Lead resistance elimination circuit and thermal mass flowmeter

Abstract

The present invention provides a lead resistance elimination circuit and a thermal mass flowmeter. In the lead resistance elimination circuit and the thermal mass flowmeter provided by the present invention, two thermal resistors (including lead resistors at both ends of the thermal resistors) of a thermal mass flowmeter sensor are respectively connected in series to two different bridge arms of the bridge through a bridge circuit powered by a constant current source. Then, a feedback circuit composed of an operational amplifier is used to feedback-limit the potentials at each point of the two bridge arms. By adjusting the resistance values of other resistors connected in series in the bridge arms, the influence of the lead resistances at both ends of the thermal resistors on the voltage signal reflecting the temperature difference can be effectively eliminated, thereby greatly improving the response speed of the sensor and the thermal mass flowmeter.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial automation, and in particular to a lead resistance elimination circuit and a thermal mass flowmeter. Background Art

[0002] Thermal gas mass flowmeter can directly measure the mass flow of the measured gas medium. Compared with gas ultrasonic wave, it eliminates the temperature and pressure compensation link, saving costs. Its basic principle is to use two platinum resistors as the core components of the sensor, one of which is used to measure the medium temperature and the other is a heating resistor. The mass flow of gas flowing through the two platinum resistors conforms to King's law: P / △θ=K1+K2(q m )K3, where P is the heating power of the heating resistor, △θ is the temperature difference between the two platinum resistors, and q m is the flow rate of the fluid, K1, K2, K3 are constants, and it can be seen that when the temperature difference △θ is a constant value, the heating power P and the flow rate q m A certain mathematical relationship is formed, and the flow rate q can be reflected by measuring the current of the heating resistor. m changes.

[0003] In actual measurements, a bridge circuit is generally used to measure the voltage difference between two platinum resistors. To improve response speed, the resistance of the heated platinum resistor is relatively small, generally 10Ω to 20Ω. For split-type flowmeters, the sensor leads can be over 30 meters long, with a lead resistance of up to 5Ω. If this lead resistance is not eliminated, significant measurement error will be introduced, seriously affecting measurement accuracy. Traditional three-wire platinum resistor measurement circuits use a constant current source to drive the platinum resistor. The platinum resistor voltage and the voltage across the lead resistor are measured separately. These are then fed into different sampling channels of an analog-to-digital converter. The difference between the two channel samples is calculated to obtain the platinum resistor voltage. This voltage is then divided by the constant current source value to obtain the platinum resistor resistance value. This method requires an analog-to-digital converter, followed by sampling and calculation by a microprocessor, which takes a long time and is not suitable for thermal mass flowmeters that require very fast response times. Therefore, it is necessary to design a hardware circuit to eliminate the lead resistance of the platinum resistor (sensor). Summary of the Invention

[0004] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a technical solution for eliminating lead resistance to solve the above-mentioned technical problems.

[0005] To achieve the above-mentioned and other related objectives, the present invention provides a lead resistance elimination circuit for eliminating the lead resistance of a sensor in a thermal mass flowmeter, wherein the sensor includes a first thermal resistor, a second thermal resistor, a first lead resistor and a second lead resistor, and the lead resistance elimination circuit includes: a bridge circuit and a feedback circuit;

[0006] The bridge circuit includes a first bridge arm and a second bridge arm arranged in parallel, one end of the first bridge arm is connected to a constant current source and the other end is grounded, and along the direction from the constant current source to the ground, the first bridge arm includes at least the first lead resistor, the first thermal resistor, and the second lead resistor connected in series in sequence, and the second bridge arm includes at least the second thermal resistor, the first resistor, and the second resistor connected in series in sequence;

[0007] A first detection point is provided at one end of the first lead resistor close to the constant current source, and a second detection point is provided at one end of the second thermal resistor close to the constant current source;

[0008] One end of the first thermal resistor away from the constant current source is connected to the feedback circuit, and one end of the first resistor away from the constant current source is connected to the feedback circuit, and the feedback circuit performs feedback limitation on the potential.

[0009] Optionally, the resistance value of the first lead resistor is equal to the resistance value of the second lead resistor, and the resistance value of the first resistor is equal to the resistance value of the second resistor.

[0010] Optionally, the first bridge arm also includes a third resistor, and the third resistor, the first lead resistor, the first thermal resistor and the second lead resistor are connected in series in sequence along the direction from the constant current source to the ground, and the first detection point is set at the common end of the third resistor and the first lead resistor.

[0011] Optionally, the second bridge arm also includes a fourth resistor and a fifth resistor, and the fourth resistor, the fifth resistor, the second thermal resistor, the first resistor and the second resistor are connected in series in sequence along the direction from the constant current source to the ground, and the second detection point is set at the common end of the fourth resistor and the fifth resistor.

[0012] Optionally, the sensor also includes a third lead resistor, and the feedback circuit includes an operational amplifier, a sixth resistor and a seventh resistor; one end of the third lead resistor is connected to the common end of the first thermal resistor and the second lead resistor, and the other end of the third lead resistor is grounded after passing through the sixth resistor; one end of the seventh resistor is connected to the common end of the third lead resistor and the sixth resistor, and the other end of the seventh resistor is connected to the non-inverting input end of the operational amplifier; the inverting input end of the operational amplifier is connected to the common end of the first resistor and the second resistor.

[0013] Optionally, the feedback circuit also includes an eighth resistor and a PNP transistor, the output end of the operational amplifier is connected to the base of the PNP transistor after passing through the eighth resistor, the collector of the PNP transistor is connected to a negative power supply, and the emitter of the PNP transistor is connected to the common end of the second thermal resistor and the first resistor.

[0014] Optionally, the operational amplifier includes a dual-power operational amplifier, a positive power pin of the dual-power operational amplifier is connected to a positive power supply, and a negative power pin of the dual-power operational amplifier is connected to the negative power supply.

[0015] Optionally, the feedback circuit further includes a unidirectional transient suppression diode, the positive electrode of the unidirectional transient suppression diode is connected to the negative power supply, and the negative electrode of the unidirectional transient suppression diode is connected to the common end of the second thermal resistor and the first resistor.

[0016] Optionally, the lead resistance elimination circuit also includes a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a fifth capacitor, a sixth capacitor and a seventh capacitor; one end of the first capacitor is grounded, and the other end is connected to the common end of the first resistor and the second thermal resistor; one end of the second capacitor is connected to the common end of the first resistor and the second resistor, and the other end is connected to the common end of the first resistor and the second thermal resistor; one end of the third capacitor is connected to the common end of the first resistor and the second resistor, and the other end is connected to the output end of the operational amplifier; one end of the fourth capacitor is connected to the positive power supply, and the other end is grounded; one end of the fifth capacitor is connected to the negative power supply, and the other end is grounded; one end of the sixth capacitor is connected to the non-inverting input end of the operational amplifier, and the other end is grounded; one end of the seventh capacitor is connected to the common end of the sixth resistor and the seventh resistor, and the other end is grounded.

[0017] In addition, to achieve the above-mentioned purpose and other related purposes, the present invention also provides a thermal mass flowmeter, which includes the lead resistance elimination circuit described in any one of the above items, and the thermal mass flowmeter also includes a constant voltage and constant current control chip, a gain circuit and an inverting proportional circuit. The constant voltage and constant current control chip serves as the constant current source, and the output end of the constant voltage and constant current control chip outputs a constant current to the outside. The two input ends of the gain circuit are respectively connected to the first detection point and the second detection point, the output end of the gain circuit is connected to the input end of the inverting proportional circuit, and the output end of the inverting proportional circuit is connected to the input end of the constant voltage and constant current control chip.

[0018] As described above, the lead resistance elimination circuit of the present invention has the following beneficial effects:

[0019] The two thermal resistors (including the lead resistances at both ends of the thermal resistors) in the thermal mass flowmeter sensor are connected in series to two different bridge arms of the bridge through a bridge circuit powered by a constant current source. The potential of each point in the two bridge arms is feedback-limited in combination with a feedback circuit. By adjusting the resistance values of other resistors connected in series in the bridge arms, the influence of the lead resistances at both ends of the thermal resistors on the voltage signal reflecting the temperature difference can be effectively eliminated, greatly improving the response speed of the sensor and the thermal mass flowmeter. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Shown is a sensor circuit diagram of a thermal mass flowmeter with a lead resistance elimination circuit according to an embodiment of the present invention.

[0021] Description of Reference Numerals

[0022] C1 first capacitor

[0023] C2 Second capacitor

[0024] C3 third capacitor

[0025] C4 fourth capacitor

[0026] C5 fifth capacitor

[0027] C6 Sixth capacitor

[0028] C7 seventh capacitor

[0029] R1 first resistor

[0030] R2 Second resistor

[0031] R3 third resistor

[0032] R4 fourth resistor

[0033] R5 fifth resistor

[0034] R6 Sixth resistor

[0035] R7 seventh resistor

[0036] R8 eighth resistor

[0037] RL1 first lead resistor

[0038] RL2 Second lead resistor

[0039] RL3 third lead resistor

[0040] RT1 first thermal resistor

[0041] RT2 Second thermal resistor

[0042] Q1 PNP transistor

[0043] TVS1 Unidirectional Transient Voltage Suppressor Diode

[0044] U1 operational amplifier

[0045] VDD positive power supply

[0046] VEE negative power supply

[0047] I1 Current flowing through the first bridge arm

[0048] I2 Current flowing through the second bridge arm

[0049] V1 first detection point potential

[0050] V2 Second detection point potential

[0051] V3 Common terminal potential of the first resistor and the second thermal resistor

[0052] 1 Output of the operational amplifier

[0053] 2 Inverting input of the operational amplifier

[0054] 3. Non-inverting input of the operational amplifier

[0055] 4 Negative power supply pin of the operational amplifier

[0056] 8 Positive power supply pin of the operational amplifier DETAILED DESCRIPTION

[0057] As mentioned above in the background technology, in existing thermal mass flowmeters, the resistance of the platinum resistor used for speed measurement is generally around 10Ω. At this time, whether it is a split structure or an integrated structure, the lead resistance of the platinum resistor (thermal resistor) cannot be ignored; the traditional three-wire measurement uses two AD sampling channels and calculates the difference between the two channel resistances as an effective signal; this method requires a series of processes such as AD sampling and holding, AD conversion, single-chip microcomputer reading, and filtering. The structure is complex and the response time is relatively long. Therefore, it is necessary to design a lead resistance elimination circuit to eliminate the influence of lead resistance on the thermal mass flowmeter sensor and improve its response time.

[0058] Based on this, the present invention proposes a technical solution for eliminating lead resistance: first, the first thermal resistor (not limited to a platinum resistor) in the thermal mass flowmeter sensor and the lead resistances at both ends are connected in series to the first bridge arm of the bridge, and the second thermal resistor (not limited to a platinum resistor) in the thermal mass flowmeter sensor is connected in series to the second bridge arm of the bridge. The first bridge arm and the second bridge arm are connected in parallel, and one end of the first bridge arm is connected to a constant current source and the other end is grounded; secondly, a feedback circuit (such as a feedback circuit composed of an operational amplifier) is introduced between the first bridge arm and the second bridge arm to feedback limit the potential of each point on the first bridge arm and the second bridge arm; finally, a suitable detection point is found on each of the first bridge arm and the second bridge arm. The potential difference between the two detection points is the voltage signal of the sensor reflecting the temperature difference. By adjusting the resistance value of other resistors connected in series in the first bridge arm or the second bridge arm, the influence of the lead resistance at both ends of the first thermal resistor on the measurement result can be eliminated.

[0059] The following describes the embodiments of the present invention through specific examples. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention.

[0060] See also Figure 1 . It should be noted that the diagrams provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner. Therefore, the diagrams only show the electronic components related to the present invention and are not drawn according to the component type, number and layout during actual implementation. During actual implementation, the type, quantity and layout of each electronic component can be changed at will, and its layout type may also be more complex. The structural form, quantity and layout of the electronic components illustrated in the drawings of this specification are only used to match the content disclosed in the specification for people familiar with this technology to understand and read. They are not used to limit the limiting conditions for the implementation of the present invention, so they have no technical significance. Any modification or simple addition or subtraction of the structural electronic components should still fall within the scope of the technical content disclosed by the present invention without affecting the efficacy and purpose that can be achieved by the present invention. At the same time, the terms such as "close" and "far away" quoted in this specification are only for the convenience of description and are not used to limit the scope of the implementation of the present invention. Changes or adjustments in their relative relationships should also be considered as the scope of the implementation of the present invention without substantially changing the technical content.

[0061] In this embodiment, if Figure 1 As shown, the present invention provides a lead resistance elimination circuit for eliminating the lead resistance of a sensor in a thermal mass flowmeter. The sensor includes a first thermal resistor RT1, a second thermal resistor RT2, a first lead resistor RL1, and a second lead resistor RL2. The lead resistance elimination circuit includes: a bridge circuit and a feedback circuit;

[0062] The bridge circuit includes a first bridge arm and a second bridge arm arranged in parallel, one end of the first bridge arm is connected to a constant current source and the other end is grounded. Along the direction from the constant current source to the ground, the first bridge arm includes at least a first lead resistor RL1, a first thermal resistor RT1 and a second lead resistor RL2 connected in series, and the second bridge arm includes at least a second thermal resistor RT2, a first resistor R1 and a second resistor R2 connected in series.

[0063] A first detection point is provided at one end of the first lead resistor RL1 close to the constant current source, and a second detection point is provided at one end of the second thermal resistor RT2 close to the constant current source;

[0064] One end of the first thermal resistor RT1 away from the constant current source is connected to the feedback circuit. The other end of the first resistor R1 away from the constant current source is connected to the feedback circuit. The feedback circuit provides feedback and limits on the potential of each point on the first bridge arm and the second bridge arm.

[0065] The current flowing through the first bridge arm is I1, and the current flowing through the second bridge arm is I2; the resistance of the first lead resistor RL1 is equal to the resistance of the second lead resistor RL2, and the resistance of the first resistor R1 is equal to the resistance of the second resistor R2.

[0066] In detail, such as Figure 1 As shown, the first bridge arm also includes a third resistor R3, the third resistor R3, the first lead resistor RL1, the first thermal resistor RT1 and the second lead resistor RL2 are connected in series in sequence along the direction from the constant current source to the ground, and the first detection point is set at the common end of the third resistor R3 and the first lead resistor RL1.

[0067] In detail, such as Figure 1 As shown, the second bridge arm also includes a fourth resistor R4 and a fifth resistor R5. The fourth resistor R4, the fifth resistor R5, the second thermal resistor RT2, the first resistor R1 and the second resistor R2 are connected in series in the direction from the constant current source to the ground, and the second detection point is set at the common end of the fourth resistor R4 and the fifth resistor R5.

[0068] In detail, such as Figure 1 As shown, the sensor also includes a third lead resistor RL3, and the feedback circuit includes an operational amplifier U1, a sixth resistor R6 and a seventh resistor R7; one end of the third lead resistor RL3 is connected to the common end of the first thermal resistor RT1 and the second lead resistor RL2, and the other end of the third lead resistor RL3 is grounded after passing through the sixth resistor R6; one end of the seventh resistor R7 is connected to the common end of the third lead resistor RL3 and the sixth resistor R6, and the other end of the seventh resistor R7 is connected to the non-inverting input terminal 3 of the operational amplifier U1; the inverting input terminal 2 of the operational amplifier U1 is connected to the common end of the first resistor R1 and the second resistor R2.

[0069] In more detail, Figure 1 As shown, the feedback circuit also includes an eighth resistor R8 and a PNP transistor Q1. The output terminal 1 of the operational amplifier U1 is connected to the base of the PNP transistor Q1 through the eighth resistor R8. The collector of the PNP transistor Q1 is connected to the negative power supply VEE. The emitter of the PNP transistor Q1 is connected to the common end of the second thermal resistor RT2 and the first resistor R1.

[0070] Optionally, the operational amplifier U1 includes a dual-power operational amplifier, a positive power pin 8 of which is connected to a positive power supply VDD, and a negative power pin 4 of which is connected to a negative power supply VEE.

[0071] Among them, the positive power supply VDD can provide a voltage of +15V, and the negative power supply VEE can provide a voltage of -5V. Flexible selection can be made according to actual conditions and needs.

[0072] Alternatively, as Figure 1 As shown, the feedback circuit also includes a unidirectional transient suppression diode TVS1. The positive electrode of the unidirectional transient suppression diode TVS1 is connected to the negative power supply VEE, and the negative electrode of the unidirectional transient suppression diode TVS1 is connected to the common terminal of the second thermal resistor RT2 and the first resistor R1. The unidirectional transient suppression diode TVS1 protects the PNP transistor Q1 from damage by various surge pulses.

[0073] Alternatively, as Figure 1 As shown, the lead resistance elimination circuit also includes a first capacitor C1, a second capacitor C2, a third capacitor C3, a fourth capacitor C4, a fifth capacitor C5, a sixth capacitor C6 and a seventh capacitor C7; one end of the first capacitor C1 is grounded, and the other end is connected to the common end of the first resistor R1 and the second thermal resistor RT2; one end of the second capacitor C2 is connected to the common end of the first resistor R1 and the second resistor R2, and the other end is connected to the common end of the first resistor R1 and the second thermal resistor RT2; one end of the third capacitor C3 is connected to the common end of the first resistor R1 and the second resistor R2, and the other end is connected to the output terminal 1 of the operational amplifier U1; one end of the fourth capacitor C4 is connected to the positive power supply VDD, and the other end is grounded; one end of the fifth capacitor C5 is connected to the negative power supply VEE, and the other end is grounded; one end of the sixth capacitor C6 is connected to the non-inverting input terminal 3 of the operational amplifier U1, and the other end is grounded; one end of the seventh capacitor C7 is connected to the common end of the sixth resistor R6 and the seventh resistor R7, and the other end is grounded.

[0074] In this embodiment, if Figure 1 As shown, the difference between the potential V1 of the first detection point and the potential V2 of the second detection point is the voltage signal reflecting the temperature difference in the sensor; specifically, the working principle of the lead resistance elimination circuit is as follows:

[0075] 1) When the circuit is stable, according to the "virtual off" principle, the currents at the non-inverting input terminal 3 and the inverting input terminal 2 of the operational amplifier U1 are both zero, and the potential of the non-inverting input terminal 3 of the operational amplifier U1 is the common terminal potential of the third lead resistor RL3 and the sixth resistor R6; however, in actual applications, the resistance of the third lead resistor RL3 is very small (on the order of 0.1Ω), and the resistance of the sixth resistor R6 is very large (on the order of 100kΩ), so the common terminal potential of the third lead resistor RL3 and the sixth resistor R6 is approximately the common terminal potential between the first thermal resistor RT1 and the second lead resistor RL2, that is, the potential of the non-inverting input terminal 3 of the operational amplifier U1 is approximately the common terminal potential between the first thermal resistor RT1 and the second lead resistor RL2, and its value is I1*RL2;

[0076] 2) When the circuit is stable, according to the "virtual short" principle, the potentials of the non-inverting input terminal 3 and the inverting input terminal 2 of the operational amplifier U1 are equal, so the common terminal potential of the first resistor R1 and the second resistor R2 is equal to I1*RL2; in addition, since the resistance values of the first resistor R1 and the second resistor R2 are equal (i.e., R1=R2), and the same current I2 flows through them, the common terminal potential of the first resistor R1 and the second thermal resistor RT2 is V3=2*I1*RL2;

[0077] 3) According to the circuit, the potential at the first detection point V1 = I1 * (RL1 + RT1 + RL2), and the potential at the second detection point V2 = V3 + I2 * (R5 + RT2); at the same time, the resistance of the first lead resistor RL1 and the resistance of the second lead resistor RL2 are equal, that is, RL1 = RL2;

[0078] 4) Combine the above equations and perform calculations

[0079] V3=2*I1*RL2 (1)

[0080] V1=I1*(RL1+RT1+RL2) (2)

[0081] V2=V3+I2*(R5+RT2) (3)

[0082] RL1=RL2 (4)

[0083] From equations (1) to (4), it can be calculated that: V1-V2=I1*RT1-I2*(R5+RT2). It can be seen that the voltage value of the voltage signal reflecting the temperature difference in the sensor is independent of the resistance of the lead resistors (first lead resistor RL1, second lead resistor RL2 or third lead resistor RL3) at both ends of the first thermal resistor RT1, thereby effectively eliminating the influence of the first lead resistor RL1, second lead resistor RL2 or third lead resistor RL3 on the measurement result, greatly improving the response speed of the sensor in the thermal mass flowmeter.

[0084] In addition, in this embodiment, if Figure 1 As shown, the present invention also provides a thermal mass flowmeter, which includes the above-mentioned lead resistance elimination circuit, and the thermal mass flowmeter also includes a constant voltage and constant current control chip, a gain circuit and an inverting proportional circuit. The constant voltage and constant current control chip serves as a controlled constant current source, and the output end of the constant voltage and constant current control chip outputs a constant current to the outside. The two input ends of the gain circuit are respectively connected to the first detection point and the second detection point, the output end of the gain circuit is connected to the input end of the inverting proportional circuit, and the output end of the inverting proportional circuit is connected to the input end of the constant voltage and constant current control chip.

[0085] To sum up, in the lead resistance elimination circuit and thermal mass flowmeter provided by the present invention, the two thermal resistors (including the lead resistances at both ends of the thermal resistors) in the thermal mass flowmeter sensor are respectively connected in series to two different bridge arms of the bridge through a bridge circuit powered by a constant current source, and then the feedback circuit composed of an operational amplifier is used to feedback limit the potential of each point in the two bridge arms, and by adjusting the resistance values of other resistors connected in series in the bridge arms, the influence of the lead resistances at both ends of the thermal resistors on the voltage signal reflecting the temperature difference can be effectively eliminated, thereby greatly improving the response speed of the sensor and the thermal mass flowmeter.

[0086] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A lead resistance elimination circuit for eliminating the lead resistance of a sensor in a thermal mass flowmeter, wherein the sensor comprises a first thermal resistor, a second thermal resistor, a first lead resistor, and a second lead resistor, characterized in that: The lead resistance elimination circuit includes: a bridge circuit and a feedback circuit; The bridge circuit includes a first bridge arm and a second bridge arm arranged in parallel, one end of the first bridge arm is connected to a constant current source and the other end is grounded, and along the direction from the constant current source to the ground, the first bridge arm includes at least the first lead resistor, the first thermal resistor, and the second lead resistor connected in series in sequence, and the second bridge arm includes at least the second thermal resistor, the first resistor, and the second resistor connected in series in sequence; A first detection point is provided at one end of the first lead resistor close to the constant current source, and a second detection point is provided at one end of the second thermal resistor close to the constant current source; One end of the first thermal resistor away from the constant current source is connected to the feedback circuit, and one end of the first resistor away from the constant current source is connected to the feedback circuit, and the feedback circuit performs feedback limitation on the potential; Wherein, the resistance value of the first lead resistor is equal to the resistance value of the second lead resistor, and the resistance value of the first resistor is equal to the resistance value of the second resistor; The sensor further includes a third lead resistor, and the feedback circuit includes an operational amplifier, a sixth resistor, and a seventh resistor; one end of the third lead resistor is connected to the common end of the first thermal resistor and the second lead resistor, and the other end of the third lead resistor is grounded after passing through the sixth resistor; one end of the seventh resistor is connected to the common end of the third lead resistor and the sixth resistor, and the other end of the seventh resistor is connected to the non-inverting input end of the operational amplifier; the inverting input end of the operational amplifier is connected to the common end of the first resistor and the second resistor; The resistance of the third lead resistor is much smaller than the resistance of the sixth resistor, and the common terminal potential of the third lead resistor and the sixth resistor is approximately the common terminal potential of the first thermal resistor and the second lead resistor; One end of the second lead resistor away from the first resistor is grounded, and one end of the second resistor away from the first resistor is grounded.

2. The lead resistance elimination circuit according to claim 1, wherein: The first bridge arm also includes a third resistor, and the third resistor, the first lead resistor, the first thermal resistor and the second lead resistor are connected in series in sequence along the direction from the constant current source to the ground, and the first detection point is set at the common end of the third resistor and the first lead resistor.

3. The lead resistance elimination circuit according to claim 2, wherein: The second bridge arm also includes a fourth resistor and a fifth resistor, and the fourth resistor, the fifth resistor, the second thermal resistor, the first resistor and the second resistor are connected in series in sequence along the direction from the constant current source to the ground, and the second detection point is set at the common end of the fourth resistor and the fifth resistor.

4. The lead resistance elimination circuit according to claim 1, wherein: The feedback circuit also includes an eighth resistor and a PNP transistor. The output end of the operational amplifier is connected to the base of the PNP transistor through the eighth resistor, the collector of the PNP transistor is connected to a negative power supply, and the emitter of the PNP transistor is connected to the common end of the second thermal resistor and the first resistor.

5. The lead resistance elimination circuit according to claim 4, characterized in that: The operational amplifier comprises a dual-power operational amplifier, a positive power pin of the dual-power operational amplifier is connected to a positive power supply, and a negative power pin of the dual-power operational amplifier is connected to the negative power supply.

6. The lead resistance elimination circuit according to claim 4, wherein: The feedback circuit further includes a unidirectional transient suppression diode, the positive electrode of the unidirectional transient suppression diode is connected to the negative power supply, and the negative electrode of the unidirectional transient suppression diode is the common end of the second thermal resistor and the first resistor.

7. The lead resistance elimination circuit according to claim 5, characterized in that: The lead resistance elimination circuit further includes a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a fifth capacitor, a sixth capacitor, and a seventh capacitor; one end of the first capacitor is grounded, and the other end is connected to a common end of the first resistor and the second thermal resistor; One end of the second capacitor is connected to the common end of the first resistor and the second resistor, and the other end is connected to the common end of the first resistor and the second thermal resistor; One end of the third capacitor is connected to the common end of the first resistor and the second resistor, and the other end is connected to the output end of the operational amplifier; one end of the fourth capacitor is connected to the positive power supply, and the other end is grounded; one end of the fifth capacitor is connected to the negative power supply, and the other end is grounded; one end of the sixth capacitor is connected to the non-inverting input end of the operational amplifier, and the other end is grounded; one end of the seventh capacitor is connected to the common end of the sixth resistor and the seventh resistor, and the other end is grounded.

8. A thermal mass flowmeter, characterized in that: The thermal mass flowmeter includes the lead resistance elimination circuit described in any one of claims 1 to 7, and the thermal mass flowmeter also includes a constant voltage and constant current control chip, a gain circuit and an inverting proportional circuit. The constant voltage and constant current control chip serves as the constant current source, and the output end of the constant voltage and constant current control chip outputs a constant current to the outside. The two input ends of the gain circuit are respectively connected to the first detection point and the second detection point, the output end of the gain circuit is connected to the input end of the inverting proportional circuit, and the output end of the inverting proportional circuit is connected to the input end of the constant voltage and constant current control chip.

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