JP2015177205A - offset cancellation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cancel an offset voltage, generated due to variation of resistive elements constituting a bridge circuit excellently, while reducing the noise contained in an output signal.SOLUTION: An offset cancellation circuit 9 includes a resistor 15 connected, respectively, between inverted input terminals of an operational amplifier 11 becoming first amplification means, and an operational amplifier 12 becoming second amplification means; and a voltage type DA converter 6 connected across the resistor 15 at its both ends. The voltage type DA converter 6 cancels the offset voltage from the output signals Vop, Von of the operational amplifier 11 becoming first amplification means, and operational amplifier 12 becoming second amplification means, by generating a potential difference depending on an offset voltage, contained in the first and second output signals Vip, Vin outputted from a bridge sensor 3, by dividing a predetermined reference voltage Vref, and applying the voltage across the resistor 15 at its both ends.

Description

  The present invention relates to an offset cancel circuit that cancels an offset voltage of a bridge-type sensor configured by bridge-connecting a plurality of resistance elements.

  Conventionally, a magnetic sensor in which a plurality of GMR elements (giant magnetoresistive elements) are bridge-connected is known (for example, Patent Document 1). In the GMR element, the electric resistance changes as the magnetization direction of the free layer rotates relative to the fixed magnetization direction of the pinned layer according to the external magnetic field. Focusing on this electrical characteristic, the GMR element is a kind of magnetoresistive element. If a bridge circuit is composed of a plurality of GMR elements, an external magnetic field is detected by a change in potential balance at two midpoint nodes of the bridge circuit. can do.

  However, since the resistance values of the magnetoresistive elements vary, even when the external magnetic field is not acting, the potential difference between the two midpoint nodes in the bridge circuit does not become zero, and an offset voltage is generated. . Therefore, when measuring the external magnetic field with the output of the magnetic sensor, it is necessary to cancel the offset voltage.

  FIG. 7 is a diagram showing two different configuration examples as a conventional offset cancel circuit. The offset cancel circuit 91 of FIG. 7A includes a magnetic sensor 80 configured by bridge-connecting four magnetoresistive elements 81, 82, 83, and 84, two operational amplifiers 86 and 87, and three resistors 88 and 89. , 90 is provided between the amplifier unit 85 and two current sources 92, 93. These current sources 92 and 93 cause an offset voltage Voff appearing between the midpoint nodes a and b by flowing preset constant currents I1 and I2 to the two midpoint nodes a and b in the bridge circuit, respectively. Cancel. That is, since the variation in the resistance value of each of the magnetoresistive elements 81, 82, 83, and 84 can be measured in advance, the offset cancel circuit 91 uses the current sources 92, By adjusting the currents I1 and I2 generated at 93, a voltage drop corresponding to the variation of the resistance value is caused in the bridge circuit to cancel the offset voltage Voff.

  On the other hand, the offset cancel circuit 94 shown in FIG. 7B has two current sources 95 for supplying a constant current to the resistor 90 connected between the inverting input terminals of the two operational amplifiers 86 and 87 of the amplifier unit 85. , 96. The offset cancel circuit 94 cancels the offset voltage Voff by causing a current I corresponding to the offset voltage Voff appearing between the two midpoint nodes a and b in the bridge circuit to flow through the resistor 90.

JP 2007-212275 A

  In the meantime, in the case of the offset cancel circuit 91 of FIG. 7A, the internal resistance for generating the constant currents I1 and I2 in the two current sources 92 and 93, and the magnetoresistive elements 81 through which the constant currents I1 and I2 flow. , 82, 83, 84 are different materials. Therefore, the temperature characteristics of the magnetoresistive elements 81, 82, 83, 84 and the temperature characteristics of the internal resistances of the current sources 92, 93 are different from each other, and when the temperature changes, the current I1 generated in the current sources 92, 93 , I2 deviates from an appropriate current value for eliminating variations in resistance values of the magnetoresistive elements 81, 82, 83, 84. Therefore, in the offset cancel circuit 91 of FIG. 7A, it is difficult to appropriately generate a voltage drop corresponding to the variation in the resistance value in an arbitrary temperature range, and in a certain temperature range. However, there is a problem that the offset voltage Voff cannot be canceled satisfactorily.

  On the other hand, in the offset cancel circuit 94 in FIG. 7B, the internal resistance provided for generating the constant current I in the current sources 95 and 96 and the resistance 90 through which the constant current I flows are formed of the same material. Therefore, the temperature characteristics of each other can be made equal. Therefore, the offset cancel circuit 94 in FIG. 7B can always cause a constant voltage drop in the resistor 90 even when the temperature changes, and can cancel the offset voltage Voff satisfactorily. is there.

  On the other hand, in any of the circuit configurations of FIGS. 7A and 7B, current source DA converters are used for the current sources 92, 93, 95, and 96, and a MOS transistor is used to generate a constant current. Since the configured current mirror circuit is included, there is a problem that flicker noise of the MOS transistor is included in the output signals Vop and Von of the amplifier unit 85. In order to reduce flicker noise, it is necessary to increase the area of the MOS transistor, which makes it difficult to reduce the size for mounting on a small information device such as a smartphone.

  Further, all of the current sources 92, 93, 95, and 96 operate at the same voltage Vref as the power supply voltage Vref of the magnetic sensor 80. The power supply voltage Vref is a voltage supplied from the outside. In such a configuration, when the power supply voltage Vref changes, the voltages at the respective midpoint nodes a and b in the magnetic sensor 80 change accordingly, whereas the current sources 92, 93, 95, and 96 output them. Since the currents I1, I2, and I do not change, there is a problem that the output signals Vop and Von from the amplifier unit 85 include noise according to the fluctuation of the power supply voltage Vref.

  The present invention has been made for the purpose of solving the above-described conventional problems, and is not affected by the temperature characteristics of each resistance element constituting the bridge circuit, and is generated by the variation of each resistance element. It is an object of the present invention to provide an offset cancel circuit that can cancel well and reduce noise included in an output signal of an amplifier unit.

  In order to achieve the above object, the present invention firstly cancels an offset voltage included in an output signal of a bridge-type sensor configured by a plurality of resistance elements being bridge-connected and operating based on a predetermined reference voltage. An offset cancel circuit, wherein a first resistor is inserted in a negative feedback path, and a first output means for amplifying a first output signal output from the bridge-type sensor by inputting it to a non-inverting input terminal; A second amplifying means for interpolating a second output signal outputted from the bridge-type sensor to a non-inverting input terminal, and a first amplifying means; A third resistor connected between the inverting input terminal of the means and the inverting input terminal of the second amplifying means, and a voltage type DA converter connected to both ends of the third resistor. The voltage type An A converter divides the predetermined reference voltage to generate a potential difference corresponding to an offset voltage included in the first and second output signals output from the bridge-type sensor, so that the third resistor A configuration in which the offset voltage is canceled from the output signals of the first and second amplifying means by applying to both ends is adopted as the solving means.

  According to the second aspect of the present invention, in the first configuration, the first output resistor connected to one output terminal of the voltage type DA converter and the other output terminal of the voltage type DA converter are connected. And a voltage-type DA converter that applies the potential difference to both ends of the third resistor via the first output resistor and the second output resistor. Is adopted as a solution.

  Thirdly, the present invention adopts a configuration in which the voltage type DA converter is an R2R ladder type DA converter in the first or second configuration as a solving means.

  Furthermore, according to the fourth aspect of the present invention, in the third configuration, the first resistor, the second resistor, the third resistor, and the resistor included in the R2R ladder DA converter are the same. A configuration formed of a material is adopted as a solution.

  According to the present invention, the voltage-type DA converter generates a potential difference corresponding to the offset voltage included in the first and second output signals output from the bridge-type sensor by dividing a predetermined reference voltage. Bridge type to cancel the offset voltage from the output signals of the first and second amplifying means by applying across the third resistor connected between the one amplifying means and the second amplifying means The offset voltage generated by the variation of each resistance element can be canceled satisfactorily without being affected by the temperature characteristics of each resistance element bridge-connected in the sensor, and is included in the output signals of the first and second amplification means. It is also possible to reduce noise.

It is a circuit diagram which shows one structural example of the sensor circuit containing the offset cancellation circuit which performs offset cancellation and signal amplification. It is a figure which shows the structural example of a voltage type DA converter. It is a figure which shows one structural example of the buffer contained in a voltage type DA converter. It is a figure which shows the output voltage of the 1st and 2nd DA converter. It is a figure which shows the equivalent circuit of the 1st and 2nd DA converter. It is a figure which shows the equivalent circuit of an offset cancellation circuit. It is a figure which shows the structural example of the conventional offset cancellation circuit.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the embodiments described below, members that are common to each other are denoted by the same reference numerals, and redundant descriptions thereof are omitted.

  FIG. 1 is a circuit diagram showing a configuration example of a sensor circuit 1 including an offset cancellation circuit 9 that performs offset cancellation and signal amplification according to the present invention. The sensor circuit 1 includes a sensor unit 2, an offset cancel circuit 9, and an AD converter 5.

  The sensor unit 2 includes three magnetic sensors 3a, 3b, and 3c for detecting external magnetic fields in three axial directions orthogonal to each other, and the magnetic sensors 3a, 3b, and 3c detect external magnetic fields in different directions. Configured to do. The sensor unit 2 sequentially shifts the three magnetic sensors 3a, 3b, and 3c to the measurement mode one by one in a time division manner, and outputs a pair of output signals from the magnetic sensors 3a, 3b, and 3c in the measurement mode. Output from terminals A and B. Each magnetic sensor 3a, 3b, 3c operates based on a reference voltage Vref supplied from an external power source.

  The magnetic sensor 3a constitutes a bridge-type sensor 3 in which four resistance elements 8a, 8b, 8c, and 8d constituted by magnetoresistive elements such as GMR elements are bridge-connected, and a connection point between the resistance elements 8a and 8b is Connected to the reference voltage Vref, the connection point between the resistance element 8c and the resistance element 8d is grounded. The connection point between the resistance elements 8b and 8d and the connection point between the resistance elements 8a and 8c form two middle point nodes a and b of the bridge circuit. The magnetic sensor 3a includes switches S1 and S1 for connecting the two midpoint nodes a and b to a pair of output terminals A and B, respectively.

  Similarly, the magnetic sensor 3b includes a bridge-type sensor 3 in which four resistance elements 8e, 8f, 8g, and 8h formed of magnetoresistance elements such as GMR elements are bridge-connected. Switches S2 and S2 for connecting the midpoint nodes a and b to the output terminals A and B are provided. Further, the same applies to the magnetic sensor 3c, and a bridge type sensor 3 in which four resistance elements 8i, 8j, 8k, and 8m are bridge-connected is formed, and two midpoint nodes a and b of the bridge circuit are formed. Switches S3 and S3 for connecting to output terminals A and B are provided.

  The sensor unit 2 sequentially switches each magnetic sensor 3a, 3b, 3c to the measurement mode by alternately closing the switches S1, S2, S3 provided on the magnetic sensors 3a, 3b, 3c. Let When each magnetic sensor 3a, 3b, 3c enters the measurement mode, it outputs the potential of the midpoint nodes a, b obtained by dividing the reference voltage Vref by each resistance element to the output terminals A, B. At this time, if an external magnetic field is applied to each magnetic sensor 3a, 3b, 3c, the resistance value of each magnetoresistive element constituting the bridge circuit changes according to the external magnetic field, so that the two output terminals A, A potential difference corresponding to the external magnetic field is generated between B. However, the output signals output from the output terminals A and B of the sensor unit 2 include not only a potential difference according to the external magnetic field but also an offset voltage Voff due to variations in resistance values of the magnetoresistive elements constituting the bridge circuit. It is.

  The offset cancel circuit 9 is a circuit that cancels the offset voltage Voff included in the output signal output from the pair of output terminals A and B of the sensor unit 2 and amplifies and outputs only the signal component corresponding to the external magnetic field. . The offset cancel circuit 9 includes an amplifier unit 4 that amplifies an output signal output from the sensor unit 2, a voltage type DA converter 6, and a first output resistor connected to one output terminal of the voltage type DA converter 6. 16, a second output resistor 17 connected to the other output terminal of the voltage type DA converter 6, and a control circuit 7 that controls the voltage type DA converter 6. Among these units, at least the amplifier unit 4, the output resistors 16 and 17, and the voltage type DA converter 6 are formed on the same semiconductor chip by a CMOS process. Note that the resistance values of the two output resistors 16 and 17 are the same.

  The amplifier unit 4 is an operational amplifier 11 serving as a first amplification unit that amplifies an output signal output from the sensor unit 2 to the output terminal A, and a second amplifier that amplifies the output signal output from the sensor unit 2 to the output terminal B. And an operational amplifier 12 as an amplification means. The operational amplifiers 11 and 12 constitute an instrumentation amplifier. That is, the operational amplifier 11 includes a first resistor 13 inserted in a negative feedback path connecting the output terminal and the inverting input terminal, and the first output signal Vip output from the sensor unit 2 to the output terminal A. Is input to the non-inverting input terminal and outputs an output signal Vop. Further, the operational amplifier 12 has a second resistor 14 inserted in a negative feedback path connecting the output terminal and the inverting input terminal, and the second output signal Vin output from the sensor unit 2 to the output terminal B. Is input to the non-inverting input terminal and outputs an output signal Von. Note that the resistance values of the first and second resistors 13 and 14 are set to equal resistance values. The amplifier unit 4 includes a third resistor 15, and the inverting input terminal of the operational amplifier 11 and the inverting input terminal of the operational amplifier 12 are connected to each other via the third resistor 15. By appropriately adjusting the resistance value of the third resistor 15 to be different from the resistance values of the first and second resistors 13 and 14, the signal amplification factor in the amplifier unit 4 can be set to a predetermined magnification. it can.

  The voltage type DA converter 6 generates output voltages V1 and V2 corresponding to a control signal CNT having a predetermined number of bits output from the control circuit 7, and applies the output voltages V1 and V2 to both ends of the third resistor 15. To do. This voltage type DA converter 6 operates based on a reference voltage Vref supplied from an external power source, as in the sensor unit 2. That is, the voltage type DA converter 6 divides the reference voltage Vref according to the control signal CNT, so that it is included in the first and second output signals Vip and Vin output from the sensor unit 2 at the two output terminals. A potential difference corresponding to the offset voltage Voff generated is generated. The voltage type DA converter 6 applies a potential difference to both ends of the third resistor 15, thereby allowing a current corresponding to the offset voltage Voff to flow only through the third resistor 15. By causing a drop, the offset voltage Voff is canceled from the output signals Vop and Von of the operational amplifiers 11 and 12.

  The control circuit 7 is a circuit that controls the operation of the sensor unit 2 and also controls the potential difference generated in the voltage type DA converter 6. That is, the control circuit 7 switches each of the switches S1, S2, and S3 of the sensor unit 2 to shift the three magnetic sensors 3a, 3b, and 3c to the measurement mode one by one in a time-division manner, and each magnetic sensor 3a. , 3b and 3c are in the measurement mode, a control signal CNT for canceling the offset voltage Voff due to variations in the resistance elements of the magnetic sensor is generated and output to the voltage DA converter 6 to thereby generate the voltage DA converter 6 is controlled to generate a potential difference corresponding to the offset voltage Voff.

  This control circuit 7 is, for example, an offset due to variations in resistance values of the resistance elements 8a to 8m in each of the magnetic sensors 3a, 3b, and 3c in a state where an external magnetic field is not applied to the sensor unit 2 before shipment of the product. The voltage Voff is measured in advance and the measured value is held. For example, the control circuit 7 holds the offset voltage Voff of each of the magnetic sensors 3 a, 3 b, 3 c based on the output signal DS of the AD converter 5 when no external magnetic field is applied to the sensor unit 2. When the external magnetic field is measured after the product is shipped, the control signal CNT is generated based on the offset voltage Voff of each magnetic sensor 3a, 3b, 3c measured in advance and is output to the voltage type DA converter 6.

  The AD converter 5 converts the potential difference between the output signals Vop and Von output from the amplifier unit 4 of the offset cancel circuit 9 into a digital signal having a predetermined number of bits, and outputs an output signal DS. The output signal DS is a signal output from the sensor circuit 1 to the outside.

  Next, the voltage type DA converter 6 will be described in detail. The voltage type DA converter 6 includes first and second DA converters 6a and 6b as shown in FIGS. 2A and 2B, and the output voltage V1 of the first DA converter 6a is output to the output resistor 16 as shown in FIG. Is applied to one end of the third resistor 15, and the output voltage V 2 of the second DA converter 6 b is applied to the other end of the third resistor 15 via the output resistor 17.

  As shown in FIG. 2A, the first DA converter 6a is configured as an R2R ladder type DA converter, and includes a buffer unit 21 to which each bit signal of the control signal CNT output from the control circuit 7 is input. ing. The buffer unit 21 has a plurality of buffers 22 corresponding to the number of bits of the control signal CNT. FIG. 3 is a diagram illustrating a configuration example of the buffer 22. The buffer 22 includes a first inverter 22a composed of a P-type MOS transistor 25 and an N-type MOS transistor 26, and a second inverter 22b composed of a P-type MOS transistor 27 and an N-type MOS transistor 28. It is prepared for. The buffer 22 outputs the reference voltage Vref as the output signal Vout when the bit signal bitN that is the input signal is “1”, and outputs the ground voltage (0 V) as the output signal Vout when the bit signal bitN is “0”. . That is, in the buffer unit 21, each of the plurality of buffers 22 outputs the reference voltage Vref or the ground voltage according to the bit signal. Therefore, the first DA converter 6a outputs an output signal V1 obtained by dividing the reference voltage Vref according to the control signal CNT.

  The 2nd DA converter 6b is comprised as a R2R ladder type DA converter like the 1st DA converter 6a, as shown in FIG.2 (b). The second DA converter 6b has an inversion unit 23 that inverts the control signal CNT in the preceding stage of the buffer unit 21 similar to the above. The inverting unit 23 includes a plurality of inverters 24 that invert each bit signal of the control signal CNT output from the control circuit 7. The buffer unit 21 outputs the reference voltage Vref or the ground voltage based on a signal obtained by inverting each bit of the control signal CNT in each of the plurality of buffers 22. Therefore, the second DA converter 6b outputs an output signal V2 obtained by dividing the reference voltage Vref according to a signal obtained by inverting each bit of the control signal CNT.

  FIG. 4 is a diagram showing output voltages V1 and V2 of the first and second DA converters 6a and 6b, respectively. Each of the first and second DA converters 6a and 6b outputs output signals V1 and V2 corresponding to the control signal CNT within the range of the ground voltage (0V) to the reference voltage Vref. The output signal V1 increases as the value of the control signal CNT increases, and the output signal V2 decreases as the value of the control signal CNT increases. As shown in FIG. 4, when the control signal CNT has an intermediate value, the output signals V1 and V2 of the first and second DA converters 6a and 6b become an intermediate voltage (Vref / 2) of the reference voltage Vref and are equal to each other. Become. When the output signal output from the sensor unit 2 to the pair of output terminals A and B does not include the offset voltage Voff, the control circuit 7 outputs the control signal CNT as an intermediate value, thereby outputting the voltage type DA converter. The two output signals output from 6 have the same potential of (Vref / 2), and no potential difference occurs between both ends of the third resistor 15. On the other hand, when the output signal output from the sensor unit 2 to the pair of output terminals A and B includes the offset voltage Voff, the control circuit 7 outputs the control signal CNT corresponding to the offset voltage Voff. As a result, the two output signals output from the voltage-type DA converter 6 deviate from the intermediate voltage (Vref / 2) and become different voltages, and a potential difference corresponding to the offset voltage Voff is generated at both ends of the third resistor 15. be able to.

  FIG. 5 is a diagram showing an equivalent circuit of the first and second DA converters 6a and 6b. Since the first DA converter 6a is configured as an R2R ladder type DA converter as described above, the voltage source 31a that changes the output voltage Vout according to the control signal CNT as shown in FIG. This is equivalent to a configuration in which a resistor 32a having a resistance value R is connected. The output voltage Vout of the voltage source 31a becomes Vref / 2 + ΔV, and ΔV is changed according to the control signal CNT. The same applies to the second DA converter 6b. As shown in FIG. 5B, a resistor 32b having a resistance value R is connected to a voltage source 31b that changes the output voltage Vout according to the control signal CNT. Is equivalent to The output voltage Vout of the voltage source 31b is Vref / 2−ΔV, and ΔV is changed according to the control signal CNT.

  When the equivalent circuit shown in FIG. 5 is applied, the offset cancel circuit 9 shown in FIG. 1 is equivalent to the circuit configuration shown in FIG. That is, the voltage type DA converter 6 described above is connected to each of the voltage adjusting unit 31 including the two voltage sources 31a and 31b whose output voltage Vout is controlled by the control circuit 7, and the two voltage sources 31a and 31b. This is equivalent to a configuration in which the resistors 32 a and 32 b are provided, the resistor 32 a is connected to one end of the third resistor 15, and the resistor 32 b is connected to the other end of the third resistor 15. In FIG. 6, the resistors 32a and 32b of FIG. 5 and the output resistors 16 and 17 of FIG. 1 are combined to form new resistors 32a and 32b, and the resistance values of the resistors 32a and 32b of FIG. 5 and the output resistor 16 of FIG. , 17 and the total resistance value are shown as R3. However, the resistance values of the resistors 32a and 32b shown in FIG. 5 may be adjusted to R3, and the output resistors 16 and 17 shown in FIG. 1 may be omitted.

  In the absence of an external magnetic field, as shown in FIG. 6, the resistance elements in the magnetic sensors 3a, 3b, 3c have resistance values R1, R2 (where R1 ≠ R2), and the offset voltage due to these resistance values R1, R2 The operation of the offset cancel circuit 9 will be described assuming that Voff appears at the output terminals A and B. In this case, the first output signal Vip of the output terminal A and the second output signal Vin of the output terminal B are expressed by the following expressions 1 and 2, respectively.

  On the other hand, when the amplifier unit 4 amplifies the first and second output signals Vip and Vin by 10 times, the first and second resistors 13 and 14 have a resistance value 9R4 as shown in FIG. 3 has a resistance value 2R4. At this time, the following Expression 3 is satisfied for the node P1 in the amplifier unit 4, and Expression 4 is satisfied for the node P2.

  In the above formulas 3 and 4, (Vref / 2 + ΔV) is the output voltage of the voltage source 31a. Further, (Vref / 2−ΔV) is an output voltage of the voltage source 31b. When calculation is performed based on these equations 3 and 4, the following equation 5 is obtained.

  In the above formula 5, when the right side becomes 0, the output signals Vop and Von of the amplifier unit 4 become Vop = Von, and the offset voltage Voff is canceled from the output signals Vop and Von of the amplifier unit 4. Therefore, when ΔV is calculated when the right side of Expression 5 becomes 0 using Expression 1 and Expression 2, the following Expression 6 is obtained.

  In other words, the voltage adjusting unit 31 outputs the output voltages ((Vref / 2 + ΔV) and (Vref / 2−ΔV)) based on Expression 6, thereby offsetting the resistance elements of the magnetic sensors 3a, 3b, and 3c due to variations. It is possible to cancel the voltage Voff. When the amplification factor in the amplifier unit 4 is different from 10 times, the coefficient of each term on the right side in Equation 6 is different from that in Equation 6.

  Based on Equation 6, the temperature characteristics of each resistor are analyzed. Each term on the right side in Equation 6 is a resistance ratio of the same material. That is, in the first term of (1 + 9R4 / 10R3), the resistors R3 and R4 are resistors formed of the same polysilicon material, and the temperature change coefficients are equal to each other, so that even if the temperature changes, (1 + 9R4 / 10R3) ) Value does not change. The same applies to the second term of (10R3 / 18R4). In the third term of ((R1-R2) / (R1 + R2)), resistors R1 and R2 are resistors formed by magnetoresistive elements of the same material, and the temperature change coefficients are equal to each other, so that the temperature changes. However, the value of ((R1-R2) / (R1 + R2)) does not change. Therefore, the offset cancel circuit 9 of the present embodiment cancels the offset voltage Voff satisfactorily in an arbitrary temperature range because the value of ΔV for canceling the offset voltage Voff does not change even when the temperature changes. Is possible.

  Moreover, although the voltage type DA converter 6 described above uses MOS transistors 25, 26, 27, and 28, these MOS transistors 25, 26, 27, and 28 are used as mere switching elements and are not used as current sources. Therefore, the configuration is not affected by flicker noise. Therefore, it is not necessary to increase the area of the MOS transistors 25, 26, 27, and 28, and the circuit scale can be reduced.

  Further, the voltage type DA converter 6 described above divides the same reference voltage Vref as the reference voltage Vref at which each of the magnetic sensors 3a, 3b, 3c operates to cancel the offset voltage Voff. Therefore, even if the reference voltage Vref supplied from the external power source fluctuates and the voltages at the midpoint nodes a and b in the magnetic sensors 3a, 3b and 3c change, the voltage type DA converter 6 The divided voltage value can be changed in conjunction with the change in Vref, and the output signals Vop and Von of the amplifier unit 4 do not generate noise due to the change in the reference voltage Vref.

  As described above, in the offset cancel circuit 9 of the present embodiment, the first resistor 13 is inserted in the negative feedback path, and the first output signal Vip output from the sensor unit 2 is input to the non-inverting input terminal. An operational amplifier 11 serving as a first amplifying means to be amplified and a second resistor 14 inserted in the negative feedback path, and a second output signal Vin output from the sensor unit 2 is input to the non-inverting input terminal. An operational amplifier 12 serving as a second amplification means to be amplified, a third resistor 15 connected between the inverting input terminal of the operational amplifier 11 and the inverting input terminal of the operational amplifier 12, and both ends of the third resistor 15 are connected. The voltage type DA converter 6 is provided. The voltage type DA converter 6 generates a potential difference corresponding to the offset voltage Voff included in the first and second output signals Vip and Vin output from the sensor unit 2 by dividing a predetermined reference voltage Vref. By applying the voltage to both ends of the third resistor 15, the offset voltage Voff is canceled from the output signals Vop and Von of the operational amplifier 11 serving as the first amplification means and the operational amplifier 12 serving as the second amplification means. Such an offset cancel circuit 9 can satisfactorily cancel the offset voltage Voff generated by the variation of each resistance element without being affected by the temperature characteristics of each resistance element constituting the bridge circuit in the sensor unit 2. It is. Further, it is possible to reduce flicker noise and noise associated with fluctuations in the reference voltage Vref, and it is also possible to suppress the inclusion of noise in the output signals Vop and Von of the amplifier unit 4. Furthermore, since it is not necessary to increase the circuit scale in order to reduce noise, it is also suitable as a circuit to be mounted on a small information device such as a smartphone.

  The offset cancel circuit 9 described above includes a first output resistor 16 connected to one output terminal of the voltage type DA converter 6 and a second output resistor connected to the other output terminal of the voltage type DA converter 6. The voltage type DA converter 6 applies a potential difference generated according to the offset voltage Voff to both ends of the third resistor 15 via the first output resistor 16 and the second output resistor 17. Apply. With such a configuration, for example, even if the voltage type DA converter 6 is not the R2R ladder type DA converter as described above but a general voltage type DA converter, the offset voltage Voff can be canceled satisfactorily. . That is, since an ideal voltage type DA converter generally has an output impedance of zero, it is possible to connect output resistors 16 and 17 to each of a pair of output terminals of the voltage type DA converter 6 as shown in FIG. The offset voltage Voff can be canceled by connecting these output resistors 16 and 17.

  If the resistance values of the output resistors 16 and 17 are changed by providing the output resistors 16 and 17, the ratio of (R3 / R4) in (Expression 6) can be adjusted relatively easily. Therefore, there is an advantage that the entire scale of ΔV in (Expression 6) can be adjusted relatively easily. That is, since the offset voltage Voff is smaller than the reference voltage Vref, the output voltages V1 and V2 of the voltage type DA converter 6 must be capable of taking a wide range of values from 0 to Vref as shown in FIG. There is no limitation as long as it can take a value within a certain range (for example, ± Vref / 4) around Vref / 2. Since such a range of the output voltages V1 and V2 can be changed by adjusting the ratio of (R3 / R4), the ratio of (R3 / R4) can be adjusted by the resistance values of the output resistors 16 and 17. This makes it easier to design the circuit.

  Further, when the voltage type DA converter 6 is configured as an R2R ladder type DA converter as described above, the resistors included in the R2R ladder type DA converter are respectively connected to the first to third resistors 13, 14, 15. By using the same material, it is possible to configure the offset cancel circuit 9 that is not easily affected by temperature changes. In the configuration in which the first output resistor 16 and the second output resistor 17 are provided, the first output resistor 16 and the second output resistor 17 are the same as the first to third resistors 13, 14, 15. It is preferable to form with a material.

  As mentioned above, although one Embodiment of this invention was described, this invention is not limited to the content mentioned above.

  For example, in the above embodiment, the magnetic sensors 3a, 3b, and 3c are illustrated as an example of the bridge-type sensor 3. However, the application range of the offset cancel circuit 9 described above is not necessarily limited to a magnetic sensor. For example, the bridge sensor 3 may be a pressure sensor or another sensor. That is, the offset cancel circuit 9 described above can cancel the offset voltage Voff included in the output signal of an arbitrary bridge-type sensor 3 configured by connecting a plurality of resistance elements in a bridge configuration.

  Moreover, in the said embodiment, although the case where the sensor part 2 was provided with the three magnetic sensors 3a, 3b, and 3c in order to detect the external magnetic field of the triaxial direction orthogonal to each other was illustrated, the sensor part 2 has three axes. It is not necessary to be able to detect an external magnetic field in the direction. In other words, the sensor unit 2 only needs to be provided with at least one bridge-type sensor 3.

  DESCRIPTION OF SYMBOLS 1 ... Sensor circuit, 2 ... Sensor part, 3 ... Bridge type sensor, 3a, 3b, 3c ... Magnetic sensor, 4 ... Amplifier part, 5 ... AD converter, 6 ... Voltage type DA converter, 7 ... Control circuit, 8a-8m DESCRIPTION OF SYMBOLS ... Resistance element, 9 ... Offset cancellation circuit, 11 ... Operational amplifier (first amplification means), 12 ... Operational amplifier (second amplification means), 13 ... First resistance, 14 ... Second resistance, 15 ... Third 16, output resistance (first output resistance), 17, output resistance (second output resistance), S 1, S 2, S 3, switch.

Claims (4)

An offset cancel circuit configured to bridge a plurality of resistance elements and cancel an offset voltage included in an output signal of a bridge-type sensor that operates based on a predetermined reference voltage,
A first amplifying unit, wherein a first resistor is inserted in a negative feedback path, and a first output signal output from the bridge-type sensor is input to a non-inverting input terminal for amplification;
A second amplifying unit, wherein a second resistor is inserted in the negative feedback path, and a second output signal output from the bridge-type sensor is input to a non-inverting input terminal and amplified;
A third resistor connected between the inverting input terminal of the first amplifying means and the inverting input terminal of the second amplifying means;
A voltage type DA converter connected to both ends of the third resistor;
With
The voltage type DA converter generates a potential difference according to an offset voltage included in the first and second output signals output from the bridge type sensor by dividing the predetermined reference voltage, thereby generating the third difference. An offset cancel circuit that cancels the offset voltage from the output signals of the first and second amplifying means by applying the voltage across the resistor. A first output resistor connected to one output terminal of the voltage type DA converter;
A second output resistor connected to the other output terminal of the voltage type DA converter;
Further comprising
2. The offset cancellation according to claim 1, wherein the voltage type DA converter applies the potential difference to both ends of the third resistor via the first output resistor and the second output resistor. circuit.   The offset cancel circuit according to claim 1, wherein the voltage type DA converter is an R2R ladder type DA converter.   The said 1st resistance, the said 2nd resistance, the said 3rd resistance, and the resistance contained in the said R2R ladder type DA converter are formed with the same material, The Claim 3 characterized by the above-mentioned. Offset cancel circuit.

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