JP2021085713A - Magnetic field detection device - Google Patents

Abstract

To provide a magnetic field detection device which has improved followability to a change of a magnetic field and thereby allowing for accurate detection of the magnetic field.SOLUTION: A magnetic field detection device is provided, comprising: a magnetic field detection unit; an amplifier amplifying the detection signal output from the magnetic field detection unit; an offset voltage generation unit generating an offset voltage applied to the amplifier; a D/A converter (DAC) applying A/D conversion to the amplified signal output from the amplifier and outputting a digital signal; and a control unit. The control unit acquires the digital signal after setting the gain of the amplifier to a first gain, and sets the gain of the amplifier to a second gain higher than the first gain while controlling the generation of the offset voltage by the offset voltage generation unit on the basis of the acquired digital signal.SELECTED DRAWING: Figure 1

Description

The present invention relates to a magnetic field detector.

Conventionally, as a magnetic field detection device, for example, a device using a Hall element (Hall type) and a device using a magnetoresistive effect element (MR type) are known (see, for example, Patent Document 1). Further, those using MI (magnetic impedance) effect elements are also known.

Japanese Unexamined Patent Publication No. 2017-181240

Here, the conventional magnetic field detection device may be used, for example, to detect a magnetic field generated by a current flowing through a conductor. When the current is an alternating current, the change in the current causes a change in the magnetic field. Therefore, the magnetic field detection device needs to detect the magnetic field by following the change in the magnetic field.

In view of the above situation, it is an object of the present invention to provide a magnetic field detection device capable of accurately detecting a magnetic field by improving the ability to follow changes in the magnetic field.

The magnetic field detection device according to one aspect of the present invention in order to achieve the above object
Magnetic field detector and
An amplifier that amplifies the detection signal output from the magnetic field detection unit, and
An offset voltage generator that generates an offset voltage to be applied to the amplifier,
A DAC (D / A converter) that A / D-converts the amplified signal output from the amplifier and outputs a digital signal,
Control unit and
With
The control unit
After setting the gain of the amplifier to the first gain, the digital signal is acquired, and the digital signal is acquired.
While controlling the generation of the offset voltage by the offset voltage generation unit based on the acquired digital signal, the gain of the amplifier is set to a second gain larger than the first gain (first). Constitution).

In the first configuration, the measurement window set by the first gain may include the entire measurable range of the magnetic field detection unit (second configuration).

Further, in the first or second configuration, the magnetic field detection unit includes an MI (magnetic impedance) effect element, a pickup coil wound around the MI effect element, and a pulse-shaped drive current around the MI effect element. Includes a pulse drive unit, and has an MI sensor
The control unit may set the first gain and then cause the pulse drive unit to generate the first pulse, and after setting the second gain, cause the pulse drive unit to generate the second pulse. (Third configuration).

Further, in the third configuration, the amplifier is a variable capacitor connected between an operational amplifier having a non-inverting input terminal to which the offset voltage is applied, and an output end of the operational amplifier and an inverting input terminal of the operational amplifier. And, including
The magnetic field detection unit includes a capacitor connected between both ends of the pickup coil, a first switch arranged between one end of the pickup coil and one end of the capacitor, the other end of the pickup coil, and the capacitor. Including a second switch and a third switch arranged between the other end of the
The other end of the capacitor is connected to the ground end and
One end of the capacitor may be connected to the inverting input end via the third switch (fourth configuration).

Further, in the fourth configuration, the magnetic field detection unit may include a short-circuit switch connected between both ends of the capacitor (fifth configuration).

Further, in the fourth or fifth configuration, the amplifier may include a discharge switch connected between both ends of the variable capacitor (sixth configuration).

Further, in any of the first to sixth configurations, the offset voltage generation unit may be a DAC (D / A converter) (seventh configuration).

Further, in any of the first to seventh configurations, the magnetic field detection unit may be one of the magnetic field detection units that detect magnetic fields in each axial direction of at least two orthogonal axes. (8th configuration).

Another aspect of the present invention is a portable device including the magnetic field detection device having the eighth configuration.

Further, another aspect of the present invention is a current sensor including a magnetic field detection device having any of the above-mentioned first to eighth configurations and a substrate on which a wiring through which an alternating current flows is formed.

According to the magnetic field detection device of the present invention, it is possible to improve the ability to follow changes in the magnetic field and accurately detect the magnetic field.

It is a block diagram which shows the whole structure of the magnetic field detection apparatus which concerns on an exemplary embodiment of this invention. It is a circuit diagram which shows the structure which outputs the magnetic field detection result which concerns on an exemplary embodiment of this invention. It is a waveform figure which shows an example of a drive current and an induced voltage. It is a figure which shows the 1st process for outputting a magnetic field detection result. It is a figure which shows the 2nd process for outputting a magnetic field detection result. It is a figure which shows the 3rd process for outputting a magnetic field detection result. It is a figure which shows the measurement window which concerns on a comparative example. It is a figure which shows the measurement window which concerns on a comparative example. It is a flowchart about the magnetic field detection operation which concerns on a comparative example. It is a figure which shows the 1st measurement window which concerns on an exemplary embodiment of this invention. It is a figure which shows the 2nd measurement window which concerns on the exemplary embodiment of this invention. It is a flowchart about the magnetic field detection operation which concerns on an exemplary embodiment of this invention. It is a waveform figure which shows the drive current which concerns on an exemplary embodiment of this invention. It is a block diagram which shows the structure of the smartphone which concerns on an exemplary embodiment of this invention. It is a top view which shows the current sensor which concerns on the exemplary embodiment of this invention. It is a waveform figure which shows an example of the magnetic field generated by the alternating current flowing through the current sensor.

An exemplary embodiment of the present invention will be described below with reference to the drawings.

<1. Overall configuration of magnetic field detector>
FIG. 1 is a block diagram showing an overall configuration of a magnetic field detection device according to an exemplary embodiment of the present invention. The magnetic field detection device 10 shown in FIG. 1 is configured as a geomagnetic sensor.

The magnetic field detection device 10 includes an X-axis MI (magnetic impedance) sensor 1, a Y-axis MI sensor 2, a Z-axis MI sensor 3, a sample / hold circuit 4, a PGA (variable gain amplifier) 5, and the like. It is a semiconductor device including an offset voltage generation unit 6, an ADC (A / D converter) 7, a serial interface 8, and a logic unit 9, and these components are integrated on one chip.

The X-axis MI sensor 1 detects a magnetic field in the X-axis direction among the X-axis, Y-axis, and Z-axis that are orthogonal to each other. The Y-axis MI sensor 2 detects a magnetic field in the Y-axis direction. The Z-axis MI sensor 3 detects a magnetic field in the Z-axis direction. The magnetic field detection is performed using the MI effect element in all of the X-axis MI sensor 1, the Y-axis MI sensor 2, and the Z-axis MI sensor 3.

The X-axis MI sensor 1 and the Y-axis MI sensor 2 detect a magnetic field in the horizontal direction. Since the tilt of the magnetic field detection device 10 from the horizontal can be detected from the magnetic field detection result by the Z-axis MI sensor 3, the detection results of the X-axis MI sensor 1 and the Y-axis MI sensor 2 are obtained based on the tilt detection result. By correcting it, a more accurate azimuth can be detected. For example, the angle of the X-axis from magnetic north can be detected.

The sample / hold circuit 4 includes a sensor signal S1 as a magnetic field detection result output from the X-axis MI sensor 1, a sensor signal S2 as a magnetic field detection result output from the Y-axis MI sensor 2, and a Z-axis MI. Each of the sensor signals S3 as the magnetic field detection result output from the sensor 3 is sampled / held. The sample / hold circuit 4 generates a sample / hold signal SH as a result of the sample / hold.

The PGA 5 amplifies the sample / hold signal SH with a predetermined gain while offsetting the sample / hold signal SH by the offset voltage Vof generated by the offset voltage generation unit 6. The predetermined gain is variable. PGA5 generates an amplified signal SA as a result of amplification.

The ADC 7 converts the amplified signal SA into A / D (analog / digital) to generate a digital signal SD. The serial interface 8 converts the digital signal SD into the serial signal SS and outputs the digital signal SD to the outside of the magnetic field detection device 10.

The logic unit 9 is a control unit that controls each unit of the magnetic field detection device 10. More specifically, the logic unit 9 is, for example, drive control of each of the X-axis MI sensor 1, the Y-axis MI sensor 2, and the Z-axis MI sensor 3, the operation control of the sample / hold circuit 4, and the PGA5. Gain adjustment and offset voltage Vof adjustment can be performed.

In addition to the configuration shown in FIG. 1, the magnetic field detection device 10 includes, for example, a regulator that generates an internal voltage Vreg.

<2. Magnetic field detection result output configuration>
Next, the configuration for outputting the magnetic field detection result composed of the MI sensor, the sample / hold circuit 4, and the PGA 5 will be described in detail with reference to FIG. In FIG. 2, for convenience, only the configuration corresponding to the X-axis MI sensor 1 is typically shown as the configuration of the MI sensor and the sample / hold circuit 4. Further, in FIG. 2, the magnetic field detection unit is configured from the X-axis MI sensor 1 and the configuration included in the sample / hold circuit 4.

As shown in FIG. 2, the X-axis MI sensor 1 includes an MI effect element 1A, a pickup coil 1B, and a pulse drive unit 1C. The MI effect element 1A is configured as, for example, an amorphous wire. The pickup coil 1B is configured to be wound around the MI effect element 1A, and outputs an induced voltage generated between both ends of the pickup coil 1B as a sensor signal S1. The pulse drive unit 1C supplies the pulse-shaped drive current Ip to the MI effect element 1A in response to a command from the logic unit 9.

Here, the principle of the magnetic field detection method using the MI effect element 1A will be described. When the MI effect element 1A is not energized, the electron spins in the MI effect element 1A are directed in the circumferential direction. The circumferential direction is a direction around the axial direction of the MI effect element 1A. Then, when an external magnetic field is applied to the non-energized MI effect element 1A, the electron spin is tilted. Here, when the MI effect element 1A is energized, the electron spins are rotated by the magnetic field in the circumferential direction generated by the electric current, and are aligned in one direction in the circumferential direction. An induced voltage generated in the pickup coil 1B is generated in the pickup coil 1B in proportion to the speed of the magnetic vector change in the axial direction of the MI effect element 1A generated during this rotation. After that, when the energization is released, the electron spin changes to a tilted state according to the external magnetic field, and an induced voltage proportional to the speed of the axial magnetic vector change of the MI effect element 1A at that time is generated in the pickup coil 1B. ..

Therefore, as shown in FIG. 3, when the MI effect element 1A is energized by the drive current Ip from the state where it is not energized (rising of the pulse wave), an induced voltage VH corresponding to the external magnetic field is generated in the pickup coil 1B. , Is output as the sensor signal S1. Further, when the energization by the drive current Ip is stopped from the state where the MI effect element 1A is energized (falling edge of the pulse wave), an induced voltage VL having a polarity opposite to the previous induced voltage is generated in the pickup coil 1B. It is output as the sensor signal S1. When the direction of the external magnetic field is reversed, the polarity of the generated induced voltage is also reversed (that is, VH is generated after the induced voltage VL is generated).

The sample / hold circuit 4 has a first switch 41, a second switch 42, a third switch 43, a fourth switch 44, a fifth switch 45, and a capacitor 4A as a configuration corresponding to the X-axis MI sensor 1. Further, the PGA 5 includes an operational amplifier 5A, a variable capacitor 5B, and a discharge switch 5C.

The first switch 41 is arranged between one end of the pickup coil 1B and one end of the capacitor 4A. The second switch 42 is arranged between the other end of the pickup coil 1B and the other end of the capacitor 4A. The other end of the capacitor 4A is connected to the ground end. A sample / hold signal SH is generated as a voltage across the capacitor 4A. The sample / hold signal SH corresponds to the detection signal of the magnetic field detection unit.

A fifth switch 45 is connected between one end and the other end of the capacitor 4A. One end of the capacitor 4A is connected to the inverting input end of the operational amplifier 5A via the third switch 43 by the connection node N1. The other end of the capacitor 4A is connected to the inverting input end of the operational amplifier 5A via the fourth switch 44 by the connection node N1.

The output end of a DAC (D / A converter) 6 as an example of the offset voltage generation unit 6 is connected to the non-inverting input end of the operational amplifier 5A. The DAC 6 D / A (digital / analog) converts the digital command output from the logic unit 9 and applies the offset voltage Vof to the non-inverting input terminal of the operational amplifier 5A.

A variable capacitor 5B is arranged between the output end of the operational amplifier 5A and the connection node N1. A discharge switch 5C is arranged between both ends of the variable capacitor 5B. An amplification signal SA is generated at the connection node N2 to which the output end of the operational amplifier 5A, the variable capacitor 5B, and the discharge switch 5C are connected.

The logic unit 9 controls the on / off of each of the first switch 41 to the fifth switch 45 and the discharge switch 5C. Further, the capacitance of the variable capacitor 5B is variable according to the control signal from the logic unit 9.

Next, the output operation of the magnetic field detection result in the configuration shown in FIG. 2 will be described with reference to FIGS. 4A to 4C.

First, as shown in FIG. 4A, with the first switch 41 and the second switch 42 turned on and the third switch 43 to the fifth switch 45 turned off, the pulse-shaped drive current Ip is generated from the pulse drive unit 1C. It is supplied to the MI effect element 1A. As a result, the capacitor 4A is charged by the sensor signal S1 having the waveform shown in FIG. The charge of the capacitor 4A is zero at the start of charging, and a sample / hold signal SH is generated according to the charging. That is, the sampling operation is performed here.

Further, at this time, the discharge switch 5C is turned on, and both ends of the variable capacitor 5B are short-circuited, so that the electric charge of the variable capacitor 5B is discharged until it becomes zero. That is, the variable capacitor 5B is initialized.

Next, as shown in FIG. 4B, the first switch 41, the fourth switch 44, and the fifth switch 45 are turned off, and the second switch 42 and the second switch 43 are turned on. As a result, the voltage value of the sample / hold signal SH generated in the capacitor 4A is held, and the hold operation is performed. The timing of performing the hold operation may be, for example, the timing of the peak PH of the induced voltage VH shown in FIG. 3 or the timing of the peak PL of the induced voltage VL.

At this time, the discharge switch 5C is turned off, and the electric charge is transferred from the capacitor 4A to the variable capacitor 5B via the third switch 43 turned on. At this time, the variable capacitor 5B is charged by the current based on the offset voltage Vof, the sample / hold signal SH, and the on-resistance of the switch 43 due to the imaginary short circuit between the non-inverting input end and the inverting input end of the operational amplifier 5A.

Then, as shown in FIG. 4C, the first switch 41, the third switch 43, and the fourth switch 44 are turned off, and the second switch 42 and the fifth switch 45 are turned on. As a result, the transfer of electric charge from the capacitor 4A to the variable capacitor 5B is stopped, and the charging of the variable capacitor 5B is stopped. At this time, the amplification signal SA generated in the connection node N2 is output to the subsequent ADC 7 (FIG. 1) as the output of the magnetic field detection result. The amplification signal SA is generated by offsetting the sample / hold signal SH with an offset voltage Vof and amplifying with a gain based on the capacitance of the variable capacitor 5B.

Further, at this time, the fifth switch 45 (short-circuit switch) turned on short-circuits between both ends of the capacitor 4A, and the capacitor 4A is discharged until the charge becomes zero. The electric charge charged in the variable capacitor 5B is discharged by the discharge switch 5C that is turned on during the next operation shown in FIG. 4A.

The configurations of the Y-axis MI sensor 2 and the Z-axis MI sensor 3 are the same as those of the X-axis MI sensor 1 shown in FIG. 2 and the same as the configuration of the sample / hold circuit 4 shown in FIG. The configuration is provided for each of the Y-axis MI sensor 2 and the Z-axis MI sensor 3. Then, the connection node corresponding to the connection node N1 in the configuration of the sample / hold circuit 4 provided for each of the Y-axis MI sensor 2 and the Z-axis MI sensor 3 is connected to the connection node N1. That is, the PGA5 is commonly provided for the three MI sensors. The magnetic field detection result output operation for each of the Y-axis MI sensor 2 and the Z-axis MI sensor 3 is the same as that of the X-axis MI sensor 1.

<3. Magnetic field detection operation according to the comparative example>
Here, for comparison with the present invention, the magnetic field detection operation by the magnetic field detection device according to the comparative example will be described. The magnetic field detection device according to the comparative example has almost the same configuration as that shown in FIGS. 1 and 2, but the gain of PGA5 may be fixed as described later, so that the variable capacitor 5B is a capacitor having a fixed capacitance. Good.

In this comparative example, as shown in FIG. 5A, the amplification signal SA corresponding to a predetermined positive magnetic field amount (+300 μT as an example) on the + side relative to the offset value is the output of the digital signal SD of the ADC 7. The amplified signal SA corresponding to a predetermined negative magnetic field amount (-300 μT as an example) corresponding to a predetermined positive digital value and relatively negative from the offset value is a predetermined negative digital value of the digital signal SD. The gain of PGA5 is set so as to correspond to. The gain is a fixed value, and the capacitance provided in place of the variable capacitor 5B is set based on the capacitance value of the fixed capacitor.

In FIG. 5A, as an example, in the 14-bit mode, the positive digital value is +7200 LSB, the negative digital value is −7200 LSB, and in the 12-bit mode, the positive digital value is +1800 LSB. The negative digital value is -1800LSB.

As a result, in this comparative example, the measurement window is in the range defined by the amount of the positive magnetic field and the amount of the negative magnetic field (± 300 μT in the example of FIG. 5A). In FIG. 5A, the offset value of the measurement window is 0 μT, which corresponds to the offset voltage Vof = 0 V. Then, by adjusting the offset voltage Vof, the measurement window can be shifted to the + side or the − side as shown by the arrow in FIG. 5A.

Here, as shown in FIG. 5A, when there is no disturbance magnetic field in the environment of the magnetic field detection device, the geomagnetism falls within the range of ± 50 μT centered on 0 μT. Therefore, when there is no disturbance magnetic field, the magnetic field to be measured fits in the measurement window even if the offset value of the measurement window is 0 μT.

Further, when the magnetic field detection device is placed in an environment where a disturbance magnetic field exists, the center value deviates from 0 μT according to the disturbance magnetic field, and the magnetic field to be measured exists within a range of ± 50 μT from the center value. When the disturbance magnetic field is relatively small, the deviation of the center value is small, so that the magnetic field to be measured fits in the measurement window even if the offset value of the measurement window is 0 μT. However, when the disturbance magnetic field is relatively large, the deviation of the center value becomes large, so that the magnetic field to be measured may be out of the range of the measurement window.

In the example of FIG. 5A, the MI sensor itself can detect a magnetic field within the range of ± 1200 μT, and when the MI sensor detects a magnetic field smaller than −300 μT, the detected magnetic field is outside the range of the measurement window of FIG. 5A. Therefore, the digital signal SD has a negative digital value (-7200 LSB or -1800 LSB). Further, when the MI sensor detects a magnetic field larger than +300 μT, the detected magnetic field is out of the range of the measurement window of FIG. 5A, so that the digital signal SD becomes a positive digital value (+7200 LSB or +1800 LSB).

In such a case, for example, as shown in FIG. 5B, the magnetic field to be measured can be accommodated in the measurement window by adjusting the measurement window to be shifted. In the example of FIG. 5B, when the magnetic field to be measured is -1000 μT, the magnetic field to be measured can be stored in the measurement window by adjusting the offset value of the measurement window to be -800 μT and shifting the measurement window to the − side. There is.

FIG. 6 is a flowchart showing a magnetic field detection operation in the magnetic field detection device according to such a comparative example.

First, in step S11, the magnetic field detection result by the MI sensor is output as the amplification signal SA by the method shown in FIGS. 4A to 4C described above, and the digital signal SD obtained by A / D converting the amplification signal SA by the ADC 7 is converted into the logic unit 9. Gets.

Next, in step S12, the logic unit 9 determines whether the acquired digital signal SD is equal to or greater than the positive digital value or equal to or less than the negative digital value. That is, it is determined whether the absolute value of the digital signal SD is equal to or more than a predetermined maximum value (7200 LSB or 1800 LSB in the example of FIG. 5A).

When the absolute value of the digital signal SD is equal to or greater than a predetermined maximum value (Y in step S12), the magnetic field to be measured is not contained in the measurement window, so the process proceeds to step S13. Here, the logic unit 9 shifts the measurement window by adjusting the offset voltage Vof by sending a control signal to the DAC6. Here, in step S12, if the digital signal SD has a negative value, the measurement window is shifted to the − side, and if the digital signal SD has a positive value, the measurement window is shifted to the + side.

After step S13, the process returns to step S11, and the logic unit 9 acquires the digital signal SD. If the absolute value of the acquired digital signal SD is not equal to or greater than the predetermined maximum value in step S12 (N in step S12), the magnetic field to be measured is within the measurement window, so the acquired digital signal SD is measured. Confirmed as.

In this way, in the comparative example, even when the disturbance magnetic field affects the magnetic field to be measured, the magnetic field can be accurately detected. However, when the magnetic field to be measured changes at high speed, the offset adjustment of the measurement window cannot follow the change of the magnetic field to be measured, the magnetic field to be measured cannot be stored in the measurement window, and accurate magnetic field detection can be performed. There is no risk. In addition, the load on the logic unit 9 may increase.

<4. Magnetic field detection operation according to the embodiment of the present invention>
As described above, since there is room for improvement in the above comparative example, an embodiment of the present invention will be described below. FIG. 8 is a flowchart showing a magnetic field detection operation according to the embodiment of the present invention. Here, the magnetic field detection operation by the X-axis MI sensor 1 will be typically described, but the magnetic field detection operation is the same for the Y-axis MI sensor 2 and the Z-axis MI sensor 3.

When the process of FIG. 8 is started, first, in step S1, the logic unit 9 sends a control signal to the variable capacitor 5B to adjust the capacitance of the variable capacitor 5B, thereby adjusting the gain of the PGA 5 to a predetermined first. Set to gain. Further, the logic unit 9 sets the offset voltage Vof to 0V by sending a digital command to the DAC6.

Here, as shown in the example of FIG. 7A, the amplification signal SA corresponding to the predetermined positive maximum magnetic field amount (+1200 μT as an example) that can be measured by the X-axis MI sensor 1 is the digital signal SD that is the output of the ADC 7. The amplification signal SA corresponding to the predetermined positive side digital value of the above and corresponding to the predetermined negative side maximum magnetic field amount (-1200 μT as an example) that can be measured by the X-axis MI sensor 1 is the predetermined negative side of the digital signal SD. The first gain is set so as to correspond to a digital value.

In FIG. 7A, as an example, in the 14-bit mode, the positive digital value is +7200 LSB, the negative digital value is −7200 LSB, and in the 12-bit mode, the positive digital value is +1800 LSB. The negative digital value is -1800LSB.

As a result, the first measurement window in the range from the maximum negative magnetic field amount to the maximum positive magnetic field amount can be set, and the entire measurable range by the X-axis MI sensor 1 can be stored in the first measurement window. .. In the example of FIG. 7A, the first measurement window in the range of ± 1200 μT centered on 0 μT can be set.

After step S1, the process proceeds to step S2, the magnetic field detection result by the X-axis MI sensor 1 is output as an amplification signal SA by the method shown in FIGS. 4A to 4C described above, and the amplification signal SA is A / D converted by the ADC 7. The logic unit 9 acquires the digital signal SD. That is, here, in response to a command from the logic unit 9, the pulse drive unit 1C supplies the drive current Ip of the first pulse PS1 shown in FIG. 9 to the MI effect element 1A.

Since the first measurement window that accommodates the entire measurable range of the X-axis MI sensor 1 is set as described above in step S1, any measurement target magnetic field can be used as long as it is within the measurable range. It is detectable. In the example of FIG. 7A, a magnetic field to be measured of −1000 μT is detected.

However, since the first measurement window is set in a relatively wide range, the magnetic field detection accuracy becomes relatively coarse. In the example of FIG. 7A, the magnetic field detection sensitivity is 1200 μT / 7200 LSB in the 14-bit mode and 1200 μT / 1800 LSB in the 12-bit mode.

Next, in step S3, the logic unit 9 adjusts the offset voltage Vof based on the digital signal SD (magnetic field detection result) acquired in step S2, and sets the gain of the PGA 5 to a predetermined second gain. As a result, the second measurement window is set.

As shown in FIG. 7B, the amplification signal SA corresponding to a predetermined positive magnetic field amount (+300 μT as an example), which is relatively + side from the offset value (−800 μT as an example), is the positive side digital of the digital signal SD. The amplified signal SA corresponding to the predetermined negative magnetic field amount (-300 μT as an example) corresponding to the value and relatively negative from the offset value corresponds to the negative digital value of the digital signal SD. The second gain is set. Since the second gain is larger than the first gain, the second measurement window defined by the positive magnetic field amount and the negative magnetic field amount is narrower than the first measurement window. In the example of FIG. 7B, the ± 300 μT second measurement window is narrower than the ± 1200 μT first measurement window shown in FIG. 7A. The offset value of the second measurement window (-800 μT in the example of FIG. 7B) is adjusted by the offset voltage Vof.

Next, in step S4, the magnetic field detection result by the X-axis MI sensor 1 is output as an amplification signal SA by the method shown in FIGS. 4A to 4C described above, and the amplification signal SA is A / D converted by the ADC 7 digital signal. Generate SD. That is, here, in response to a command from the logic unit 9, the pulse drive unit 1C supplies the drive current Ip of the second pulse PS2 shown in FIG. 9 to the MI effect element 1A. Therefore, as shown in FIG. 9, the second pulse PS2 is continuously generated in the first pulse PS1.

Since the offset value of the second measurement window is adjusted in step S3 so as to include the magnetic field detection result (-1000 μT in the example of FIG. 7A) acquired in step S2, the magnetic field detection result (-1000 μT in the example of FIG. 7A) is accurately adjusted in step S4. In the case of the example of FIG. 7B, −1000 μT) can be obtained. At this time, the detection accuracy by the second measurement window is higher than the detection accuracy by the first measurement window. In the example of FIG. 7B, the magnetic field detection sensitivity is 300 μT / 7200 LSB in the 14-bit mode and 300 μT / 1800 LSB in the 12-bit mode. Therefore, a more accurate magnetic field detection result can be obtained.

According to this embodiment, even when the magnetic field change is high speed, the magnetic field is detected with coarse accuracy by the relatively wide first measurement window, and then the second measurement window is relatively narrow based on the detection result. Is set to detect the magnetic field with high accuracy, so that it is possible to improve the followability to the change in the magnetic field and detect the magnetic field accurately.

<5. First application example of magnetic field detection device>
Here, an example in which the magnetic field detection device according to the embodiment of the present invention is applied to a smartphone will be described. A smartphone is an example of a mobile device.

FIG. 10 is a block diagram showing a configuration of a smartphone 15 according to an exemplary embodiment of the present invention. The smartphone 15 includes a communication unit 151, a camera unit 152, a display unit 153, a voice input / output unit 154, a geomagnetic sensor 155, an operation unit 156, a storage unit 157, and a control unit 158.

The communication unit 151 performs wireless communication with a base station in the mobile communication network. Using this wireless communication, voice data, video data, e-mail data and the like are transmitted and received, and Web data and streaming data and the like are received.

The camera unit 152 electronically captures an image using an image sensor such as a CMOS image sensor, and can compress the captured image in, for example, a JPEG format, and store the compressed data in the storage unit 157.

The display unit 153 is composed of, for example, a liquid crystal panel and a backlight, and displays various images. The audio input / output unit 154 includes a microphone that collects the input surrounding sound and converts it into an electric signal, and a speaker that converts the input audio signal into sound and outputs it to the outside.

The geomagnetic sensor 155 is a sensor that detects the geomagnetism for detecting the orientation, and the magnetic field detection device 10 according to the embodiment of the present invention described above can be used.

The operation unit 156 is composed of keys for various operations such as power supply operation and volume operation arranged on the housing of the smartphone 15, and a touch panel arranged on the surface of a liquid crystal panel of the display unit 153, for example.

The storage unit 157 includes control programs and control data for the control unit 158, address data associated with the name and telephone number of the communication partner, transmitted / received e-mail data, Web data downloaded by Web browsing, and downloaded contents. It stores data and temporarily stores streaming data and the like. The storage unit 157 is composed of, for example, a flash memory, a RAM, a ROM, or the like.

The control unit 158 operates according to the control program and control data stored in the storage unit 157, and controls each unit of the smartphone 15 in an integrated manner.

In such a smartphone 15, a permanent magnet is provided in the microphone and the speaker included in the voice input / output unit 154, and a disturbance magnetic field by the permanent magnet is applied to the geomagnetic sensor 155 in addition to the geomagnetism. However, by using the magnetic field detection device 10 that performs the magnetic field detection operation described above as the geomagnetic sensor 155, the magnetic field can be detected accurately immediately.

<6. Second application example of magnetic field detection device>
Next, an example in which the magnetic field detection device according to the embodiment of the present invention is applied to a current sensor will be described. FIG. 11 is a schematic plan view showing the configuration of the current sensor 25 according to the exemplary embodiment of the present invention. In the current sensor 25, the magnetic field detection device 10 is used not as a geomagnetic sensor but for detecting a magnetic field generated by an electric current.

The current sensor 25 shown in FIG. 11 includes a magnetic field detection device 10 according to an embodiment of the present invention and a printed circuit board 20. The printed circuit board 20 has a wiring 201 formed of a conductor (copper foil or the like) inside. The magnetic field detection device 10 is placed on the surface of the printed circuit board 20.

An alternating current Ia flows through the wiring 201. Since the current value of the alternating current Ia changes with time, the amount of the magnetic field generated by the alternating current Ia also changes with time. Further, the direction of the magnetic field generated by the alternating current Ia is switched according to the switching of the polarity of the alternating current Ia. Such a magnetic field generated by the alternating current Ia is applied to the magnetic field detection device 10.

The alternating current Ia generates, for example, a magnetic field as shown in FIG. 12, where there is a period during which the magnetic field exceeds ± 300 μT. In this case, when the magnetic field detection operation according to the comparative example described above is performed, the shift position of the measurement window is searched, but the timings t1, t2, etc. (magnetic field> + 300 μT), timings t3, t4, etc. shown in FIG. The magnetic field <-300 μT) cannot follow the change in the magnetic field, and there is a possibility that an accurate magnetic field cannot be detected.

On the other hand, if the magnetic field detection device 10 that performs the magnetic field detection operation according to the embodiment of the present invention described above is used, the magnetic field can be accurately generated by following the change in the magnetic field at the timings t1 and t2 and the timings t3 and t4. It becomes possible to detect.

<7. Others>
Although the embodiments of the present invention have been described above, the embodiments can be changed in various ways within the scope of the gist of the present invention.

The present invention can be used, for example, in a geomagnetic sensor, a current sensor, or the like.

1 X-axis MI sensor 1A MI effect element 1B Pickup coil 1C Pulse drive unit 2 Y-axis MI sensor 3 Z-axis MI sensor 4 Sample / hold circuit 41 1st switch 42 2nd switch 43 3rd switch 44 4th switch 45 5th switch 4A capacitor 5 PGA
5A Operational amplifier 5B Variable capacitor 5C Discharge switch 6 Offset voltage generator (DAC)
7 ADC
8 Serial interface 9 Logic part 10 Magnetic field detector 15 Smartphone 20 Printed circuit board 25 Current sensor

Claims (10)

Magnetic field detector and
An amplifier that amplifies the detection signal output from the magnetic field detection unit, and
An offset voltage generator that generates an offset voltage to be applied to the amplifier,
A DAC (D / A converter) that A / D-converts the amplified signal output from the amplifier and outputs a digital signal,
Control unit and
With
The control unit
After setting the gain of the amplifier to the first gain, the digital signal is acquired, and the digital signal is acquired.
A magnetic field detection device that sets the gain of the amplifier to a second gain larger than the first gain while controlling the generation of the offset voltage by the offset voltage generation unit based on the acquired digital signal. The magnetic field detection device according to claim 1, wherein the measurement window set by the first gain includes the entire measurable range of the magnetic field detection unit. The magnetic field detection unit is an MI sensor having an MI (magnetic impedance) effect element, a pickup coil wound around the MI effect element, and a pulse drive unit that supplies a pulsed drive current to the MI effect element. Including
The control unit sets the first gain and causes the pulse drive unit to generate a first pulse, and after setting the second gain, causes the pulse drive unit to generate a second pulse. Alternatively, the magnetic field detection device according to claim 2. The amplifier includes an operational amplifier having a non-inverting input end to which the offset voltage is applied, and a variable capacitor connected between the output end of the operational amplifier and the inverting input terminal of the operational amplifier.
The magnetic field detection unit includes a capacitor connected between both ends of the pickup coil, a first switch arranged between one end of the pickup coil and one end of the capacitor, the other end of the pickup coil, and the capacitor. Including a second switch and a third switch arranged between the other end of the
The other end of the capacitor is connected to the ground end and
The magnetic field detection device according to claim 3, wherein one end of the capacitor is connected to the inverting input end via the third switch. The magnetic field detection device according to claim 4, wherein the magnetic field detection unit includes a short-circuit switch connected between both ends of the capacitor. The magnetic field detection device according to claim 4 or 5, wherein the amplifier includes a discharge switch connected between both ends of the variable capacitor. The magnetic field detection device according to any one of claims 1 to 6, wherein the offset voltage generation unit is a DAC (D / A converter). The magnetic field detection device according to any one of claims 1 to 7, wherein the magnetic field detection unit is one of the magnetic field detection units that detect a magnetic field in each axial direction of at least two orthogonal axes. .. A portable device including the magnetic field detection device according to claim 8. A current sensor comprising the magnetic field detection device according to any one of claims 1 to 8 and a substrate on which a wiring through which an alternating current flows flows.

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