Signal processing circuit and sensor unit

Abstract

The technical problem of the present invention is to provide a signal processing circuit capable of suppressing power consumption even when the amplitude range of an output signal of a bridge resistance type sensor becomes large. The signal processing circuit is provided with: a first element and a second element which are connected in series between a first DC power supply and a ground, and which vary in respective resistance values according to a change in a physical quantity of an observation target; an operational amplifier having an inverting input terminal connected to an intermediate point between the first element and the second element, a non-inverting input terminal connected to a second DC power source, and an output terminal; and a feedback resistor connected between the output terminal and the inverting input terminal. The operational amplifier of the signal processing circuit connected in such a way acts negative feedback control so that the input voltage input to the inverting input terminal is equal to the voltage of the second DC power supply input to the non-inverting input terminal, and therefore, the amplitude range of the voltages input to both input terminals is suppressed to an extremely small range, and the output voltage of the second DC power supply is suppressed to an extremely small range. Consumed power can be greatly suppressed.

Description

Technical Field

[0001] The present invention relates to a signal processing circuit and a sensor unit. Background Art

[0002] There is known a bridge resistor type sensor in which a plurality of resistor elements such as MR (Magneto Resistive) elements for detecting the magnitude of a magnetic field are arranged and bridged. As a signal processing circuit for the bridge resistor type sensor, there is known a circuit that connects an output signal thereof to a non-inverting input terminal having a high input resistance among input terminals of an operational amplifier to obtain an amplified voltage signal (for example, refer to Patent Document 1).

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2014-89087 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] In the case of a signal processing circuit that connects an output signal of a bridge resistor type sensor to a non-inverting input terminal of an operational amplifier, if the amplitude range (AC voltage component + DC voltage component) that the output signal can obtain becomes large, there is a problem that the power required to control the noise level within a certain range increases.

[0008] The present invention has been made to solve such problems, and provides a signal processing circuit that can suppress power consumption even when the amplitude range of an output signal of a bridge resistor type sensor becomes large.

[0009] Technical Means for Solving the Problems

[0010] The signal processing circuit according to the first aspect of the present invention includes: a first element and a second element that are connected in series with each other between a first DC power supply and ground and whose respective resistance values change according to a change in a physical quantity of an observation object; an operational amplifier having an inverting input terminal connected to a midpoint between the first element and the second element, a non-inverting input terminal connected to a second DC power supply, and an output terminal; and a feedback resistor connected between the output terminal and the inverting input terminal.

[0011] The operational amplifier of the signal processing circuit connected in this way performs negative feedback control in such a manner that even if the output voltage output from the midpoint between the first element and the second element fluctuates, the input voltage input to the inverting input terminal is equal to the voltage of the second DC power supply input to the non-inverting input terminal. As a result, since the amplitude range of the voltages input to the two input terminals is suppressed within an extremely small range, the power consumption of the signal processing circuit can be significantly suppressed.

[0012] In the above-described signal processing circuit, preferably, the minimum value of the input resistance value to the inverting input terminal determined by the respective resistance values of the first element and the second element is greater than the resistance value of the feedback resistor. When this relationship holds, the followability of the output voltage output from the output terminal of the operational amplifier with respect to the change in the output voltage output from the midpoint between the first element and the second element is good.

[0013] Further, in the above-described signal processing circuit, it may also be configured by two sets of first elements and second elements that form a full-bridge circuit as a whole, two operational amplifiers, and two feedback resistors, and the non-inverting input terminals of the two operational amplifiers are both connected to a common second DC power supply. In this way, also in a signal processing circuit in which four resistance elements are connected in a so-called full-bridge connection, the power consumption of the signal processing circuit can be significantly suppressed in the same manner as in a signal processing circuit in a half-bridge connection. Thus, whether it is a signal processing circuit in which two resistance elements are connected in a half-bridge connection or a signal processing circuit in which four resistance elements are connected in a full-bridge connection, it is more practical in the case of a resistance element, particularly a magnetic sensor represented by an MR element.

[0014] Further, in the above-described signal processing circuit, the operational amplifier used may be a cascode operational amplifier. Originally, since the cascode operational amplifier has a small allowable amplitude range of the input voltage, it is difficult to use it for the amplitude range of the output voltage envisioned in the present invention. However, in the present invention, since the amplitude range of the voltages input to the input terminals is suppressed within an extremely small range, a cascode operational amplifier can be used, and its low power consumption characteristics can be enjoyed.

[0015] In addition, the signal processing circuit according to the second aspect of the present invention includes: a full-bridge circuit formed integrally by a combination of a first element and a second element and a combination of a third element and a fourth element, the first element and the second element being connected in series with each other between a first DC power supply and ground and each having a resistance value that varies according to a change in the physical quantity of the observation object, the third element and the fourth element being connected in series with each other between the first DC power supply and ground and each having a resistance value that varies according to the change in the physical quantity; a differential operational amplifier having an inverting input terminal connected to the midpoint between the first element and the second element, a non-inverting input terminal connected to the midpoint between the third element and the fourth element, a first output terminal, and a second output terminal; a first feedback resistor connected between the first output terminal and the inverting input terminal; and a second feedback resistor connected between the second output terminal and the non-inverting input terminal.

[0016] Thus, even in a signal processing circuit in which the output terminals of the full-bridge circuit are respectively connected to the inverting input terminal and the non-inverting input terminal of the differential operational amplifier, the power consumption of the signal processing circuit can be significantly suppressed.

[0017] In addition, the sensor unit according to the third aspect of the present invention includes the above-described signal processing circuit. If the above-described signal processing circuit is unitized as a sensor unit, it is easy to assemble into various devices and utilized.

[0018] Advantages of the Invention

[0019] According to the present invention, it is possible to provide a signal processing circuit that can suppress power consumption even when the amplitude range of the output signal of a bridge resistance type sensor becomes large. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 FIG. is an example showing a usage mode of the magnetic sensor unit according to the present embodiment.

[0021] Figure 2 FIG. is a circuit diagram of the signal processing circuit according to the first embodiment of the present embodiment.

[0022] Figure 3 FIG. is a diagram showing the circuit structure of a telescopic type operational amplifier.

[0023] Figure 4 FIG. is a circuit diagram of the signal processing circuit according to the second embodiment of the present embodiment.

[0024] Figure 5 FIG. is a circuit diagram of the signal processing circuit according to the third embodiment of the present embodiment.

[0025] DESCRIPTION OF REFERENCE NUMERALS

[0026] 10…Magnetic sensor unit, 20…Sensor head, 30…Control unit, 40…Cable, 90…Busbar, 101…First element, 102…Second element, 103…Third element, 104…Fourth element, 111…First operational amplifier, 112…Second operational amplifier, 113…Differential operational amplifier, 121…First feedback resistor, 122…Second feedback resistor Detailed implementation mode

[0027] Hereinafter, the present invention will be described by way of embodiments of the invention. However, the invention covered by the scope of the claims is not limited to the following embodiments. In addition, not all the structures described in the embodiments are necessarily means for solving the problems.

[0028] Figure 1 FIG. 9 is a diagram showing an example of the usage mode of the magnetic sensor unit 10 of the present embodiment. The magnetic sensor unit 10 mainly includes a sensor head 20 having a signal processing circuit described later, a control unit 30 that controls the sensor head 20 and processes a detection signal output from the sensor head 20, and a cable 40 that connects the sensor head 20 and the control unit 30.

[0029] The sensor head 20 is provided, for example, within the range of a magnetic field Mf generated by a current Ig flowing through a busbar 90, and outputs a detection signal corresponding to the intensity of the magnetic field Mf. The control unit 30 receives the detection signal from the sensor head 20 via the cable 40, and performs, for example, AD conversion and sends it to an external device such as a current analysis device. Since the intensity of the magnetic field Mf changes according to the current Ig, the magnetic sensor unit 10 can be used as a current sensor for detecting the current Ig flowing through the busbar 90. In addition, in the present embodiment, the sensor head 20 and the control unit 30 are separately configured, but they may also be integrally configured.

[0030] Figure 2 FIG. 16 is a circuit diagram of the signal processing circuit of the first embodiment of the present embodiment. The signal processing circuit constitutes the sensor head 20 and mainly includes: a first element 101, a second element 102, a first operational amplifier 111, and a first feedback resistor 121.

[0031] The first element 101 and the second element 102 are, respectively, magnetoresistive effect elements (MR elements) of, for example, a spin valve type, whose resistance values change according to the change in the physical quantity of the observation object, i.e., magnetism. The first element 101 and the second element 102 are connected in series with each other between a first DC power supply and ground to form a half-bridge circuit. The first DC power supply supplies a constant supply voltage V SUP . The first element 101 and the second element 102 are arranged such that their magnetic detection axes face opposite to each other, and the resistance value R of the first element 101BP1 and the resistance value R of the second element 102 BP2 vary complementarily with each other according to the applied magnetism. Therefore, the voltage at the midpoint between the first element 101 and the second element 102 varies corresponding to the intensity of the applied magnetism. That is, the first element 101 and the second element 102 constituting a half-bridge circuit function as a magnetic sensor that outputs a voltage signal corresponding to the intensity of the magnetism applied at the midpoint therebetween. In addition, the "midpoint" means a point on the connection line connecting the first element 101 and the second element 102.

[0032] The first operational amplifier 111 has: an inverting input terminal, a non-inverting input terminal, and an output terminal. The inverting input terminal is connected to the midpoint between the first element 101 and the second element 102. The non-inverting input terminal is connected to a second DC power supply. The second DC power supply supplies a constant supply voltage V to the non-inverting input terminal. REF The first feedback resistor 121 is connected between the output terminal and the inverting input terminal of the first operational amplifier 111 and is a fixed resistor having a constant resistance value R2.

[0033] If the resistance value at the midpoint between the first element 101 and the second element 102, that is, the bridge output resistance value that becomes the input resistance value to the inverting input terminal, is set as R1, the closed-loop gain of the inverting input terminal is -R2 / R1. At this time, the first signal voltage V input to the inverting input terminal IP1 , according to the characteristics of the first operational amplifier 111 as an operational amplifier, acts negative feedback control in such a way as to be equal to the supply voltage V REF supplied from the second DC power supply to the non-inverting input terminal. As a result, the first output voltage V OP1 output from the output terminal of the first operational amplifier 111 becomes a voltage signal that varies corresponding to the intensity of the magnetism applied to the first element 101 and the second element 102.

[0034] In addition, the lowest value R of the varying bridge output resistance value R1 1MIN is preferably greater than the resistance value R2 of the first feedback resistor 121. When this relationship holds, the followability of the first output voltage V OP1 output from the output terminal of the first operational amplifier 111 with respect to the change in the output voltage output from the midpoint between the first element 101 and the second element 102 is good.

[0035] In addition, by making the first signal voltage V IP1 input to the inverting input terminal equal to the supply voltage V as a constant voltage REFIn the feedback control in an equal manner, the amplitude range of the voltages input to the two input terminals of the operational amplifier is suppressed within an extremely small range. In a signal processing circuit having such characteristics, it is preferable to use a cascode operational amplifier as the first operational amplifier 111.

[0036] Figure 3 FIG. is a diagram showing the circuit configuration of the cascode operational amplifier. In the cascode operational amplifier, since the difference between V DD and V in is small, the allowable amplitude range of V in is restricted to be smaller than that of other types of operational amplifiers. On the other hand, the current path between V DD and the ground is fewer than that of other types of operational amplifiers, and the power consumption for operating the operational amplifier can be suppressed.

[0037] As described above, in the signal processing circuit of the first embodiment, the amplitude range of the voltages at the two input terminals is suppressed within an extremely small range. Therefore, the disadvantages related to the amplitude range of the cascode operational amplifier can be ignored. Thus, if a cascode operational amplifier is used as the first operational amplifier 111, the advantage of low power consumption can be enjoyed without any particular inconvenience.

[0038] Next, the signal processing circuit of the second embodiment of the present embodiment will be described. The signal processing circuit of the first embodiment has a structure in which a magnetic sensor configured as a half-bridge circuit is combined with an inverting single-ended amplifier, but the signal processing circuit of the second embodiment has a structure in which a magnetic sensor configured as a full-bridge circuit is combined with a differential inverting amplifier. Figure 4 FIG. is a circuit diagram of the signal processing circuit of the second embodiment of the present embodiment. Since the signal processing circuit of the second embodiment is in a relationship including the signal processing circuit of the first embodiment, the same reference numerals are assigned to the common elements, and the description thereof is omitted except in special cases.

[0039] The signal processing circuit, similarly to the signal processing circuit of the first embodiment, forms a sensor head 20 and mainly includes: a first element 101, a second element 102, a third element 103, a fourth element 104, a first operational amplifier 111, a second operational amplifier 112, a first feedback resistor 121, and a second feedback resistor 122.

[0040] The third element 103 and the fourth element 104 are also magnetoresistive effect elements (MR elements) of, for example, a spin valve type, whose resistance values change according to magnetic changes, like the first element 101 and the second element 102. The first element 101 and the second element 102 are connected in series with each other between the first DC power supply and the ground, and the third element 103 and the fourth element 104 are also connected in series with each other between the first DC power supply and the ground, jointly forming a full-bridge circuit. The first DC power supply supplies a constant supply voltage V to the full-bridge circuit SUP .

[0041] The third element 103 and the fourth element 104 are arranged such that their magnetic detection axes face in opposite directions to each other, and the third element 103 and the second element 102 are arranged in the same direction, and the fourth element 104 and the first element 101 are arranged in the same direction. With the resistance value R of the first element 101 BP1 and the resistance value R of the second element 102 BP2 changing complementarily to each other according to the applied magnetism, the resistance value R of the third element 103 BN1 and the resistance value R of the fourth element 104 BN2 also change complementarily to each other according to the applied magnetism. Therefore, similar to the voltage at the midpoint between the first element 101 and the second element 102 changing corresponding to the intensity of the applied magnetism, the voltage at the midpoint between the third element 103 and the fourth element 104 also changes corresponding to the intensity of the applied magnetism. Moreover, since the orientations of the magnetic detection axes of the respective elements are adjusted and arranged as described above, the voltages at the respective midpoints change in such a way that if one rises, the other decreases. That is, the first element 101 to the fourth element 104 constituting the full-bridge circuit function as a magnetic sensor that outputs anti-correlated voltage signals according to the intensity of the applied magnetism at two midpoints respectively.

[0042] The second operational amplifier 112 has the same structure as the first operational amplifier 111, and in addition, the connection relationship with other elements is the same as that of the first operational amplifier 111. Specifically, the second operational amplifier 112 has: an inverting input terminal, a non-inverting input terminal, and an output terminal. The inverting input terminal is connected to the midpoint between the third element 103 and the fourth element 104. The non-inverting input terminal is connected to the second DC power supply. That is, the non-inverting input terminals of the two operational amplifiers are both connected to the common second DC power supply. The second feedback resistor 122 is connected between the output terminal and the inverting input terminal of the second operational amplifier 112 and is a fixed resistor having a constant resistance value R4.

[0043] If the resistance value at the midpoint of the third element 103 and the fourth element 104, that is, the bridge output resistance value that becomes the input resistance value to the inverting input terminal, is set as R3, the closed-loop gain of the inverting input terminal is -R3 / R4. At this time, for the second signal voltage V input to the inverting input terminal IN1 , according to the characteristics of the second operational amplifier 112 as an operational amplifier, negative feedback control is applied in such a way as to be equal to the supply voltage V supplied from the second DC power supply to the non-inverting input terminal REF . As a result, the second output voltage V output from the output terminal of the second operational amplifier 112 ON1 becomes a voltage signal that varies corresponding to the intensity of the magnetism applied to the third element 103 and the fourth element 104. In addition, the lowest value R of the varying bridge output resistance value R3 3MIN is preferably greater than the resistance value R4 of the second feedback resistor 122. In addition, it is preferable that the resistance value R2 of the first feedback resistor 121 is equal to the resistance value R4 of the second feedback resistor 122. In addition, it is preferable that both the first operational amplifier 111 and the second operational amplifier 112 are cascode operational amplifiers.

[0044] Furthermore, the signal processing circuit of the third embodiment of the present embodiment will be described. The signal processing circuit of the second embodiment has a structure in which the outputs of a full-bridge circuit are received by two operational amplifiers, but the signal processing circuit of the third embodiment has a structure in which the output of a magnetic sensor configured as a full-bridge circuit is received by one differential operational amplifier and differentially output from the two output terminals. Figure 5 is the circuit diagram of the signal processing circuit of the third embodiment of the present embodiment. Since the signal processing circuit of the third embodiment is in a relationship of partially including the signal processing circuit of the first embodiment, the same reference numerals are assigned to the common elements, and the description thereof is omitted except in special cases.

[0045] Similar to the signal processing circuit of the first embodiment, the signal processing circuit of the third embodiment forms a sensing head 20 and mainly includes: a first element 101, a second element 102, a third element 103, a fourth element 104, a differential operational amplifier 113, a first feedback resistor 121, and a second feedback resistor 122.

[0046] The third element 103 and the fourth element 104 are also, like the first element 101 and the second element 102, magnetoresistive effect elements (MR elements), such as spin valve type, whose resistance values change according to the change in magnetism. The first element 101 and the second element 102 are connected in series with each other between the first DC power supply and the ground, and the third element 103 and the fourth element 104 are also connected in series with each other between the first DC power supply and the ground, jointly forming a full-bridge circuit. The first DC power supply supplies a constant supply voltage V SUP to the full-bridge circuit.

[0047] The third element 103 and the fourth element 104 are configured such that their respective magnetic detection axes face in opposite directions, and the third element 103 and the second element 102 are configured to face in the same direction, and the fourth element 104 and the first element 101 are configured to face in the same direction. With the resistance value R of the first element 101 BP1 and the resistance value R of the second element 102 BP2 changing complementarily with each other according to the applied magnetic field, the resistance value R of the third element 103 BN1 and the resistance value R of the fourth element 104 BN2 also change complementarily with each other according to the applied magnetic field. Therefore, similar to the voltage at the midpoint between the first element 101 and the second element 102 changing corresponding to the intensity of the applied magnetism, the voltage at the midpoint between the third element 103 and the fourth element 104 also changes corresponding to the intensity of the applied magnetism. Moreover, since the orientations of the magnetic detection axes of the respective elements are adjusted and configured as described above, the voltages at the respective midpoints change in such a way that if one increases, the other decreases. That is, the first element 101 to the fourth element 104 constituting the full-bridge circuit function as a magnetic sensor that outputs anti-correlated voltage signals at two midpoints respectively according to the intensity of the applied magnetic field.

[0048] The differential operational amplifier 113 includes: a first output terminal and a second output terminal that differentially output an input signal. The midpoint between the first element 101 and the second element 102 is connected to the inverting input terminal, and the first feedback resistor 121 is connected between the first output terminal and the inverting input terminal. The first feedback resistor 121 is a fixed resistor having a constant resistance value R2. In addition, the midpoint between the third element 103 and the fourth element 104 is connected to the non-inverting input terminal, and the second feedback resistor 122 is connected between the second output terminal and the non-inverting input terminal. The second feedback resistor 122 is a fixed resistor having a constant resistance value R4.

[0049] If the resistance value at the midpoint between the first element 101 and the second element 102, that is, the bridge output resistance value that becomes the input resistance value to the inverting input terminal, is set as R1, the closed-loop gain of the inverting input terminal is -R2 / R1. In addition, if the resistance value at the midpoint between the third element 103 and the fourth element 104, that is, the bridge output resistance value that becomes the input resistance value to the non-inverting input terminal, is set as R3, the closed-loop gain of the non-inverting input terminal is R3 / R4. At this time, according to the characteristics of the differential operational amplifier 113 as an operational amplifier, negative feedback control is applied in such a way that the first signal voltage V input to the inverting input terminal IP1 and the second signal voltage V input to the non-inverting input terminal IN1 are equal to each other. As a result, V output from the first output terminal of the differential operational amplifier 113OP1 and the second output voltage V output from the second output terminal ON1 respectively become voltage signals corresponding to the difference between the first signal voltage V IP1 and the second signal voltage V IN1 in a differential manner.

[0050] In addition, the minimum value R of the variable bridge output resistance value R1 1MIN is preferably greater than the resistance value R2 of the first feedback resistor 121. Similarly, the minimum value R of the variable bridge output resistance value R3 3MIN is preferably greater than the resistance value R4 of the second feedback resistor 122. In addition, it is preferable that the resistance value R2 of the first feedback resistor 121 is equal to the resistance value R4 of the second feedback resistor 122. In addition, the differential operational amplifier 113 preferably uses a cascode operational amplifier.

[0051] In the present embodiment described above, it is assumed that the magnetic sensor unit 10 is used as a current sensor for detecting the current Ig flowing through the bus bar 90, but the magnetic sensor using the signal processing circuit of the first to third embodiments is not limited to being used as a current sensor. In addition to current sensors, angle sensors, azimuth sensors (compasses), position sensors, etc. can also be used. In particular, it is useful for sensors that require low power consumption.

[0052] In addition, in the present embodiment described above, since a magnetic sensor is used as an application example, the resistance element is a magnetoresistive element, but the resistance element incorporated into the signal processing circuit may be any element whose resistance value changes according to the change in the physical quantity of the observation object. In this case, the signal processing circuit functions as a sensor for detecting the physical quantity of the observation object.

Claims

1. A signal processing circuit, wherein: It includes: A first element and a second element, which are connected in series with each other between a first DC power supply and ground, and their respective resistance values change according to the change in the physical quantity of the object to be observed; An operational amplifier having an inverting input terminal connected to the midpoint of the first element and the second element, a non-inverting input terminal connected to a second DC power supply, and an output terminal; And A feedback resistor connected between the output terminal and the inverting input terminal.

2. The signal processing circuit according to claim 1, wherein: The minimum value of the input resistance value to the inverting input terminal determined by the respective resistance values of the first element and the second element is greater than the resistance value of the feedback resistor.

3. The signal processing circuit according to claim 1, wherein: It is composed of two sets of the first elements and the second elements that form a full-bridge circuit as a whole, Two of the operational amplifiers, and Two of the feedback resistors. The non-inverting input terminals of the two operational amplifiers are both connected to the common second DC power supply.

4. The signal processing circuit according to claim 1, wherein: The first element and the second element are respectively magnetic sensors.

5. The signal processing circuit according to claim 1, wherein: The operational amplifier is a cascode operational amplifier.

6. A signal processing circuit, wherein: It includes: A full-bridge circuit formed as a whole by a combination of a first element and a second element and a combination of a third element and a fourth element. The first element and the second element are connected in series with each other between a first DC power supply and ground, and their respective resistance values change according to the change in the physical quantity of the object to be observed. The third element and the fourth element are connected in series with each other between the first DC power supply and the ground, and their respective resistance values change according to the change in the physical quantity; A differential operational amplifier having an inverting input terminal connected to the midpoint of the first element and the second element, a non-inverting input terminal connected to the midpoint of the third element and the fourth element, a first output terminal, and a second output terminal; A first feedback resistor connected between the first output terminal and the inverting input terminal; And A second feedback resistor connected between the second output terminal and the non-inverting input terminal.

7. The signal processing circuit according to claim 6, wherein: The first element to the fourth element are respectively magnetic sensors.

8. The signal processing circuit according to claim 6, wherein: The differential operational amplifier is a cascode operational amplifier.

9. A sensor unit, wherein: It includes the signal processing circuit according to any one of claims 1 to 8.

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