JP2014102095A - Sensor threshold decision circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sensor threshold decision circuit capable of reducing a residual offset with simple circuit design without increasing arrangement area.SOLUTION: A K-to-1 current mirror circuit is composed of a driving current detection resistance RS for detecting a sensor driving current I, a bias current output resistance RB, and an operational amplifier 131, a sensor bias current IB which is 1/K time as large as the sensor driving current I is generated, and a digital output having hysteresis characteristics to a sensor external input BIN is obtained by utilizing a voltage drop which is the product of sensor resistance R as output impedance of a four-terminal type sensor 3 and the sensor bias current IB. At this time, a residual offset is reduced by adding or subtracting an offset current to or from only the sensor bias current IB supplied to only one output terminal, specified based upon the digital output, between two output terminals of the four-terminal type sensor 3.

Description

  The present invention relates to a sensor threshold value determination circuit, and more particularly to a sensor threshold value determination circuit in which an offset generated in a sensor is reduced.

There is a sensor threshold value determination circuit as a circuit for determining a threshold value necessary for digitizing the outputs of various sensors. First, a circuit configuration of a conventional sensor threshold value determination circuit 300 will be described with reference to FIG. 2 (see, for example, Patent Document 1).
2 includes a sensor drive current detection circuit 120, a sensor bias current output circuit 130, a bias current switching circuit 140, and an offset cancellation circuit 200. The offset cancel circuit 200 includes a chopper circuit 100 including the four-terminal sensor 3.

  The four-terminal sensor 3 is a four-terminal sensor having two sensor input terminals and two sensor output terminals, or a sensor having a circuit configuration equivalent to the four-terminal sensor. An element, a magnetoresistive element, a strain sensor, a pressure sensor, a temperature sensor, an acceleration sensor, or the like. The four-terminal sensor 3 has sensor resistances R1 to R4. For example, based on the displacement of two systems of output current I1 flowing through the sensor resistances R1 and R2 and current I2 flowing through the sensor resistances R3 and R4, for example, The absolute value of the magnetic flux density generated from a permanent magnet, a coil, or the like is detected, or the magnetic resistance or distortion is detected.

The sensor drive current detection circuit 120 includes a drive current detection resistor RS to which the sensor drive voltage Vcc is applied at one end. The other end of the driving current detection resistor RS is connected to the chopper circuit 100.
This drive current detection resistor RS detects a sensor drive current I for driving the four-terminal sensor 3.
The sensor bias current output circuit 130 includes an operational amplifier 131 for amplifying the sensor drive current I detected by the sensor drive current detection circuit 120, a PMOS transistor 132 for switching switching for performing an amplification operation, and a sensor drive voltage Vcc. Is configured to have a bias current output resistor RB applied to one end. The other end of the bias current output resistor RB is connected to the bias current switching circuit 140 through the PMOS transistor 132.

The inverting input terminal of the operational amplifier 131 is connected between the bias current output resistor RB and the source of the PMOS transistor 132, and the non-inverting input terminal is connected between the driving current detection resistor RS and the chopper circuit 100. . The output terminal of the operational amplifier 131 is connected to the gate of the PMOS transistor 132.
That is, the sensor bias current output circuit 130 detects the sensor drive current I using the drive current detection resistor RS, and outputs the sensor drive current I as the bias current output of the drive current detection resistor RS and the sensor bias current output circuit 130. A current mirror circuit that outputs a current amplified at a ratio of K to 1 determined by a combination of the resistor RB and the operational amplifier 131 is configured. The current mirror circuit generates a sensor bias current IB that is a predetermined multiple of the sensor drive current I, and supplies the sensor bias current IB to the bias current switching circuit 140.

  The bias current switching circuit 140 is one of the sensor drive currents I generated by the sensor bias current output circuit 130 in accordance with an output voltage VO (HIGH level or LOW level) of a later-described comparison amplifier 11 included in the offset cancellation circuit 200. The operation is switched so as to discharge (flow in the positive direction) or draw (flow in the negative direction) / K times sensor bias current IB. 1 / K is 1 / K = RS / RB.

  In other words, the bias current switching circuit 140 includes a switch SW1 having one end connected to the source of the PMOS transistor 132 of the sensor bias current output circuit 130 and the other end connected to the negative output end 100N of the chopper circuit 100, as well as a PMOS transistor. The switch SW2 has one end connected to the source 132 and the other end connected to the positive output end 100P of the chopper circuit 100, and an inverter 141 that inverts the output voltage VO of the comparison amplifier 11. The switches SW1 and SW2 are composed of PMOS transistors. The output voltage VO of the comparison amplifier 11 is input to the gate of the PMOS transistor constituting the switch SW2, and the signal obtained by inverting the output voltage VO of the comparison amplifier 11 by the inverter 141 is the switch SW1. Is input to the gate of the PMOS transistor.

  The offset cancel circuit 200 includes a chopper circuit 100 including the four-terminal sensor 3, a gain amplifier 10 that amplifies the output signal of the chopper circuit 100, a capacitor 5 having a negative output terminal of the gain amplifier 10 connected to one end, A comparison amplifier 11 and a switch 4 are provided. The positive output terminal of the gain amplifier 10 is connected to the non-inverting input terminal of the comparison amplifier 11, and the other end of the capacitor 5 is connected to the inverting input terminal. One end of the switch 4 is connected to the output terminal of the comparison amplifier 11, and the other end of the switch 4 is connected between the capacitor 5 and the inverting input terminal of the comparison amplifier 11.

  The chopper circuit 100 includes a four-terminal sensor 3 and a switch circuit 101. The switch circuit 101 switches the connection destination of the four terminals of the four-terminal sensor 3 so that two opposing terminals of the four terminals operate as sensor input terminals and the remaining two terminals operate as sensor output terminals. The connection destination is switched, and the four-terminal sensor 3 is operated by a two-way chopper method that switches the direction of the current flowing through the four-terminal sensor 3 in two directions.

  Two corresponding terminals operating as sensor output terminals are connected to the output terminals 100P and 100N of the chopper circuit 100, and applied from the sensor output Vh of the four-terminal sensor 3, that is, from the outside of the four-terminal sensor 3. An electromotive force corresponding to the sensor external input BIN such as a magnetic field is obtained. This sensor output Vh is output as a voltage between the output terminals 100P and 100N of the chopper circuit 100.

  The output terminal 100P of the chopper circuit 100 is connected to the non-inverting input terminal of the gain amplifier 10, and the output terminal 100N of the chopper circuit 100 is connected to the inverting input terminal of the gain amplifier 10. The source of the PMOS transistor constituting the switch SW2 is connected between the positive output terminal 100P of the chopper circuit 100 and the non-inverting input terminal of the gain amplifier 10, and the negative output terminal 100N of the chopper circuit 100 and the gain amplifier are connected. A source of a PMOS transistor constituting the switch SW1 is connected between the 10 inverting input terminals.

Next, the operation of the conventional sensor threshold value determination circuit 300 will be described.
In the conventional sensor threshold value determination circuit 300 shown in FIG. 2, the sensor drive current I is detected using the drive current detection resistor RS, and the sensor drive current I is converted into the drive current detection resistor RS and the bias current output resistor RB. A current mirror circuit that outputs a current amplified at a ratio of K to 1 determined by the combination with the operational amplifier 131 is configured to generate a sensor bias current IB that is 1 / K of the sensor driving current I.

  This sensor bias current IB is connected to the positive output terminal 100P of the chopper circuit 100 and the gain amplifier via the bias current switching circuit 140 depending on whether the output voltage VO of the comparison amplifier 11 is HIGH level or LOW level. 10 or between the negative output terminal 100N and the gain amplifier 10.

  As a result, the sensor bias current IB is caused to flow through the sensor resistance R of the four-terminal sensor 3 to generate a sensor threshold voltage Vx represented by Vx = IB × R. Here, the sensor resistance R is an output impedance viewed from the output terminal of the four-terminal sensor 3, that is, the positive and negative output terminals 100P and 100N of the chopper circuit 100. The sensor resistance R (output impedance) ) Can be expressed by R1 × R2 / (R1 + R2) or R3 × R4 / (R3 + R4) from the sensor resistances R1 to R4. Here, the sensor resistance R is assumed on the assumption that R1 = R4 and R2 = R3.

  A sensor output Vh, which is an electromotive force generated in the four-terminal sensor 3, is input to the gain amplifier 10 by a sensor external input BIN such as a magnetic field, and a voltage “A × Vh” obtained by multiplying the sensor output Vh by a gain A is obtained. The sensor threshold voltage Vx is compared by the comparison amplifier 11. As a result, a digital output having hysteresis characteristics independent of variations in sensor resistances R1 to R4, sensor drive voltage Vcc, and sensor drive current I can be obtained with respect to the sensor external input BIN.

To be precise, since the signal is doubled, that is, the sensor output Vh is doubled in the offset cancel circuit 200, the comparison amplifier 11 compares “2 × A × Vh” with the sensor threshold voltage Vx. .
At this time, assuming that the offset of the 4-terminal sensor 3 is canceled to “0” in the offset cancel circuit 200, the magnetic sensitivity Bop of the 4-terminal sensor 3 is Bop = IB × R = 2 × A ×. Vh can be expressed.

When the polarity of the sensor external input BIN is reversed to “−BIN”, the electromotive force of the four-terminal sensor 3 is also “−Vh”, and the magnetic sensitivity Brp of the four-terminal sensor 3 is Brp = −IB × R = -2 * A * Vh. Since the offset of the four-terminal sensor 3 is not related (independent) to the sensor external input BIN, the polarity does not change (Bop + Brp) / 2 = (IB × R + (− IB × R)) / 2 = 0.
FIG. 3 is a diagram in which the offset cancel circuit 200 of FIG. 2 is extracted.

  As described above, the offset cancel circuit 200 includes the chopper circuit 100 including the four-terminal type sensor 3, and the gain amplifier 10 that amplifies the chopper output signal (HoutP, HoutN) and the output voltage of the gain amplifier 10 are at one end. The supplied output voltage of the capacitor 5 and the gain amplifier 10 is input to the non-inverting input terminal, the comparison amplifier 11 having one end of the capacitor 5 connected to the inverting input terminal, and the output voltage VO of the comparison amplifier 11 is supplied to one end. The other end is composed of a switch 4 connected between the capacitor 5 and the inverting input terminal of the comparison amplifier 11, and constitutes an offset cancel circuit by a two-way chopper method.

The offset cancel circuit 200 using the two-way chopper method has two states (PH1 and PH2) when the switch 4 is switched.
FIG. 4 shows the state of the offset cancellation circuit 200 when in the state PH1, and FIG. 5 shows the state of the offset cancellation circuit 200 when in the state PH2.
In the state PH1 of FIG. 4, the four-terminal sensor 3 obtains an output signal “−Vh” according to, for example, the magnetic flux density.

When the magnetic flux density is “0”, the output of the four-terminal sensor 3 should ideally be “0”, but there is actually an offset. If the offset at this time is Voff1, the sensor output of the four-terminal sensor 3 is represented by “−Vh + Voff1”.
The sensor output of the four-terminal sensor 3 becomes an input signal of the gain amplifier 10. When the gain of the gain amplifier 10 is A times, the output “Vh1−Vh2” of the gain amplifier 10 is Vh1−Vh2 = A × (−Vh + Voff1). “Vh1” is a positive output of the gain amplifier 10 and “Vh2” is a negative output of the gain amplifier 10.

In the state PH1, the switch 4 is controlled to be ON, and the comparison amplifier 11 operates as a voltage follower. Therefore, the potential “Vh3” of the node N11 in FIG. 4, that is, the node to which one end of the switch 4 is connected between the capacitor 5 and the inverting input terminal of the comparison amplifier 11, is Vh3 = VO = Vh1.
When the capacitance value of the capacitor 5 is C, the charge Q1 charged in the capacitor 5 is Q1 = C × (Vh3−Vh2) = C × (Vh1−Vh2) = C × A × (−Vh + Voff1). .

When the state PH1 is switched to the state PH2, the offset cancel circuit 200 enters the state shown in FIG.
The four-terminal sensor 3 is driven by the two-way chopper method in the chopper circuit 100 and the connection is switched from the state PH1, so that the sensor output changes in polarity to “Vh”.
When the magnetic flux density is “0”, the output of the four-terminal sensor 3 should ideally be “0”, but there is actually an offset. The offset at this time is Voff2.

As in the case of the state PH1, when the gain of the gain amplifier 10 is A times, the positive output of the gain amplifier 10 is Vh1 ′, and the negative output is Vh2 ′, Vh1′−Vh2 ′ = A × (Vh + Voff2) It becomes.
In the state PH2, the switch 4 is controlled to be OFF, and the comparison amplifier 11 operates as a comparator. Therefore, the output voltage VO of the comparison amplifier 11 becomes a high level or a low level.

At this time, the potential Vh3 ′ of the node N11 is determined by Vh3 ′ = Vh2 ′ + Q1 / from the charge Q1 charged in the capacitor 5 in the state PH1 and the negative output voltage Vh2 ′ output from the gain amplifier 10 in the state PH2. C = Vh2 ′ + A × (−Vh + Voff1).
If the difference “Vh1′−Vh3 ′” between “Vh1 ′” and “Vh3 ′” input to the comparison amplifier 11 is a positive value, the output voltage VO of the comparison amplifier 11 is VO = High level and a negative value. If so, Vo = Low level.

The threshold voltage at which the output voltage VO of the comparison amplifier 11 is switched is a voltage at which Vh1′−Vh3 ′ = 0. That is, Vh1′−Vh3 ′ can be expressed by the following equation.
Vh1'-Vh3 '
= Vh1 '-(Vh2' + Q / C)
= Vh1'-Vh2 '-(Vh1-Vh2)
= A * (Vh + Voff2) -A * (-Vh + Voff1)
= 2 × A × Vh + A × (Voff2−Voff1)

At this time, “Voff2−Voff1” ideally becomes “0” and the offset disappears. This is an effect obtained by driving the four-terminal sensor 3 by the two-way chopper method.
However, since the terminals of the four-terminal sensor 3 connected to the input terminal of the gain amplifier 10 are different from each other, Voff2 and Voff1 do not take equivalent values, and in reality, they are not completely “0”. “Voff2−Voff1” after the offset cancellation is called a residual offset.

Here, in the offset cancel circuit 200, if the offset of the four-terminal sensor 3 is controlled to “0”, the magnetic sensitivity Bop of the four-terminal sensor 3 is Bop = IB × R = 2 × A × Vh. Can be represented.
However, since there is a residual offset, the magnetic sensitivity Bop is Bop = IB × R = 2 × A × Vh + A × (Voff2−Voff1).

When the polarity of the sensor external input BIN is reversed to “−BIN”, the electromotive force in the four-terminal sensor 3 is also “−Vh”, and the magnetic sensitivity Brp is Brp = −IB × R = −2 × A × Vh + A. X (Voff2-Voff1).
Since the offset of the 4-terminal sensor 3 is not related to (or does not depend on) the sensor external input BIN, the polarity does not change, and the hysteresis offset (Bop + Brp) / 2 = (IB × R + (− IB × R) due to the influence of the residual offset. )) / 2 = A * (Voff2-Voff1).

In order to solve this problem, for example, in Patent Document 2, a configuration in which offset is canceled by driving a four-terminal sensor by chopper driving by switching in two directions is configured to be chopper driven by switching in all directions. To reduce the offset.
That is, the connection of each terminal of the 4-terminal sensor is switched, and the direction of the current flowing between the opposing terminals of the 4-terminal sensor is switched by 90 degrees. Thereby, the offset cancellation circuit can have four states from the first state to the fourth state according to the direction of the current flowing through the four-terminal sensor. The output signal of the four-terminal sensor output in each of the four states is held in different capacitors, and the offset can be canceled by adding the charging voltage of each capacitor.

JP 2001-108480 A JP 2010-281864 A

However, when a four-terminal sensor is driven by a chopper method in all directions, the offset cancel circuit requires capacitors for holding the output signal of the four-terminal sensor in each of four states, and is connected to these capacitors. Switch is required. When holding the charge in the capacitor by switching the switch, considering the influence of charge injection due to switching, the capacitor for holding the charge preferably has a large capacitance value. This leads to an increase in circuit scale. There is also a problem that the circuit becomes complicated because the four states are controlled.
Accordingly, an object of the present invention is to provide an offset cancel circuit capable of reducing a residual offset with a simple circuit design without greatly increasing the layout area.

  One aspect of the present invention is a sensor threshold value determination circuit that determines a threshold value for digitizing a sensor output by a sensor based on a product of an output impedance of a sensor (for example, the four-terminal sensor 3 in FIG. 1) and a bias current. A sensor drive current detection circuit for detecting a sensor drive current for driving the sensor (for example, the sensor drive current detection circuit 120 in FIG. 1) and a sensor drive current detected by the sensor drive current detection circuit are multiplied by a predetermined amount. A sensor bias current output circuit that outputs a bias current (for example, the sensor bias current output circuit 130 in FIG. 1), and a control circuit that controls the current value of the bias current output from the sensor bias current output circuit (for example, the offset in FIG. 1). Current addition / subtraction circuit 150), and a sensor threshold value determination circuit.

  The sensor is a four-terminal sensor (for example, the four-terminal sensor 3 in FIG. 1), and the connection destinations of the four terminals of the four-terminal sensor so that the direction of the current flowing through the four-terminal sensor is switched in two directions. The sensor output voltage is extracted from two of the four terminals that operate as output terminals in the two states, respectively, and operates as the output terminal. A voltage comparator (for example, the comparison amplifier 11 in FIG. 1) that compares the sensor output voltages taken out from two terminals and outputs the sensor output voltage as a digital value, and a bias current switching circuit (for example, bias current switching in FIG. 1). The bias current switching circuit based on the digital output of the voltage comparator. Of the two terminals operating as the output terminal of the four-terminal type sensor, the bias current is supplied only to any one of the terminals specified by the digital output, and the control circuit is configured to supply the bias current to the voltage comparator. Based on the digital output, the current value of only the bias current supplied to one of the two terminals operating as the output terminal of the four-terminal sensor may be controlled. .

  The sensor drive current detection circuit detects the sensor drive current using a first resistor (for example, the drive current detection resistor RS in FIG. 1), and the sensor bias current generation circuit has a second resistor (for example, FIG. 1 bias current output resistor RB) and an operational amplifier (for example, operational amplifier 13 in FIG. 1), and the first resistor is a combination of the first resistor, the second resistor, and the operational amplifier. And a K: 1 current mirror circuit determined by the ratio of the resistance values of the second resistors, and a bias current that is 1 / K times the sensor driving current is output as the bias current. The circuit may add or subtract an offset current for adjusting a current value of the bias current with respect to a bias current that is 1 / K times the sensor driving current.

  According to the present invention, the residual offset can be reduced with a simple circuit design without significantly increasing the elements and the layout area, and the offset of the four-terminal sensor can be further reduced.

It is a circuit diagram which shows an example of the sensor threshold value determination circuit in embodiment of this invention. It is a circuit diagram which shows an example of the conventional sensor threshold value determination circuit. It is a circuit diagram which shows an example of the offset cancellation circuit of a two-way chopper system. FIG. 4 is a circuit diagram showing a state of the offset cancel circuit of FIG. 3 when in a state PH1. FIG. 4 is a circuit diagram showing a state of the offset cancel circuit of FIG. 3 when in a state PH2.

Embodiments of the present invention will be described below with reference to the drawings. However, in all the drawings in the present specification, the same reference numerals are given to portions corresponding to each other, and description of the overlapping portions will be omitted as appropriate.
FIG. 1 is a circuit diagram showing an example of a sensor threshold value determination circuit 1 according to the present invention.
The sensor threshold value determination circuit 1 according to the present invention is obtained by adding an offset current addition / subtraction circuit 150 to the conventional sensor threshold value determination circuit 300 shown in FIG.

  That is, as shown in FIG. 1, the sensor threshold value determination circuit 1 according to the present invention includes a sensor drive current detection circuit 120, a sensor bias current output circuit 130, a bias current switching circuit 140, an offset current addition / subtraction circuit 150, and an offset And a cancel circuit 200, and further includes a control unit 20 that controls each switch of the offset cancel circuit 200. The configurations of the sensor drive current detection circuit 120, the sensor bias current output circuit 130, the bias current switching circuit 140, and the offset cancellation circuit 200 are the same as the conventional configuration shown in FIG.

  That is, the offset cancel circuit 200 includes a chopper circuit 100 including the four-terminal sensor 3 and the switch circuit 101, a gain amplifier 10 that amplifies the sensor output Vh of the four-terminal sensor 3 by A times, and a comparison amplifier that performs a comparison operation. 11, a capacitor 5 connected between the gain amplifier 10 and the comparison amplifier 11, and a switch 4 connected between the output terminal of the comparison amplifier 11 and the inverting input terminal. 101 is controlled by the control unit 20.

  The four-terminal sensor 3 is a four-terminal sensor having two sensor input terminals and two sensor output terminals, or a sensor having a circuit configuration equivalent to the four-terminal sensor, the Hall element, A magnetoresistive element, a strain sensor, a pressure sensor, a temperature sensor, an acceleration sensor, or the like. The four-terminal sensor 3 has sensor resistances R1 to R4. For example, based on the displacement of two systems of output current I1 flowing through the sensor resistances R1 and R2 and current I2 flowing through the sensor resistances R3 and R4, For example, the absolute value of the magnetic flux density generated from a permanent magnet, a coil, or the like is detected, or the magnetic resistance or distortion is detected. The switch circuit 101 switches the connection destinations of the four terminals of the four-terminal sensor 3 to drive the four-terminal sensor 3 by the two-way chopper method, and changes the current flow direction alternately in two directions. Of the two terminals, switching is performed so that two opposing terminals operate as input terminals and the other two terminals operate as output terminals.

The sensor drive current detection circuit 120 and the sensor bias current output circuit 130 constitute a current mirror circuit, and the sensor bias current output circuit 130 is a sensor detected by a drive current detection resistor RS included in the sensor drive current detection circuit 120. A sensor bias current IB obtained by multiplying the drive current I by a predetermined value (1 / K = RS / RB times) is output.
The offset current addition / subtraction circuit 150 includes a PMOS transistor 151 and an NMOS transistor 152 in which drains are connected in series, and the source of the PMOS transistor 151 is connected between the bias current output resistor RB and the PMOS transistor 132, and the NMOS transistor The source of the transistor 152 is grounded. The output of the operational amplifier 131 is input to the gate of the PMOS transistor 151, and the output voltage VO of the comparison amplifier 11 is input to the gate of the NMOS transistor 152.

  The offset current Ioffset is supplied only when the output voltage VO of the comparison amplifier 11 is at the HIGH level, and the sensor bias current IB output from the sensor bias current output circuit 130 is adjusted to IB ′ = IB−Ioffset. On the other hand, when the output voltage VO of the comparison amplifier 11 is at the LOW level, both the PMOS transistor 151 and the NMOS transistor 152 of the offset current addition / subtraction circuit 150 are in the OFF state, so the sensor bias current IB is not changed and remains IB. . Note that the current value of the offset current Ioffset is smaller than the sensor bias current IB. For example, a residual offset is obtained in advance by experiment, a current value that can reduce the residual offset is obtained, and this is used as the offset current Ioffset. Set.

  The offset current adding / subtracting circuit 150 is formed so as to generate the offset current Ioffset set in this way. For example, a size ratio capable of generating a predetermined offset current Ioffset is set as a size ratio between the PMOS transistor 151 and the PMOS transistor 132 of the sensor bias current output circuit 130, and the size ratio between the PMOS transistor 151 and the PMOS transistor 132 is set to a predetermined value. This is realized by fabricating these PMOS transistors 151 and 132 so as to have a size ratio set as a size ratio that can generate the offset current Ioffset.

  Alternatively, the offset current Ioffset may be finely adjusted when the target four-terminal sensor 3 is actually operated. For example, a plurality of PMOS transistors that are PMOS transistors 151 are provided in parallel, and the residual offset becomes zero when the target four-terminal sensor 3 is actually operated. This is realized by selecting one or more of the transistors as the PMOS transistor 151, and thereafter configuring the selected one or more PMOS transistors to operate as the PMOS transistor 151. At this time, one or a plurality of PMOS transistors operating as the PMOS transistor 151 may be configured to be connectable / disconnectable by a switch or the like, or connected to the sensor bias current output circuit 130 without using a switch or the like. You may comprise so that a line may be cut | disconnected.

  The sensor bias current IB ′ or IB adjusted by the offset current adding / subtracting circuit 150 is output from the positive output terminal 100P of the chopper circuit 100 and the gain amplifier 10 by the bias current switching circuit 140 according to the output voltage VO of the comparison amplifier 11. Between the negative output terminal 100N of the chopper circuit 100 and the inverting input terminal of the gain amplifier 10. As a result, the sensor bias current IB flows through the sensor resistance R of the four-terminal sensor 3 (output impedance viewed from the output terminal of the chopper circuit 100), and the sensor threshold voltage Vx (= IB × R or IB ′ × R) is generated. .

  The electromotive force generated in the four-terminal sensor 3 by the sensor external input BIN is multiplied by the gain A by the gain amplifier 10 as the sensor output Vh, and the voltage A × Vh obtained by multiplying the sensor output Vh output from the gain amplifier 10 by A. The sensor threshold voltage Vx is compared with the comparison amplifier 11, whereby a digital output having hysteresis characteristics independent of the sensor resistance R, the sensor drive voltage Vcc, and the sensor drive current I is compared with the sensor external input BIN. 11 output voltage VO.

  At this time, the sensor bias current IB is adjusted by the offset current addition / subtraction circuit 150, and two types of sensor bias currents are set. Therefore, the hysteresis offset (Bop + Brp) / 2 is (Bop + Brp) / 2 = (IB × R + (− IB ′ × R)) / 2, and | −IB ′ | is | −IB ′ | <| IB | Therefore, the hysteresis offset (Bop + Brp) / 2 is reduced.

  That is, in the offset current addition / subtraction circuit 150, the offset current addition / subtraction is performed in accordance with the output voltage VO of the comparison amplifier 11 that compares the sensor threshold voltage Vx with the voltage A × Vh obtained by multiplying the sensor output Vh output from the gain amplifier 10 by A. By operating the circuit 150, the offset current Ioffset is added to or subtracted from the sensor bias current IB in accordance with the polarity of the sensor output Vh of the four-terminal sensor 3, and as a result, the sensor drive is performed as the sensor bias current. Two types are set: “IB” obtained by multiplying the current I by 1 / K, and a new sensor bias current IB ′ = IB ± Ioffset obtained by adding or subtracting the offset current Ioffset. Therefore, the hysteresis offset (Bop + Brp) / 2 = (IB × R + (− IB ′ × R)) / 2 can be reduced.

  In FIG. 1, the case where the offset current Ioffset is added to or subtracted from the sensor bias current IB supplied between the negative output terminal 100N of the chopper circuit 100 and the inverting input terminal of the gain amplifier 10 has been described. However, the present invention is not limited to this, and an offset current Ioffset with respect to a reverse bias current, that is, a sensor bias current IB supplied between the positive output terminal 100P of the chopper circuit 100 and the non-inverting input terminal of the gain amplifier 10. Can be added or subtracted, and in this case, the same effect can be obtained.

  As described above, the sensor threshold value determination circuit 1 shown in FIG. 1 takes into account the offset (residual offset) that cannot be removed even if the four-terminal sensor 3 is driven by the two-way chopper method to cancel the offset. The offset current Ioffset corresponding to the residual offset is added to or subtracted from the sensor bias current IB that determines the sensor threshold value, so that a new sensor bias current IB ′ = IB ± Ioffset is obtained. Since the sensor threshold value is determined using the two types of bias currents of the sensor bias current IB ′ and the sensor bias current IB, the residual offset can be reduced.

  Here, when the four-terminal sensor 3 is, for example, a Hall element, the peripheral portion of the N-type impurity region serving as a sensitive part of the Hall element is surrounded by a P-type impurity region for isolation, so When a voltage is applied to the sensor 3, a depletion layer spreads at the boundary between the Hall element sensing part and its peripheral part. In the region where the depletion layer extends, the hole current is suppressed and the resistance increases. Therefore, when the resistance values of the sensor resistances R1 to R4 of the four-terminal sensor 3 change and the four-terminal sensor 3 is driven by the two-way chopper method, the sensor resistances R1 to R4 are changed between the state PH1 and the state PH2. The fluctuation of the resistance value contributes to the residual offset.

Therefore, if the sensor drive voltage Vcc for driving the four-terminal sensor 3, the sensor size, the sensor shape, and the direction of current flow on the wafer are the same, the residual offset amount is almost the same in the state PH1 and the state PH2. Thus, the offset can be reduced.
As described above, by configuring the sensor threshold value determination circuit 1 described in the above embodiment, the offset of the four-terminal sensor 3 can be further reduced. By providing a compensation circuit or the like, the output accuracy of the sensor can be further improved.

In the above embodiment, the offset current adding / subtracting circuit 150 has been described with respect to the case where the offset current Ioffset is generated depending on the sensor driving current I. However, the present invention is not limited to this, and the offset current Ioffset is a preset fixed value. Also good.
Further, the scope of the present invention is not limited to the illustrated and described exemplary embodiments, but includes all embodiments that provide an effect equivalent to that of the present invention. Further, the scope of the invention can be defined by any desired combination of particular features among all the disclosed features.

DESCRIPTION OF SYMBOLS 1 Sensor threshold value determination circuit 3 4 terminal type sensor 4 Switch 5 Capacitor 10 Gain amplifier 11 Comparison amplifier 100 Chopper circuit 101 Switch circuit 120 Sensor drive current detection circuit 130 Sensor bias current output circuit 131 Operational amplifier 132 PMOS transistor 140 Bias current switching circuit 150 Offset current addition / subtraction circuit 151 PMOS transistor I Sensor drive current IB Sensor bias current Ioffset Offset current RB Bias current output resistor RS Drive current detection resistor VO Output voltage Vx Sensor threshold voltage

Claims (3)

In a sensor threshold value determining circuit for determining a threshold value for digitizing the sensor output by the sensor by a product of the output impedance of the sensor and a bias current,
A sensor drive current detection circuit for detecting a sensor drive current for driving the sensor;
A sensor bias current output circuit for outputting a bias current obtained by multiplying the sensor drive current detected by the sensor drive current detection circuit by a predetermined value;
And a control circuit for controlling the current value of the bias current output from the sensor bias current output circuit. The sensor is a four-terminal sensor, and the connection destination of the four terminals of the four-terminal sensor is switched to two states so that the direction of the current flowing through the four-terminal sensor is switched to two directions. Among the two terminals, the sensor output voltage is extracted from two terminals that operate as output terminals in the two states,
A voltage comparator that compares the sensor output voltages extracted from two terminals operating as the output terminals and outputs the sensor output voltage as a digital value;
A bias current switching circuit,
The bias current switching circuit is
Based on the digital output of the voltage comparator, the bias current is supplied to only one of the two terminals operating as the output terminal of the four-terminal type sensor specified by the digital output,
The control circuit is based on the digital output of the voltage comparator, only for the bias current supplied to one of the two terminals operating as the output terminal of the four-terminal sensor, The sensor threshold value determination circuit according to claim 1, wherein the current value is controlled. The sensor drive current detection circuit detects the sensor drive current using a first resistor,
The sensor bias current generation circuit includes a second resistor and an operational amplifier, and the first resistor, the second resistor, and the operational amplifier are combined with each other. Forming a K: 1 current mirror circuit determined by the resistance value ratio of the resistors, and outputting a bias current 1 / K times the sensor driving current as the bias current;
3. The sensor threshold according to claim 2, wherein the control circuit adds or subtracts an offset current for adjusting a current value of the bias current to a bias current that is 1 / K times the sensor driving current. Decision circuit.

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