the measurement target component is selected. For example, if the offset magnetic field strength is b _OFF and the fluctuation range of the measurement target component superimposed thereon is Δb, the intersection of the characteristic curve 70 and the B axis has the B coordinate closest to the offset magnetic field strength b _OFF. If the corresponding characteristic curve 70 is selected, the fluctuation range Δb is included in the magnetic field measurement range 72. In the example shown in FIG. _{5, b OFF} is closest to the intersection point _{P B4,} the purpose bias magnetic field data _{D B,} a value corresponding to the characteristic curve 70-4 is set to the register 32 of the control unit 30.

  In the example shown in FIG. 5, the current DAC 28 is 2 bits. However, in the present invention, the number of bits can be arbitrarily set.

It is a schematic circuit block diagram of the magnetic measuring device which concerns on embodiment of this invention. It is a timing diagram regarding processing of a magnetic sensor and its output signal. It is a timing diagram which shows another example regarding the process of a magnetic sensor and its output signal. It is a typical graph which shows the characteristic of the magnetic field measurement signal with respect to an external magnetic field. It is a schematic diagram which shows the input / output characteristic of a magnetic measuring device.

Explanation of symbols

  2 Magnetic measurement device, 4 Magnetic sensor unit, 6 Magnetic sensor control circuit, 8 Magnetic sensor, 10 Coil, 12 Polarity inversion unit, 20 Differential preamplifier, 22 ΔΣ modulator, 24 Digital filter, 26 Output interface, 28 Current DAC, 30 control units, 32 registers.

Claims (8)

A magnetic sensor having a linear region for generating a magnetic field measurement signal that linearly changes according to the intensity of an external magnetic field in input / output characteristics, and the polarity of the magnetic field measurement signal is switched according to the polarity of an applied voltage, and measurement of the magnetic sensor A magnetic sensor control circuit used in a magnetic sensor unit including a bias magnetic field generation unit that generates a bias magnetic field having a strength corresponding to a drive signal at a position and shifts the position of the linear region on the strength axis of the external magnetic field. There,
A circuit for generating the drive signal to be supplied to the bias magnetic field generating means, and by discretely setting the strength of the drive signal, a plurality of steps shifted by predetermined setting steps on the strength axis of the external magnetic field A measurement range setting circuit that selects any one of the setting positions and arranges the magnetic field measurement range included in the linear region at the selected setting position;
An AD converter that converts the magnetic field measurement signal obtained in the magnetic field measurement range into digital data, and generates magnetic field measurement data that expresses the magnetic field strength in the magnetic field measurement range with higher resolution than the setting step;
A controller for switching the polarity of the applied voltage;
The external magnetic field is obtained by subtracting the first magnetic field measurement data obtained by setting the applied voltage to the first polarity and the second magnetic field measurement data obtained by setting the applied voltage to the second polarity. A data synthesis means for obtaining a measured value of
A magnetic sensor control circuit comprising: The magnetic sensor control circuit according to claim 1,
The AD converter includes a ΔΣ modulator and a digital filter having a low-pass characteristic that removes quantization noise shifted to a high frequency region by the ΔΣ modulator,
The control unit includes a first polarity period in which the applied voltage is set to the first polarity within a measurement period for acquiring the one measured value, and the applied voltage having the same length as the first polarity period. A second polarity period for setting the second polarity to the second polarity,
The data synthesizing unit is an adder provided in the digital filter, and switches addition / subtraction of the magnetic field measurement data in synchronization with switching between the first polarity period and the second polarity period;
A magnetic sensor control circuit. The magnetic sensor control circuit according to claim 2,
The control unit alternately sets the first polarity period and the second polarity period a plurality of times within the measurement period;
A magnetic sensor control circuit. In the magnetic sensor control circuit according to any one of claims 1 to 3,
Data holding means for holding target bias magnetic field data determined based on an offset component that is present in the external magnetic field and superimposed on a measurement target component;
The measurement range setting circuit includes:
A DA converter that converts the target bias magnetic field data that is digital data into the drive signal that is an analog signal;
Arranging the magnetic field measurement range at a position including the fluctuation range of the measurement target component;
A magnetic sensor control circuit. A magnetic sensor control circuit according to any one of claims 1 to 4,
The magnetic sensor unit;
A magnetic field measuring apparatus comprising: The magnetic field measurement apparatus according to claim 5, wherein
The magnetic sensor includes a magnetoresistive element that exhibits a change in resistance that monotonously changes with respect to a magnetic field. The magnetic field measuring apparatus according to claim 6.
The magnetic field measuring device, wherein the magnetoresistive element is a GMR element. In the magnetic field measurement apparatus according to any one of claims 5 to 7,
The magnetic sensor is constituted by a bridge circuit.

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