A resistance value conversion circuit

Abstract

The present invention belongs to the field of resistance signal detection and analog circuit design, and particularly relates to a resistance value conversion circuit, including: a constant current source generated by a constant current element; a current element and a driving current that are the same as those that generate the constant current source, used to convert the resistance value into a voltage value of the resistance to be measured, and the voltage value generated by this current at both ends of the resistance to be measured is the conversion value of the resistance to be measured. In the present invention, the resistance to be measured is supplied with a constant current, which eliminates the conversion error caused by the constant voltage bridge power supply, and the measurement is accurate; the single power supply and signal difference output reduce the voltage complexity and the noise introduced by the power supply; the constant current power supply driven by the transistor has a large output signal adjustment margin, and can realize the amplification of the signal to be measured; the range of the output conversion value is (0.4V~2.8V), which can be directly connected to most digital-to-analog converters without the need for a special amplification circuit.

Description

Technical Field

[0001] The invention belongs to the field of resistance signal detection and analog circuit design, and in particular relates to a resistance value conversion circuit. Background Art

[0002] Resistance sensor is a widely used electrical parameter sensor, such as temperature-sensitive resistor, thermistor, magnetoresistor, photoresistor and stress-sensitive resistor. Its basic principle is to convert the measured physical quantity into a corresponding resistance value, and then pass it through the corresponding measurement circuit to form a sensor for measuring temperature, heat, eddy current, light intensity, stress, pressure, weight, displacement, acceleration, torque, etc. It is widely used in various fields of national economy and people's livelihood, such as electricity, metallurgy, transportation, petrochemical, commerce, biomedicine and national defense. It is a commonly used sensing technology for realizing process detection and production process automation.

[0003] Resistive sensors need to convert resistance values ​​into voltage or current before subsequent signal processing can be performed. For applications where accuracy is not high, the bridge structure can meet the requirements. However, for resistive sensors with high accuracy requirements, such as PT100 and PT1000, the bridge structure cannot meet the accuracy requirements. In the bridge structure, Figure 3 , the relationship between the output voltage and each parameter is Reference Figure 4 When △R / R varies in the range of (10% to -10%), the conversion error range is (-5% to 5.5%). When △R / R = 0.5, the conversion error is (1-1 / 1.25)*100% = 20%. Moreover, the conversion errors of resistance values ​​in different ranges of change are inconsistent, resulting in a large conversion error of the bridge constant voltage circuit and difficulty in piecewise linearity.

[0004] In summary, a conversion circuit is needed at this stage to reduce the conversion error of the bridge constant voltage circuit. Summary of the invention

[0005] In order to solve or improve the problem of excessive conversion error of the above-mentioned bridge constant voltage circuit, the present invention provides a resistance value conversion circuit, and the specific technical solution is as follows:

[0006] The present invention provides a resistance value conversion circuit, comprising: a constant current source generated by a constant current element; a current element that generates the same driving current as the constant current source, and a resistance to be measured that converts a resistance value into a voltage value, wherein the voltage value generated by the current at both ends of the resistance to be measured is the conversion value of the resistance to be measured.

[0007] Preferably, the constant current element includes a reference voltage source VR, an operational amplifier A and resistors (R1, R3).

[0008] Preferably, the current element includes a transistor T and a resistor RT to be measured.

[0009] Preferably, the reference voltage source VR outputs a constant voltage V S , this voltage is input to the non-inverting terminal “+” of operational amplifier A, and resistor R1 is connected between the inverting terminal “-” of operational amplifier A and the ground, and the current flowing through R1 remains constant.

[0010] Preferably, a resistor R3 is connected between the output end of the operational amplifier A and the base of the transistor T1, the collector of the transistor T1 and the inverting end "-" of the operational amplifier A are connected to the resistor RT to be measured, the collector output current of the transistor T is the same as the current flowing through R1, and the voltage signal VT across RT is the converted value of the RT resistance.

[0011] Preferably, the relationship between the resistance value R(T) of RT, the value V(T) of VT, the output voltage value Vs(0) of the voltage source and the resistance value R1(0) of R1 is as follows:

[0012]

[0013] Preferably, for the thermal resistor PT1000 with a variation range of 980-1200Ω, it is necessary to construct a mirror circuit with the same structure as the resistance value conversion circuit.

[0014] Preferably, the mirror circuit includes a resistor R3` connected between the output end of the operational amplifier A` and the base of the transistor T1`, the collector of the transistor T1` and the inverting end "—" of the operational amplifier A` face the measuring resistor RT(0), the collector output current of the transistor T is the same as the current flowing through R1`, the parameters of the operational amplifier A` are the same as the parameters of the operational amplifier A, and R1`=R1, R3`=R3.

[0015] Preferably, when the measured signal does not change, the value of RT is RT(0). When the RT value changes from RT(0) to RT(0)+△R, the relationship between the output voltage value Vo(0) and the resistance value R1(0) of R1 is as follows:

[0016]

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The measured resistance is supplied with constant current, which eliminates the conversion error caused by constant voltage bridge power supply and ensures accurate measurement.

[0019] 2. Single power supply and signal difference output reduce voltage complexity and noise introduced by power supply;

[0020] 3. The transistor-driven constant current power supply has a large output signal adjustment margin and can amplify the signal to be measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The circuit diagram of the resistance value conversion of the present invention is shown in FIG.

[0022] Figure 2 The circuit diagram of the resistance change value conversion of the present invention is shown in FIG.

[0023] Figure 3 This is a schematic diagram of a constant voltage bridge.

[0024] Figure 4 It is a comparison diagram of the conversion error of the circuit of the present invention and the bridge conversion error. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0027] It should also be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an" and "the" are intended to include plural forms unless the context clearly indicates otherwise.

[0028] It should be further understood that the term "and / or" used in the present description and the appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0029] In the first embodiment, the thermal resistor is RT100:

[0030] In order to solve the problem of excessive conversion error in the bridge constant voltage circuit, the following is proposed Figure 1The resistance value conversion circuit shown in the figure includes a constant current source generated by a reference voltage source VR, an operational amplifier A and resistors (R1, R3); the transistor T and the resistor to be measured RT have the same driving current as the constant current source, and the voltage value generated by this current at both ends of the resistor to be measured RT is the conversion value of the resistor to be measured RT. The reference voltage source VR outputs a constant voltage V S , the voltage is input to the non-inverting terminal "+" of operational amplifier A, and resistor R1 is connected between the inverting terminal "-" of operational amplifier A and the ground, and the current flowing through R1 remains constant. Resistor R3 is connected between the output terminal of operational amplifier A and the base of transistor T1, and the collector of transistor T1 and the inverting terminal "-" of operational amplifier A are connected to the resistor RT to be measured. The collector output current of transistor T is the same as the current flowing through R1, and the voltage signal VT across RT is the conversion value of RT resistance. The relationship between the resistance value R(T) of RT and the value V(T) of VT, the output voltage value Vs(0) of the voltage source and the resistance value R1(0) of R1 is as follows:

[0031]

[0032] Transistor T can be selected as 9013, operational amplifier A can be selected as AD8028, the resistor R3 between the output terminal of operational amplifier A and the base of transistor T1 can be selected as 1000Ω, the output voltage of reference voltage source VR can be selected as 0.4V, constant VS, this voltage is input to the in-phase terminal "+" of operational amplifier A, the resistor R1 connected between the inverting terminal "-" of operational amplifier A and ground can be selected as 100Ω, VCC is powered by a single power supply of 3.3V, then the voltage variation range on RT can reach 3.3V-0.3V-0.4V=2.5V, and the constant current flowing through RT is: 400mV / 100Ω=10mA, then the maximum resistance value of RT is: 3.3V / 10mA=330Ω

[0033] Therefore, the parameter values ​​in this example fully meet the needs of accurate conversion of the full range of thermal resistors such as PT100, and have the following advantages:

[0034] 1. Constant current 10mA can amplify the value of PT100 resistance by 10 times. For example, the conversion value of 101.11 (Ω) resistance is 1011.1 (mV).

[0035] 2. The output conversion value range is (0.4V ~ 2.8V), which can be directly connected to most digital-to-analog converters without the need for a special amplifier circuit.

[0036] Embodiment 2: Embodiment 2 is different from Embodiment 1 in that the thermal resistor used is a thermal resistor PT1000 with a variation range of 980-1200Ω.

[0037] refer to Figure 2For the thermal resistor PT1000 with a variation range of 980-1200Ω, in addition to the resistance value conversion circuit in the first embodiment, it is also necessary to construct a mirror circuit with the same structure as the resistance value conversion circuit. The mirror circuit includes a resistor R3' connected between the output end of the operational amplifier A' and the base of the transistor T1', the collector of the transistor T1' and the inverting end "-" of the operational amplifier A' facing the measured resistor RT(0), the collector output current of the transistor T is the same as the current flowing through R1', the parameters of the operational amplifier A' are the same as the parameters of the operational amplifier A, and R1'=R1, R3'=R3, the value of RT when the measured signal does not change is RT(0), then when the RT value changes from RT(0) to RT(0)+△R, the relationship between the output voltage value Vo(0) and the resistance value R1(0) of R1 is as follows:

[0038]

[0039] Transistors T and T` can be selected from 9013; operational amplifiers A and A` can be selected from AD8028; Vs = 15V; R1 = R1` = 100Ω; RT(0) = R3 = R3` = 1000Ω;

[0040] When the thermal resistance value of the thermal resistor PT1000 is 1119.99 (Ω), △R = 119.99 (Ω), the output value of the mirror conversion circuit is 1199.9, the output voltage is 1.1999mV, and it can also be indirectly connected to the AD input terminal of the 3.3V power supply.

[0041] Those of ordinary skill in the art will appreciate that the units of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition of each example has been generally described in terms of function in the above description. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0042] In the embodiments provided in the present application, it should be understood that the division of units is merely a logical function division, and there may be other division methods in actual implementation, for example, multiple units may be combined into one unit, one unit may be split into multiple units, or some features may be ignored.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. A resistance value conversion circuit, comprising: a constant current source generated by a constant current element; The current element generates the same driving current as the constant current source, which is used to convert the resistance value into the voltage value of the resistance to be measured. The voltage value generated by this current across the two ends of the resistance to be measured is the conversion value of the resistance to be measured; The constant current element includes a reference voltage source VR, an operational amplifier A and resistors (R1, R3); the current element includes a transistor T and a resistor to be measured RT; the reference voltage source VR outputs a constant voltage V S , the voltage is input to the in-phase terminal "+" of the operational amplifier A, a resistor R1 is connected between the inverting terminal "-" of the operational amplifier A and the ground, and the current flowing through R1 is kept constant; a resistor R3 is connected between the output terminal of the operational amplifier A and the base of the transistor T1, the collector of the transistor T1 and the inverting terminal "-" of the operational amplifier A are connected to the resistor RT to be measured, the collector output current of the transistor T is the same as the current flowing through R1, and the voltage signal VT across RT is the conversion value of the RT resistance value; it is characterized in that: the resistor RT to be measured adopts a thermal resistor PT1000, the variation range of the thermal resistor is 980-1200Ω, and a mirror circuit with the same structure as the resistance value conversion circuit is constructed; The mirror circuit includes a resistor R3` connected between the output end of the operational amplifier A` and the base of the transistor T1`, the collector of the transistor T1` and the inverting end "-" of the operational amplifier A` facing the measuring resistor RT(0), the collector output current of the transistor T is the same as the current flowing through R1`, the parameters of the operational amplifier A` are the same as the parameters of the operational amplifier A, and it can be obtained that R1`=R1, R3`=R3; When the measured signal does not change, the value of RT is RT(0). When the RT value changes from RT(0) to RT(0)+△R, the relationship between the output voltage value Vo(0) and the resistance value R1(0) of R1 is as follows:

Images