CN217738500U - A conditioning circuit for a piezoresistive pressure sensor - Google Patents

Abstract

The utility model discloses a piezoresistive pressure sensor's conditioning circuit belongs to sensor technical field. The design comprises an amplifying circuit design for outputting signals to a pressure sensor, wherein the amplifying circuit design adopts three operational amplifiers OP77 to form an instrument amplifier module, a front-stage two operational amplifier circuit mainly carries out interference removal and preliminary amplification on the signals, a rear-stage operational amplifier carries out high-gain amplification on the signals, one pin of two input ends of the instrument amplifier is connected with the output signals of a sensor chip, the other end of the two input ends of the instrument amplifier is connected with a voltage regulating circuit, a sliding rheostat is used for carrying out voltage division on the voltages, and then proper voltage is taken out and input to the input end of the amplifying circuit to serve as a clamping voltage for fixing the output voltage range of the module. Compared with the prior art, this application still designs the power supply and the circuit temperature drift suppression of circuit to pressure sensor temperature characteristic, and the measuring error that effectual suppression temperature arouses in the circuit compares in software compensation, and hardware circuit temperature compensation is more convenient and real-time compensation.

Description

A conditioning circuit for a piezoresistive pressure sensor

technical field

The utility model relates to the technical field of sensors, in particular to a conditioning circuit of a piezoresistive pressure sensor.

Background technique

Sensors play a very important role in the fields of automatic detection and automatic control systems, and their function is to sense and collect signals. For example, in the relay valve brake system test bench, the most important thing is to realize the electro-pneumatic signal conversion. As an important part of the pressure sensor module, the accuracy and stability of the signal are very important. The original output signal of the piezoresistive pressure sensor is a weak signal, which is easily interfered by noise signals, and is not conducive to subsequent acquisition and utilization. At the same time, it is easily affected by temperature during the test. Therefore, suitable conditioning methods are needed to optimize solutions to these characteristics. At present, in the design of the conditioning circuit of the pressure sensor, more attention is paid to its stability. Most of the commonly used ones are temperature compensation or gain compensation on the software algorithm, and most of them are relatively complicated and the compensation accuracy is limited.

The amplification gain of the sensor output signal is also very important. When sampling and outputting the signal, the radio frequency interference between the sensor and the multi-stage amplifier circuit and the offset voltage at the input terminal of the amplifier also need to be considered.

Utility model content

Aiming at the deficiencies of the prior art, the utility model proposes a conditioning circuit for a piezoresistive pressure sensor. The temperature compensation is carried out on the hardware circuit, and the influence of the temperature on the input signal of the sensor is eliminated by selecting a suitable power supply circuit, thereby ensuring the stability of the signal acquisition, and further improving the amplification gain module of the system, and the instrument amplifier is composed of three operational amplifiers The circuit is used to enhance the acquisition signal, and through the filter circuit, effectively filter the radio frequency interference between the sensor and the instrument amplifier; and select the ADC analog-to-digital conversion circuit, so as to collect the amplified signal of the sensor and provide it to the single chip microcomputer for processing and display .

The purpose of the utility model can be realized through the following technical solutions: including instrument amplifier module, power supply circuit and ADC signal acquisition circuit;

The instrumentation amplifier module adopts a three-stage op-amp circuit design, the pre-stage op-amp circuit de-interferences and initially amplifies the signal, and the latter stage op-amp circuit performs high-gain amplification on it;

The instrumentation amplifier module includes a voltage regulating circuit, an instrumentation amplifier, a sliding rheostat, a low-pass filter circuit, and a switching diode. One input terminal and one pin of the two op-amp circuits in the front stage of the instrumentation amplifier are connected to the output signal of the sensor chip. The other input end of the two operational amplifier circuits in the preceding stage of the instrumentation amplifier is connected to a voltage regulating circuit, and the output terminals of the two operational amplifier circuits in the preceding stage of the instrumentation amplifier are connected to the two inputs of the rear stage operational amplifier circuits of the instrumentation amplifier. terminal connection, the instrumentation amplifier is a differential input, the two input terminals of the two operational amplifier circuits in the front stage of the instrumentation amplifier are connected to a low-pass filter circuit, and the rear-stage operational amplifier circuit of the instrumentation amplifier is connected to a switching diode.

Further, the power supply circuit is the power input circuit of the instrumentation amplifier module, and the input power signal is subjected to anti-interference processing to generate various power supply signals required by the instrumentation amplifier module;

The power supply circuit includes a power conversion module. The 12V power supply is input to the power conversion module, and the input voltage signal is converted through a fixed voltage three-terminal integrated voltage regulator to output a 5V voltage. The output terminal of the power conversion module is connected to the constant current The source circuit is connected, and the peripheral circuit of the constant current source circuit is connected with two resistors and a diode to form a temperature compensation circuit, and a resistor and a diode are connected in parallel with the other resistor.

Further, the ADC signal acquisition circuit is used to acquire and convert the output signal of the pressure sensor module; the ADC signal acquisition circuit is connected to the output terminal of the instrumentation amplifier module, and the analog-to-digital conversion chip used in the ADC signal acquisition circuit , supports the acquisition of 8-channel signals, and converts the output voltage of the instrumentation amplifier.

Further, the voltage regulating circuit uses a sliding rheostat to divide the voltage, and then takes out a suitable voltage and inputs it to the input terminal of the instrumentation amplifier module as a clamping voltage, which is used to fix the output voltage range of the module.

Further, the instrumentation amplifier module adopts a symmetrical design, which is used to suppress the interference of the common mode signal and suppress the voltage imbalance.

Furthermore, the switching diode protects the output signal and sets a range for the output voltage to prevent damage to the previous stage circuit, resulting in damage to the lower circuit due to excessive output voltage. After adding the switching diode to stabilize the voltage, when the output voltage is too large, it will The diode is turned on so that no voltage signal is given to the lower circuit.

Further, the instrumentation amplifier needs positive and negative power supplies. The instrumentation amplifier module uses a chip that inverts the voltage to provide negative voltage. The 5V voltage signal output by the fixed voltage three-terminal integrated voltage regulator is connected to the chip circuit that inverts the voltage. At the input end, the polarity of the voltage is converted by the chip that inverts the voltage so that the output voltage of the instrumentation amplifier module is equal to the voltage at the input end of the chip that inverts the voltage.

Furthermore, before connecting to the constant current source, the circuit provides a 5V power supply voltage through the linear regulated power supply 78L05 chip circuit. The chip used in the design of the constant current source circuit is LM134. According to the chip characteristic manual, the current and temperature generated by the chip It is related, so when designing the current source, it is necessary to design the peripheral circuit to eliminate the influence of temperature. It is realized by adding a resistor and a diode. According to the temperature coefficient calculation of the diode, the temperature drift of the diode 1N457 and LM134 can cancel each other out. In addition, the pressure sensor MPX5999D selected in this design has its own temperature compensation function in the range of 0-85 °C. Therefore, the two together cancel the effect of temperature on the pressure signal.

Further, there is a split-board design between the instrumentation amplifier module and the ADC signal acquisition circuit, and the design of the integrated circuit and the external environment will form certain interference signals; the instrumentation amplifier module is connected with a low-pass filter circuit at the signal input part, The low-pass filter circuit adopts a second-order RC low-pass filter circuit. The second-order RC low-pass filter circuit attenuates the input high-frequency signal, thereby ensuring that the low-frequency signal can pass through stably.

The beneficial effects of the utility model:

(1) According to the temperature characteristics of the pressure sensor, the power supply of the circuit and the suppression of the temperature drift of the circuit are designed, which can effectively suppress the measurement error caused by the temperature in the circuit, and compared with the software compensation, the hardware circuit temperature compensation is more convenient And it can be compensated in real time.

(2) In order to better ensure the validity of the signal of the pressure sensor, the instrument amplifier composed of three operational amplifiers is designed to amplify the signal, which is a differential amplifier circuit with a very high common-mode rejection ratio. The generated radio frequency interference signal is suppressed in the amplifying circuit part, and a variable resistor is added to the input end to complete the control of the multiple of the amplifying circuit, which effectively solves the problem of low output precision and susceptible In case of external interference and low conversion gain, protective measures are added to the output to prevent abnormal output from affecting the use of lower-level circuits.

(3) In order to ensure that the single-chip microcomputer can collect accurate pressure signals, a 12-bit high-precision AD7606 sampling chip is designed, which has low energy consumption and can complete error correction without external adjustment. Its internal reference voltage is used in the design, and no additional reference voltage circuit is needed to ensure the integration and miniaturization of the circuit board. And considering the external interference and aliasing phenomenon, the second-order RC circuit is used to filter the signal at high frequency, which can effectively control the module cost while ensuring the working accuracy requirements.

Description of drawings

Below in conjunction with accompanying drawing, the utility model is further described.

Fig. 1 is the flow chart diagram of whole conditioning circuit design of the present application;

Fig. 2 is the schematic diagram of the circuit design of the power supply part of the present application;

Fig. 3 is a schematic diagram of a voltage inversion circuit inside the circuit of the present application;

Fig. 4 is the circuit design schematic diagram of the instrumentation amplifier module that three operational amplifiers of the present application are formed;

Fig. 5 is the A/D analog-to-digital conversion circuit diagram of the present application;

Fig. 6 is the 8-way second-order RC filtering circuit diagram of the present application;

FIG. 7 is a schematic diagram of the conditioning circuit of the present application for soldering a PCB.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. example. Based on the embodiments of the present utility model, all other embodiments obtained by persons of ordinary skill in the art without creative efforts belong to the scope of protection of the present utility model.

In a specific embodiment, when designing the piezoresistive pressure sensor signal acquisition module, it is first necessary to select the sensors and other chips used in the module. After the selection of the sensor chip is completed, it is necessary to design its driver Circuit and signal conversion circuit, as shown in Figure 1, the main part of the design lies in the design of the constant current source power supply and the effective amplification of the sensor signal by the instrumentation amplifier.

The power supply circuit is the power input circuit of the module. After the input power signal is subjected to anti-interference processing, various power signals required by the circuit module are generated. The processed power signal and the voltage signal converted by the sensor are further processed through the instrument amplifier to obtain the voltage signal. Then pass it to the single-chip microcomputer for processing through the AD analog-to-digital conversion module, and finally display the real-time pressure curve on the host computer. The working mode of the module will be described in detail below.

Under the condition that the total power supply is normal, the 12V power supply is input to the power conversion module, and the input voltage signal is converted by 78L05, and the output voltage is 5V. Compared with the direct power supply of the general constant voltage source, it is difficult to eliminate the temperature of the circuit. Therefore, this design connects the output terminal of 78L05 to the constant current source circuit, processes it through the LM134 chip, and then adds a diode through two resistors in its peripheral circuit to form a temperature compensation circuit design. By bringing in the temperature coefficient of the diode (-2.5mV /°C) calculation and analysis, when the circuit satisfies the following formula, it means that the temperature drift of LM134 and its peripheral circuit is offset, please refer to attached drawing 2, the circuit in Figure 2 is composed of two parts, namely the total 5V power output circuit and the sensor current source power supply circuit.

Among them, I ₁ is the current flowing through R ₁₁ , which is approximately expressed as the current flowing through the diode, and I ₂ is the current flowing through R _12. When the following formula is satisfied, plus the selected sensor chip can have a certain range Temperature compensation, both of which together eliminate the influence of temperature on the pressure signal.

In addition, the power supply part needs a negative polarity power supply for the op amp, so it is necessary to connect the 5V voltage signal output by the 78L05 to the input terminal of the chip ICL7660 circuit. The polarity of the voltage can be converted through the ICL7660 so that VOUT=-VIN, as shown in the attached As shown in Figure 3, the peripheral electrolytic capacitors E ₁ and E ₂ are external energy storage capacitors of the chip, and C ₉ and C ₁₀ are filter capacitors.

The LM314 outputs a constant current signal to supply power to the sensor. Further, the signal generated by the sensor by measuring the air pressure is transmitted to the instrument amplifier circuit module. The instrument amplifier is a differential input, and one of the negative input ports (IN1) is connected to a sliding rheostat and then connected to a 5v power supply. , so as to input an adjustable clamping voltage to the port to control the output range of the entire module. The other end of the amplifier (IN2) is connected to the sensor signal input, and the low-pass filter circuit design is connected to the two ports first to filter out radio frequency interference signals. , the entire amplifying circuit adopts a symmetrical design, that is, the internal resistors R ₃ ＝R ₇ , R ₄ ＝R ₆ , R ₂ ＝R ₈ , R ₉ ＝R ₁₀ , C ₁ ＝C ₃ , and R ₀ are used to adjust the instrument amplifier The magnification factor, the role of C ₂ is to reduce the matching requirements of the two low-pass filter cut-off frequencies, and its value is about 10 times that of C _1. The calculation of the output voltage and the calculation of the differential bandwidth and common-mode bandwidth are as follows, The operational amplifiers at the two input ends of the module perform basic amplification first, and make the input impedance of the entire instrumentation amplifier circuit tend to be infinite. Therefore, compared with ordinary operational amplifier circuits, the instrumentation amplifier circuit used in this design has high input impedance and high Excellent characteristics such as common mode rejection ratio, and also suppresses radio frequency interference on the circuit, which improves the accuracy and stability of the output signal. Into MMBD7000, MMBD7000 consists of two diodes, when the output voltage is too large, the diode will be turned on to prevent the output voltage from damaging the lower circuit. In the following formula, Vout ₁ , Vout ₂ , Vin ₁ , Vin ₂ , and Vout are respectively represented as the output voltage of U ₁ and U ₂ shown in Figure 1, the input voltage and the output voltage of the entire instrumentation amplifier, D ₁ represents the differential bandwidth, D ₂ represents the common mode bandwidth, please refer to Figure 2.

Furthermore, after the amplifier is output, the signal is connected to a second-order filter circuit. Due to the design of the integrated circuit and the external environment, certain interference signals will be formed, and its output information is also easily affected by noise. Therefore, in this design, the The signal input part is designed with a second-order RC low-pass filter to ensure that low-frequency signals can pass through stably. As shown in Figure 4, the design is symmetrical, that is, the values of the capacitors and resistors in the upper and lower parts are the same. In order to compare with the ADC sampling, it involves a total of 8 input and output channels, as shown in Figure 6, the input ports corresponding to the analog-to-digital conversion circuit are subjected to a high-frequency filtering process on eight input ports, and then lead to the ADC module.

In another step, the filtered signal is input to the ADC7606 module for analog-to-digital conversion, and finally calculated and output to the microcontroller for the next step of processing and display. As shown in Figure 5, after the sensor signal is amplified, the output port of the instrument sensor is connected to AD7606 for selective analog-to-digital conversion, which can simultaneously support the input conversion of 8 sensor signals. The input and output calculations are shown below.

In the formula, Vout represents the voltage signal output by the instrumentation amplifier, U _ADC represents the reference voltage of the ADC, the number of digits is N, and the measurement range is D ₁ .

As shown in Figure 7, it is a schematic diagram of boarding of the conditioning circuit, in which there are front and back parts, which are convenient for packaging for integration and miniaturization.

In the description of the present invention, it should be understood that the terms "opening", "upper", "lower", "thickness", "top", "middle", "length", "inner", "surrounding" Indicating orientations or positional relationships, etc., are only for the convenience of describing the utility model and simplifying the description, rather than indicating or implying that the components or elements referred to must have a specific orientation, and be constructed and operated in a specific orientation, so it cannot be understood as a reference to the utility model. New types of restrictions.

In the description of this specification, descriptions referring to the terms "one embodiment", "example", "specific example" and the like mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in the description of the present invention. In at least one embodiment or example. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

The basic principles, main features and advantages of the present utility model have been shown and described above. Those skilled in the industry should understand that the utility model is not limited by the above-mentioned embodiments. The above-mentioned embodiments and descriptions only illustrate the principle of the utility model. Without departing from the spirit and scope of the utility model, the utility model The new model also has various changes and improvements, and these changes and improvements all fall within the scope of the claimed utility model.

Claims (9)

1. a conditioning circuit of piezoresistive pressure sensor, is characterized in that, comprises instrumentation amplifier module, power supply circuit and ADC signal acquisition circuit; The instrumentation amplifier module adopts a three-op-amp design, the two op-amp circuits in the front stage are used to de-interference and initially amplify the signal, and the op-amp circuit in the rear stage is performing high-gain amplification on it; The instrumentation amplifier module includes a voltage regulating circuit, an instrumentation amplifier, a sliding rheostat, a low-pass filter circuit and a switching diode, and one input end and one pin of the two op-amp circuits in the front stage of the instrumentation amplifier are connected to the output signal of the sensor chip, so The other input end of the two operational amplifier circuits in the preceding stage of the instrumentation amplifier is connected to a voltage regulating circuit, and the output terminals of the two operational amplifier circuits in the preceding stage of the instrumentation amplifier are connected to the two inputs of the rear stage operational amplifier circuits of the instrumentation amplifier. terminal connection, the instrumentation amplifier is a differential input, the two input terminals of the two operational amplifier circuits in the front stage of the instrumentation amplifier are connected to a low-pass filter circuit, and the rear-stage operational amplifier circuit of the instrumentation amplifier is connected to a switching diode. 2. The conditioning circuit of a kind of piezoresistive pressure sensor according to claim 1, is characterized in that, described power supply circuit is the power supply input circuit of instrumentation amplifier module, and the power supply signal of input produces instrumentation amplifier module after anti-interference processing Various power signals required; The power supply circuit includes a power conversion module. The 12V power supply is input to the power conversion module, and the input voltage signal is converted through a fixed voltage three-terminal integrated voltage regulator to output a 5V voltage. The output terminal of the power conversion module is connected to the constant current The source circuit is connected, and the peripheral circuit of the constant current source circuit is connected with two resistors and a diode to form a temperature compensation circuit, and a resistor and a diode are connected in parallel with the other resistor. 3. The conditioning circuit of a piezoresistive pressure sensor according to claim 1, wherein the ADC signal acquisition circuit is used to collect and convert the output signal of the pressure sensor module; the ADC signal acquisition circuit It is connected with the output end of the instrumentation amplifier module, and the analog-to-digital conversion chip adopted by the ADC signal acquisition circuit supports the acquisition of 8-channel signals, and converts the output voltage of the instrumentation amplifier. 4. The conditioning circuit of a piezoresistive pressure sensor according to claim 1, characterized in that, the voltage regulating circuit uses a sliding rheostat to divide the voltage, and then takes out a suitable voltage and inputs it to the input of the instrument amplifier module The terminal is used as a clamp voltage to fix the output voltage range of the module. 5 . The conditioning circuit of a piezoresistive pressure sensor according to claim 1 , wherein the instrumentation amplifier module adopts a symmetrical design, which is used to suppress interference of common-mode signals and suppress voltage imbalance. 6 . 6. A conditioning circuit for a piezoresistive pressure sensor according to claim 1, characterized in that the switching diode protects the output signal and defines a range for the output voltage. 7. The conditioning circuit of a piezoresistive pressure sensor according to claim 1, characterized in that the instrumentation amplifier needs positive and negative power supplies, and the instrumentation amplifier module uses a chip that reverses the voltage to provide a negative voltage and a fixed voltage The 5V voltage signal output by the three-terminal integrated voltage regulator is connected to the input terminal of the chip circuit for inverting the voltage, and the polarity of the voltage is converted through the chip for inverting the voltage so that VOUT=-VIN. 8. The conditioning circuit of a piezoresistive pressure sensor according to claim 1, wherein the piezoresistive pressure sensor includes a Wheatstone bridge inside, when the sensor uses a constant current source to supply power to the sensor. 9. the conditioning circuit of a kind of piezoresistive pressure sensor according to claim 1, it is characterized in that, between the instrument amplifier module and the ADC signal acquisition circuit is a sub-board, and the low-pass filter circuit adopts a second-order RC low-pass The filter circuit, the second-order RC low-pass filter circuit is to attenuate the input high-frequency signal.

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