Abstract:
A magnetometer provides a newly designed signal processing circuit that can reduce noise and increase the magnetic sensitivity by means of reducing the noise of GND electrode potential occurred at the timing when the pulse current passes through the magnetic wire. It is characterized by preparing a differential element with a neutral coil electrode, a differential sample holding circuit and a differential amplifier circuit.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Japanese Patent Application No. 2015-246972 filed on Dec. 18, 2015, entitled “Magnetometer with a Differential Type Integrated Circuit,” which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    Technical Field 
         [0003]    The present invention relates to a magnetometer with a differential-type integrated circuit that can improve its performance in sensitivity, noise, size and power consumption. 
         [0004]    Background Art 
         [0005]    High sensitive micro-magnetometers have been used to detect bio-magnetism, the earth magnetism and so on. Nowadays, they are widely used for mobiles phones, smartphones, etc. MI sensors and GSR sensors operated by high frequency pulse current are well known as common types of those micro-magnetometers. They comprise a magnetic sensitive part made by an amorphous wire composed of CoFeSiB and a coil wound around its magnetic wire. 
         [0006]    The MI sensor is based on a magneto-impedance effect caused by a surface effect of high frequency current from 1 MHz to 100 MHz when it passes through the amorphous wire with a circular anisotropic magnetic field on the surface. There are two types of MI sensors. One directly detects the change of wire impedance proportional to an external magnetic field. The other detects it indirectly using the coil wound around the wire. As used herein, MI is an abbreviation for magneto-impedance. 
         [0007]    The GSR sensor is based on the GSR effect caused by spin rotations excited by a current pulse with frequency of from 0.5 GHz to 4 GHz when it passes through the amorphous wire with a circular spin aliment domain on the surface. The micro coil wound around the wire can detect the coil voltage proportional to the external magnetic field caused by the GSR effect. As used herein, GSR is an abbreviation for GHz-spin rotation. 
         [0008]    The coil voltages of MI sensors and GSR sensors depend on the current strength of pulse current. Saturation tends to occur, offering stable output when the current strength is large. As an example, a current strength of over 200 mA causes saturation applied to the amorphous wires with the anisotropy field of 20 Gauss and the diameter of 10 μm. Another example is that the current strength of over 400 mA causes saturation applied to the amorphous wires with the anisotropy field of 20 Gauss and the diameter of 20 μm. 
         [0009]    The pulse current with proper frequency and proper current strength makes a large coil voltage that can improve the magnetic sensitivity of the magnetometer. But when it passes through the wire, large variations of a grand electrode potential (referred to as “GND” hereinafter) occur to make large noise in the electronics circuit for the sensor. 
         [0010]    An integrated electronic circuit (referred to as “ASIC” hereinafter) is more sensitive to this problem than a discrete electronic circuit assembled from separate electronic parts. The reason is that an ASIC receives interference from the GND of the wire and the GND of the coil more strongly than that of the discrete circuit and the GND of the circuit fluctuates to make large noise in the circuit. 
         [0011]    Prior-art inventions for solving the problem are, for example, those described in Patent Document 1 and Patent Document 2. Patent Document 1 relates to an impedance-type MI sensor that measures the wire impedance change excited by pulse current with a frequency of from 1 MHz to 100 MHz. The invention in Patent Document 1 detects a differential voltage with two MI elements with a right turn bias coil and a left turn bias coil that make the same but opposite voltages sensitive to the external magnetic field. The differential voltage is detected by a differential sample hold circuit and by a differential amplified circuit. When the pulse current passes through the wire, the difference between two wire GND potentials of the two MI elements become so large that the noise caused by GND fluctuations becomes not so small. 
         [0012]    Patent Document 2 relates to a coil type MI sensor that measures the coil voltage proportional to the wire impedance change indirectly excited by pulse current with a frequency of from 1 MHz to 100 MHz. The invention in Patent Document 2 detects the coil voltage by separating three GNDs of the wire GND, the coil GND and the processing circuit GND to decrease the coil GND fluctuations caused by the big wire GND fluctuations when the pulse current passes through the wire. At the same time, the invention described in this document decreases noise with the same phase using a differential amplified circuit. The GND of the coil and the GND of the differential amplified circuit are connected directly, but the noise of both GND is not exactly the same so that the invention described in this document cannot decease the noise so effectively especially when the GND receives large fluctuations excited by large pulse current. 
         [0013]    Recently, there is a growing demand for highly sensitive micro-magnetometers for improvement in the sensitivity, noise, size, current consumption, etc. Efforts for improving their performance are being made by some means such as increasing the pulse frequency of from 10 MHz to 1 GHz, making small size coils with a coil pitch from 30 μm to 3 μm, decreasing the wire diameter from 40 μm to 10 μm with the ASIC process rules changed from 350 nm to 130 nm. The noise is apt to become larger due to a higher frequency of the pulse current, a smaller size of the pickup coil and finer process rules applied when a large pulse current passes. 
       Patent Documents 
       [0014]    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-365349 
         [0015]    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2008-134236 
       BRIEF SUMMARY OF THE INVENTION 
     Technical Problems 
       [0016]    The object of the present invention is to improve the performance of magnetometers in sensitivity, noise, size, power consumption, etc., by developing a newly designed electronic circuit that can remove undesirable effect on the noise caused by big GND electrical potential fluctuates when large pulse current passes through the wire of a MI sensor or a GSR sensor. 
       Means to Solve Technical Problems 
       [0017]    The inventor carried out numerous experiments for solving the problem and then created a newly designed circuit that can remove undesirable effect on the noise to signal ratio caused by electrode potential fluctuations of the wire GND, the coil GND, the sample hold circuit GND and the amplified circuit GND. The newly designed circuit comprises a differential element with one wire and two coils divided by a middle coil point with two wire electrodes, two coil electrodes and a neutral coil electrode, a differential sample hold circuit and a differential amplifier or a differential ADC. It can obtain the difference coil voltage between the voltages of two coils without any undesirable effect from the large GND fluctuations at the moment of pulse current passage. 
         [0018]    Both coil voltages of the differential coils show the same value with opposite signs against the neutral coil electrode serving as the reference potential. The voltage values are a little different due to the GND fluctuations. However, when both coil voltages are held in the differential sample hold circuit and successively transferred to the differential amplifier or the differential ADC to output the sensor signal, undesirable effect on the signal can be almost perfectly eliminated from the signal to provide an exact output voltage proportional to the external magnetic field. 
         [0019]    The electrode potential of the neutral coil electrode fluctuates significantly because of the pulse current passage and as a result, both coil voltages of the differential element are also varied. Nevertheless the difference of both coil voltages, in other words, the total voltage of the differential coil is kept constant free from undesirable effect caused by GND fluctuations. 
       Effect of the Invention 
       [0020]    The present magnetometer with a differential-type integrated circuit can detect the coil voltage proportional to the external magnetic field free from the bad influence to the signal voltage caused by the fluctuations of GND electrical potential at the moment of the pulse current passage. Therefore, it can improve its signal-noise ratio, resulting in an excellent high sensitive micro-magnetometer having high magnetic sensitivity, low noise, small size and small power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  shows a circuit block with a differential amplifier circuit related to Example 1. 
           [0022]      FIG. 2  shows a circuit block with a differential ADC related to Example 2. 
           [0023]      FIG. 3  shows a circuit block with a buffer circuit related to Example 3. 
           [0024]      FIG. 4  shows a circuit block related to a current MI sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    A magnetometer with a differential-type integrated circuit according to the present invention comprises a differential element with a magnetic wire sensitive to a magnetic field, a coil wound around its magnetic wire to pick up the change of the wire longitudinal magnetizing and five electrodes that consists of two wire electrodes, two coil electrodes and a neutral coil electrode, a pulse generator circuit that supplies the pulse current to the magnetic wire, an adjustment circuit for detection timing, a differential sample holding circuit that holds two coil output voltage with the same absolute voltage value of opposite signs against a reference electrode potential set up by the neutral coil electrode, a differential amplifier circuit that amplifies the difference voltage between two sample holding coil output voltages and a neutral potential stabilizing circuit for keeping the voltage of the neutral coil electrode around a middle value of an electronics circuit power source. 
         [0026]    When the coil voltage is considerably large, a differential ADC can be used instead of the differential amplifier circuit. Moreover, in the case that the pickup coil has a resistance of over 100Ω, it is desirable that two coil voltage of the differential element is inputted to the differential sample hold circuit through a buffer circuit. 
         [0027]    The coil of the differential element is divided to a plus coil and a minus coil at its center point that connects to the neutral coil electrode. Both pickup coil voltages detected between the output electrodes and the neutral coil electrode have the same voltage value with opposite signs. 
         [0028]    The differential element has a magnetic wire composed of CoFeSiB, a wire length of 0.2 mm, a diameter of 10 μm and a wire resistance of 4Ω, and the pickup coil with 25 coil turns and a coil resistance of 65Ω. 
         [0029]    The electrode potential of the neutral coil electrode  114  is set to be a half of the potential of the electronics circuit power source  15  to make the output voltage of the differential amplifier circuit zero. It is adjusted by the neutral potential stabilizing circuit to make the output voltage of the differential amplifier circuit zero when the external magnetic field strength is zero. 
         [0030]    The pulse generator  12  generates a pulse current to supply it to the magnetic wire and can control the rising time or the falling time of the pulse current to 0.5 nsec, that is, a pulse frequency of 1 GHz. It can also control the pulse current strength to 200 mA. The power source voltage is preferably kept between 1 V and 15 V for supplying the pulse current with a current strength of from 10 mA to 500 mA. The power source voltage is 2 V for supplying the pulse current with a current strength of more than 200 mA. 
         [0031]    Two electronic switches  131   a,    131   b  turn off at the moment when the coil output voltages reach the peak voltage. The peak voltage is held in the holding capacitors  132   a,    132   b.  The adjustment circuit  16  for detection timing can control the detection time range from 0.2 nsec to 5 nsec according to the delayed time of the peak coil voltage from the switch off timing. 
         [0032]    The capacity of the holding capacitors is 4 pF and the average number of times of the final output signal is 16 times to suppress the noise. 
         [0033]    The electrode potential of the neutral coil electrode  114  fluctuates significantly at the moment of pulse current passage but the difference between the two voltages of the two sample hold circuits  13  composing two electronics switch  131   a,    131   b  and two capacitors  132   a,    132   b  and the differential amplifier circuit  14  amplifies the difference coil voltage between the plus coil  115  and minus coil  116  without any undesirable effect from large GND fluctuations at the moment of pulse current passage and outputs it as the sensor signal. The variation of the output voltage decreases from 5 mV to less than 1 mV. 
         [0034]    Compared with a conventional GSR sensor having a single type GSR element processed by a single circuit shown in  FIG. 4 , Example 1 having the differential-type GSR element processing the differential circuit, can achieve 4 times better σ-noise from 2 mG to 0.5 mG than that of the conventional GSR sensor. 
       EXAMPLE 2 
       [0035]    Example 2 relates to a magnetometer using a GSR sensor applicable to an electronic compass and its circuit block is shown in  FIG. 2 . 
         [0036]    The magnetometer  2  of Example 2 comprises a differential element  11  with a magnetic wire sensitive to a magnetic field, a pulse generator circuit  12  for supplying a pulse current to the magnetic wire; an adjustment circuit  16  for detection timing, a differential sample holding circuit  13  for holding two coil output voltages with the absolute voltage value of opposite signs against a reference electrode potential set up by the neutral coil electrode  114 , a differential ADC  24   14  for amplifying the difference voltage between two sample holding coil output voltages, and a potential stabilizing circuit  15  for keeping the voltage of the neutral coil electrode around a middle value of an electronics circuit power source. 
         [0037]    Example 2 is characterized by a differential ADC instead of the differential amplifier circuit of Example 1. 
         [0038]    The differential element has a magnetic wire  110  sensitive to the magnetic field, a coil wound around its magnetic wire  113   a,    113   b  to pick up the change of the wire longitudinal magnetizing and five electrodes that consists of two wire electrodes  111 ,  112 , two coil electrodes  115 ,  116  and a neutral coil electrode  114 . 
         [0039]    The coil of the differential element  11  is divided into a plus coil  113   a  and a minus coil  113   b  at its center point which connects to the neutral coil electrode  114 . Both pickup coil voltages  115 ,  116  detected between the output electrode and the neutral coil electrode  114  have the same voltage value with the opposite signs. 
         [0040]    The differential element has a magnetic wire composed of CoFeSiB, the anisotropic field of 30 G, a wire length of 0.2 mm, a diameter of 10 μm and a wire resistance of 4Ω and the pickup coils  113   a,    113   b  with 42 coil turns and a coil resistances of 80Ω respectively. 
         [0041]    The electric potential of the neutral coil electrode  114  is set to make the output voltage of the differential ADC zero by the electronics circuit power source  15  when the external magnetic field strength is zero. 
         [0042]    The pulse generator  12  generates a pulse current to supply it to the magnetic wire with a wire length of 0.2 mm and a wire resistance of 4Ω. It can control the rising time or the falling time of the pulse current to 0.5 nsec, that is, a pulse frequency of 1 GHz. It can also control the pulse current strength to 200 mA. The power source voltage is 2V which can supply a pulse current with a current strength of more than 200 mA. 
         [0043]    Two electronic switches  131   a,    131   b  turn off at the moment when the coil output voltage reaches the peak voltage. The peak voltage is held in the holding capacitors  132   a,    132   b.  The adjustment circuit  16  for detection timing can control the detection time range from 0.2 nsec to 5 nsec according to the delayed time of the peak coil voltage from the switch off timing. 
         [0044]    The capacity of the holding capacitors  132   a,    132   b  capacitors is 4 pF and the average number of times of the final output signal is 16 times to suppress the noise. 
         [0045]    The electrode potential of the neutral coil electrode  114  fluctuates significantly at the moment of pulse current passage, but the difference between the two voltages of the two sample hold circuits  13  composing two electronics switch  131   a,    131   b  and two capacitors  132   a,    132   b  and the differential ADC  24  amplifies the difference coil voltage between the plus coil  115  and minus coil  116  without any undesirable effect from large GND fluctuations at the moment of pulse current passage and outputs it as the sensor signal. The variation of the output voltage decreases from 5 mV to less than 1 mV. 
         [0046]    Compared with a conventional GSR sensor having a single type GSR element processed by a single circuit shown in  FIG. 4 , Example 2 having the differential-type GSR element processing the differential circuit, can achieve 4 times better σ-noise from 2 mG to 0.5 mG than that of the conventional GSR sensor. 
       EXAMPLE 3 
       [0047]    Example 3 relates to a magnetometer using GSR sensor for detecting the magnetism of a living body and its circuit block is shown in  FIG. 3 . 
         [0048]    The magnetometer  3  of Example 3 comprises a differential element  11  that has a magnetic wire sensitive to a magnetic field, a pulse generator circuit  12  for supplying a pulse current to the magnetic wire, an adjustment circuit  16  for detection timing, a buffer circuit before a differential sample holding circuit  13  for holding two coil output voltages with the same absolute voltage value of opposite signs against a reference electrode potential set up by the neutral coil electrode  114 , a differential amplifier circuit  14  for amplifying the difference voltage between two sample holding coil output voltages, and a potential stabilizing circuit  15  for keeping the voltage of the neutral coil electrode around a middle value of an electronics circuit power source. 
         [0049]    The differential element has a magnetic wire  110  sensitive to the magnetic field, a coil wound around its magnetic wire  113   a,    113   b  to pick up the change of the wire longitudinal magnetizing and five electrodes that consists of two wire electrodes  111 ,  112 , two coil electrodes  115 ,  116  and a neutral coil electrode  114 . 
         [0050]    The coil of the differential element  11  is divided into a plus coil  113   a  and a minus coil  113   b  at its center point which connects to the neutral coil electrode  114 . Both pickup coil voltages  115 ,  116  detected between the output electrodes and the neutral coil electrode  114  have the same voltage value with the opposite signs. 
         [0051]    The differential element has a magnetic wire composed of CoFeSiB, an anisotropic field of 30 G, a wire length of 2 mm, a diameter of 10 μm and a wire resistance of 20Ω and the pickup coils  113   a,    113   b  with 600 coil turns and a coil resistances of 1 KΩ respectively. 
         [0052]    The electrode potential of the neutral coil electrode  114  is set to make the output voltage of the differential ADC zero by the electronics circuit power source  34  when the external magnetic field strength is zero. 
         [0053]    The pulse generator  12  generates a pulse current to supply it to the magnetic wire with a wire length of 2 mm and a wire resistance of 20Ω. It can control the rising time or the falling time of the pulse current to 0.5 nsec, that is, a pulse frequency of 1 Ghz. It can also control the pulse current strength to 200 mA. The power source voltage is 5V which can supply the pulse current with a current strength of 200 mA. 
         [0054]    When the coil resistance is 1 KΩ accompanied with a large number of coil turns, a buffer circuit  31  must be set between the differential element  11  and the differential sample hold circuit for suppressing IR voltage drop of the coil voltage caused by the coil current and the coil resistance. The buffer circuit used in this example is disclosed in Japanese Patent No. 5678358. 
         [0055]    Two electronic switches  131   a,    131   b  turn off at the moment when the coil output voltage reaches the peak voltage. The peak voltage is held in the holding capacitors  132   a,    132   b.  The adjustment circuit  16  for detection timing can control the detection timing time within a range of from 0.2 nsec to 5 nsec according to the delayed time of the peak coil voltage from the switch off timing. 
         [0056]    The capacity of the holding capacitors  132   a,    132   b  is 20 pF and the average number of times of the final output signal is 16 times to suppress the noise. 
         [0057]    The electrode potential of the neutral coil electrode  114  fluctuates significantly at the moment of pulse current passage but the difference between the two voltages of the two sample hold circuits  13  composing two electronics switch  131   a,    131   b  and two capacitors  132   a,    132   b  and it is amplified in the differential amplifier circuit  32  and transferred to ADC  33  subsequently converted to output digital signal 
         [0058]    The digital output signal is free from any undesirable effect from large GND fluctuations at the moment of pulse current passage and outputs it as the sensor signal. The variation of the output voltage decreases from 5 mV to less than 1 mV at the moment when the pulse passes through the wire. 
         [0059]    The comparison with a conventional GSR sensor having single type GSR element processed by single circuit shown in  FIG. 4  and Example 3 having the differential-type GSR element processing the differential circuit shows that Example 3 can decrease the noise density at 1 Hz from 10 pT to 4 pT. 
       INDUSTRIAL APPLICABILITY 
       [0060]    The present invention makes improvements in the sensitivity, the noise, size, power consumption, etc., and contributions to increase the accuracy of electronic compasses used for smartphones, tablets, car navigation, digital cameras,  3 -dimensional mice and so on. It also can detect the magnetism of a living body and make contributions to develop a wearable-type MCG and MEG systems. 
       LIST OF REFERENCE NUMERALS 
       [0061]      1 : circuit of Example  1 , 
         [0062]      11 : differential element,  110 : magnetic wire,  111 : wire input electrode,  112 : wire GND,  113   a,    113   b:  pickup coil,  114 : neutral coil electrode,  115 : plus coil electrode,  116 : minus coil electrode 
         [0063]      12 : pulse generator circuit 
         [0064]      13 : sample holding circuit,  131   a,    131   b:  electronics switch,  132   a,    132   b    
         [0065]      14 : differential amplifier,  15 : potential stabilizing circuit,  16 : an adjustment circuit,  17 : 
         [0066]    VDD power source, 
         [0067]      18 : GND,  181 : pulse circuit GND,  182 : signal processing circuit GND 
         [0068]      2 : circuit of Example  2 ,  24 : differential ADC, 
         [0069]      3 : circuit of Example  3 ,  31 : buffer circuit  32 : differential amplifier circuit, 
         [0070]      33 : ADC circuit 34 : a potential stabilizing circuit, 
         [0071]      4 : current circuit of Example  4 , 
         [0072]      41 : element,  410 : magnetic wire sensitive,  411 : pickup coil,  42 : pulse generator circuit, 
         [0073]      43 : sample holding circuit,  44 : differential amplifier,  47 : VDD power source, 
         [0074]      48 : GND 
         [0075]      481 : pulse circuit GND,  482 : signal processing circuit GND.