Signal processing circuit and sensor unit

Abstract

To provide a signal processing circuit for restraining power consumption even if an amplitude range of an output signal of a bridge resistance type sensor becomes large.SOLUTION: A signal processing circuit has: a first element and a second element that are connected in series between a first DC power supply and the ground while each resistance value varies according to a change in a physical quantity of an observation object; an operational amplifier having an inverted input terminal connected to a middle point between the first element and the second element, a non-inverted input terminal connected to a second DC power supply, and an output terminal; and a feedback resistor connected between the output terminal and the inverted input terminal. In the operational amplifier of a signal processing circuit connected in this manner, negative feedback control operates such that an input voltage inputted to the inverted input terminal becomes equal to the voltage of the second DC power supply inputted to the non-inverted input terminal, such that the amplitude range of voltage inputted to both the input terminals is suppressed to an extremely small range, thus greatly suppressing consumed power.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

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

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the case of a signal processing circuit that connects the output signal of a bridge resistance type sensor to the non-inverting input terminal of an operational amplifier, when the amplitude range (AC voltage component + DC voltage component) that the output signal can take becomes large, there is a problem that the power required to keep the noise level within a certain range increases.

[0005] The present invention has been made to solve such a problem, and provides a signal processing circuit in which power consumption is suppressed even when the amplitude range of the output signal of a bridge resistance type sensor becomes large.

Means for Solving the Problems

[0006] In the signal processing circuit according to the first aspect of the present invention, a first element and a second element are connected in series between a first DC power supply and ground, and each resistance value varies according to a change in the physical quantity to be observed, and 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 supply, and an operational amplifier having an output terminal, and a feedback resistor connected between the output terminal and the inverting input terminal.

[0007] In the operational amplifier of the signal processing circuit connected in this way, even if the output voltage output from the intermediate point between the first element and the second element fluctuates, negative feedback control acts so that the input voltage input to the inverting input terminal becomes equal to the voltage of the second DC power supply input to the non-inverting input terminal. As a result, the amplitude range of the voltages input to both input terminals is suppressed to a very small range, so that the power consumed by the signal processing circuit can be significantly suppressed.

[0008] In the above signal processing circuit, it is desirable that 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 larger than the resistance value of the feedback resistor. When such a 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 intermediate point between the first element and the second element is good.

[0009] Further, the above signal processing circuit is composed of two sets of a first element and a second element 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 may both be connected to a common second DC power supply. Also in the signal processing circuit in which the four resistance elements are connected as a so-called full bridge, the power consumed by the signal processing circuit can be significantly suppressed in the same manner as in the signal processing circuit connected as a half bridge. Thus, both the signal processing circuit in which two resistance elements are connected as a half bridge and the signal processing circuit in which four resistance elements are connected as a full bridge are more practical when the resistance element is a magnetic sensor typified by an MR element.

[0010] Also, the operational amplifier employed in the above signal processing circuit is preferably a telescopic operational amplifier. Originally, since the telescopic operational amplifier has a small allowable input voltage amplitude range, it was difficult to adopt it for the output voltage amplitude range assumed by the present invention. However, in the present invention, since the amplitude range of the voltage input to the input terminal is suppressed to an extremely small range, it becomes possible to adopt a telescopic operational amplifier, and its low power consumption characteristics can be enjoyed.

[0011] Also, the signal processing circuit in the second aspect of the present invention includes a combination of 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 vary in response to a change in the physical quantity of the observation target, and a combination of a third element and a fourth element that are connected in series with each other between the first DC power supply and ground and whose respective resistance values vary in response to a change in the physical quantity. It includes a full-bridge circuit formed by the whole, 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.

[0012] Even in a signal processing circuit in which the output terminals of the full-bridge circuit are connected to the inverting input terminal and the non-inverting input terminal of the differential operational amplifier in this way, the power consumed by the signal processing circuit can be significantly suppressed.

[0013] Also, the sensor unit in the third aspect of the present invention includes the above signal processing circuit. By unitizing the above signal processing circuit as a sensor unit, it becomes easier to incorporate it into various devices and use it.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a signal processing circuit in which power consumption is suppressed even when the amplitude range of the output signal of the bridge resistance type sensor becomes large.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Also, not all of the configurations described in the embodiments are essential as means for solving the problems.

[0017] FIG. 1 is a diagram showing an example of the usage mode of the magnetic sensor unit 10 according to the present embodiment. The magnetic sensor unit 10 mainly includes a sensor head 20 including 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.

[0018] The sensor head 20 is installed, for example, within the range of the magnetic field Mf generated by the current Ig flowing through the bus bar 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, performs, for example, AD conversion, and transmits 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 bus bar 90. Note that, in the present embodiment, the sensor head 20 and the control unit 30 are configured as separate bodies, but they may be integrally configured.

[0019] FIG. 2 is a circuit diagram of a signal processing circuit according to 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.

[0020] Each of the first element 101 and the second element 102 is, for example, a spin valve type magnetoresistive effect element (MR element) whose resistance value varies according to a change in magnetism, which is a physical quantity to be observed. 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 to form a half-bridge circuit. The first DC power supply supplies a constant supply voltage V SUP to the half-bridge circuit. The first element 101 and the second element 102 are arranged such that their respective magnetic detection axes face opposite to each other, and the resistance value R BP1 of the first element 101 and the resistance value R BP2 of the second element 102 vary complementarily to 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 configured as a half-bridge circuit function as a magnetic sensor that outputs a voltage signal corresponding to the intensity of the applied magnetism at the midpoint thereof. Note that the "midpoint" means a point on the connection line connecting the first element 101 and the second element 102.

[0021] 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 REF to the non-inverting input terminal. 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.

[0022] Assuming that 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 R1, the closed-loop gain at the inverting input terminal is -R2 / R1. At this time, the first signal voltage V IP1 input to the inverting input terminal, due to the characteristics of the operational amplifier of the first operational amplifier 111, negative feedback control acts so 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.

[0023] Note that the minimum value R 1MIN of the varying bridge output resistance value R1 is preferably larger than the resistance value R2 of the first feedback resistor 121. When such a 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.

[0024] Also, by performing feedback control so that the first signal voltage V IP1 input to the inverting input terminal is equal to the supply voltage V REF which is a constant voltage, the amplitude range of the voltages input to both input terminals of the operational amplifier is suppressed to an extremely small range. In a signal processing circuit having such characteristics, it is preferable to employ a telescopic type operational amplifier as the first operational amplifier 111.

[0025] FIG. 3 is a diagram showing the circuit configuration of a telescopic type operational amplifier. Since the difference between V DD and V in is small, the allowable amplitude range of V in is limited to a small value with respect to other types of operational amplifiers. On the other hand, the current path between V DD and the ground is smaller than that of other types of operational amplifiers, and the power consumption for operating the operational amplifier can be suppressed.

[0026] As described above, in the signal processing circuit according to the first embodiment, since the amplitude range of the voltages input to both input terminals is suppressed to an extremely small range, the demerit regarding the amplitude range of the telescopic type operational amplifier can be ignored. Therefore, if a telescopic type operational amplifier is adopted as the first operational amplifier 111, the merit of low power consumption can be enjoyed without any particular inconvenience.

[0027] Next, a signal processing circuit according to a second embodiment of the present embodiment will be described. The signal processing circuit according to the first embodiment was configured by combining a magnetic sensor configured as a half-bridge circuit with an inverting single-ended amplifier, but the signal processing circuit according to the second embodiment adopts a configuration in which a magnetic sensor configured as a full-bridge circuit is combined with an inverting differential amplifier. FIG. 4 is a circuit diagram of the signal processing circuit according to the second embodiment of the present embodiment. Since the signal processing circuit according to the second embodiment includes the signal processing circuit according to the first embodiment, the same reference numerals are given to the common elements, and the description thereof is omitted unless otherwise particularly mentioned.

[0028] The signal processing circuit configures the sensor head 20 in the same manner as the signal processing circuit of the first embodiment, 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.

[0029] 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) of, for example, a spin valve type, whose resistance values vary in response to a 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, together constituting a full-bridge circuit. The first DC power supply supplies a constant supply voltage V SUP to the full-bridge circuit.

[0030] The third element 103 and the fourth element 104 are arranged such that their respective magnetic detection axes are opposite to each other, and the third element 103 and the second element 102 are in the same direction, and the fourth element 104 and the first element 101 are in the same direction. The resistance value R BP1 of the first element 101 and the resistance value R BP2 of the second element 102 vary complementarily with each other according to the applied magnetism. Similarly, the resistance value R BN1 of the third element 103 and the resistance value R BN2 of the fourth element 104 also vary complementarily with 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 varying corresponding to the intensity of the applied magnetism, the voltage at the midpoint between the third element 103 and the fourth element 104 also varies corresponding to the intensity of the applied magnetism. And since the direction of the magnetic detection axis of each element is adjusted and arranged as described above, the voltage at each midpoint varies such that if one increases, the other decreases. That is, the first element 101 to the fourth element 104 configured as a full-bridge circuit function as a magnetic sensor that outputs an anti-correlated voltage signal at each of the two midpoints corresponding to the intensity of the applied magnetism.

[0031] The second operational amplifier 112 has the same configuration as the first operational amplifier 111, and its 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 with a certain resistance value R4.

[0032] Assuming that the resistance value at the midpoint between the third element 103 and the fourth element 104, that is, the bridge output resistance value which becomes the input resistance value to the inverting input terminal, is R3, the closed-loop gain at the inverting input terminal is -R3 / R4. At this time, the second signal voltage V IN1 input to the inverting input terminal, due to the characteristics of the second operational amplifier 112 as an operational amplifier, negative feedback control acts so that it becomes equal to the supply voltage V REF supplied from the second DC power supply to the non-inverting input terminal. As a result, the second output voltage V ON1 output from the output terminal of the second operational amplifier 112 becomes a voltage signal that varies corresponding to the intensity of the magnetism applied to the third element 103 and the fourth element 104. Note that the minimum value R 3MIN of the varying bridge output resistance value R3 is preferably larger than the resistance value R4 of the second feedback resistor 122. Also, the resistance value R2 of the first feedback resistor 121 and the resistance value R4 of the second feedback resistor 122 are preferably equal. Further, it is preferable that both the first operational amplifier 111 and the second operational amplifier 112 adopt telescopic-type operational amplifiers.

[0033] Next, a signal processing circuit according to the third embodiment of the present embodiment will be described. The signal processing circuit according to the second embodiment is configured to receive the output of the full-bridge circuit with two operational amplifiers, while the signal processing circuit according to the third embodiment adopts a configuration in which the output of a magnetic sensor configured as a full-bridge circuit is received by one differential operational amplifier and differential output is performed from its two output terminals. FIG. 5 is a circuit diagram of the signal processing circuit according to the third embodiment of the present embodiment. Since the signal processing circuit according to the third embodiment partially includes the signal processing circuit according to the first embodiment, the same reference numerals are assigned to the common elements, and the description thereof is omitted unless otherwise particularly mentioned.

[0034] The signal processing circuit according to the third embodiment configures the sensor head 20 in the same manner as the signal processing circuit of the first embodiment, 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.

[0035] The third element 103 and the fourth element 104 are also, for example, spin valve type magnetoresistive effect elements (MR elements) whose resistance values vary in response to a change in magnetism in the same manner as 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 to form a full-bridge circuit together. The first DC power supply supplies a constant supply voltage V SUP to the full-bridge circuit.

[0036] The third element 103 and the fourth element 104 are arranged such that their respective magnetic detection axes are opposite to each other, and the third element 103 and the second element 102 are in the same direction, and the fourth element 104 and the first element 101 are in the same direction. The resistance value R BP1 of the first element 101 and the resistance value R BP2 of the second element 102 vary complementarily with each other according to the applied magnetism, so that the resistance value R BN1 of the third element 103 and the resistance value R BN2They also vary complementarily with 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 varying according to the intensity of the applied magnetism, the voltage at the midpoint between the third element 103 and the fourth element 104 also varies according to the intensity of the applied magnetism. And since the orientation of the magnetic detection axis of each element is adjusted and arranged as described above, the voltages at the respective midpoints vary such that when one increases, the other decreases. That is, the first element 101 to the fourth element 104 configured as a full-bridge circuit function as a magnetic sensor that outputs an anti-correlated voltage signal according to the intensity of the applied magnetism at each of the two midpoints.

[0037] The differential amplifier 113 includes a first output terminal and a second output terminal that perform differential output with respect to the 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. Also, 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.

[0038] Assuming that 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 R1, the closed-loop gain at the inverting input terminal is -R2 / R1. Also, assuming that 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 R3, the closed-loop gain at the non-inverting input terminal is R3 / R4. At this time, due to the characteristics of the differential amplifier 113 as an amplifier, the first signal voltage V IP1 input to the inverting input terminal and the second signal voltage V IN1 input to the non-inverting input terminal are subject to negative feedback control so as to be equal to each other. As a result, V OP1 output from the first output terminal of the differential amplifier 113 and the second output voltage V output from the second output terminalON1 becomes a voltage signal corresponding to the difference between the first signal voltage V IP1 and the second signal voltage V IN1 respectively.

[0039] Note that the minimum value R of the fluctuating 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 fluctuating bridge output resistance value R3 3MIN is preferably greater than the resistance value R4 of the second feedback resistor 122. Also, the resistance value R2 of the first feedback resistor 121 and the resistance value R4 of the second feedback resistor 122 are preferably equal. Further, the differential operational amplifier 113 preferably employs a telescopic type operational amplifier.

[0040] In the present embodiment described above, the case where the magnetic sensor unit 10 is used as a current sensor for detecting the current Ig flowing through the bus bar 90 is assumed. However, the magnetic sensors employing the signal processing circuits of the first to third embodiments are not limited to being used as current sensors. In addition to current sensors, they can also be employed in angular sensors, azimuth sensors (compasses), position sensors, etc. In particular, they are useful for sensors that require low power consumption.

[0041] Also, in the present embodiment described above, since a magnetic sensor is described as an application example, the resistance element was a magnetoresistive effect element. However, the resistance element incorporated in the signal processing circuit may be any element whose resistance value varies in accordance with a change in the physical quantity to be observed. In that case, the signal processing circuit functions as a sensor for detecting the physical quantity of the observation target.

Description of Reference Numerals

[0042] 10... Magnetic sensor unit, 20... Sensor head, 30... Control unit, 40... Cable, 90... Bus bar, 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

Claims

1. A first element and a second element, which are connected in series with each other between a first DC power supply and ground and each have a resistance value that varies in response to a change in a physical quantity to be observed; 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 supply, and an output terminal; A feedback resistor connected between the output terminal and the inverting input terminal And a signal processing circuit comprising.

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

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

4. The signal processing circuit according to claim 1, wherein each of the first element and the second element is a magnetic sensor.

5. The signal processing circuit according to claim 1, wherein the operational amplifier is a telescopic type operational amplifier.

6. A combination of a first element and a second element, which are connected in series with each other between a first DC power supply and ground and each have a resistance value that varies in response to a change in a physical quantity to be observed, and a combination of a third element and a fourth element, which are connected in series with each other between the first DC power supply and the ground and each have a resistance value that varies in response to the change in the physical quantity, and a full-bridge circuit formed by the whole; A differential 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 an intermediate point 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; A second feedback resistor connected between the second output terminal and the non-inverting input terminal And a signal processing circuit comprising.

7. The signal processing circuit according to claim 6, wherein each of the first element to the fourth element is a magnetic sensor.

8. The signal processing circuit according to claim 6, wherein the differential operational amplifier is a telescopic type operational amplifier.

9. A sensor unit comprising the signal processing circuit according to any one of claims 1 to 8.

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