A calibrable absorbance measurement circuit

Abstract

A calibrable absorbance measurement circuit comprises a signal detection unit, a signal amplification unit and a signal compensation unit; the signal detection unit comprises an incident light detection circuit, a transmitted light detection circuit and a logarithmic operation circuit; the incident light detection circuit is used to detect the light intensity of the incident light; the transmitted light detection circuit is used to detect the light intensity of the transmitted light; the first input end of a logarithmic amplifier IC2 of the logarithmic operation circuit is connected to the output end of the incident light detection circuit via a resistor R3 and an adjustable resistor P1; the second input end of the logarithmic amplifier IC2 is connected to the output end of the transmitted light detection circuit via a resistor R4; the second input end of the logarithmic amplifier IC2 is connected to the output end via a capacitor C3; the output end of the logarithmic amplifier IC2 is connected to the signal amplification unit; the output end of the signal amplification unit is connected to the signal compensation unit; the absorbance is output through the signal compensation unit.

Description

Technical Field

[0001] The invention relates to the technical field of absorbance measurement, and in particular to a calibrable absorbance measurement circuit. Background Art

[0002] Absorbance: refers to the logarithm of the ratio of the incident light intensity before the light passes through a solution or a substance to the transmitted light intensity after the light passes through the solution or the substance (i.e. Log(I0 / I1)), where I0 is the incident light intensity and I1 is the transmitted light intensity. Absorbance is a physical quantity used to measure the degree to which light is absorbed. Determining the concentration of the target component by measuring absorbance is an important means in the field of modern analytical instruments. The absorption coefficient is related to the wavelength of the incident light and the properties of the material through which the light passes. Due to the different properties of different substances, the spectra they can absorb are also different. Therefore, after determining the substance to be tested, you only need to select a light source corresponding to the spectrum that the substance to be tested can absorb, and then you can calculate the content of the substance to be tested by absorbance.

[0003] The existing technical solutions for measuring absorbance usually adopt the traditional AD sampling circuit plus single-chip microcomputer calculation solution. The voltage signals of the incident light and the reference light are sampled by the high-precision AD sampling circuit, and then sent to the single-chip microcomputer, and then the absorbance is calculated by software programming. This solution is not only complex to implement, but also has high hardware and software development costs. At the same time, the noise interference introduced by the AD sampling circuit and the distortion of the sampling waveform will greatly affect the actual measurement accuracy. Summary of the invention

[0004] The purpose of the present invention is to provide a calibrable absorbance measurement circuit in view of the deficiencies in the prior art. The circuit structure is simple, and absorbance measurement can be achieved through a pure analog signal. Zero point calibration of absorbance measurement can be achieved through an adjustable resistor, and the requirements under different working conditions can be met, thereby greatly improving the versatility of the measurement circuit.

[0005] The technical solution of the present invention is: a calibrable absorbance measurement circuit, comprising a signal detection unit, a signal amplification unit, and a signal compensation unit;

[0006] The signal detection unit includes an incident light detection circuit, a transmitted light detection circuit, and a logarithmic operation circuit. The incident light detection circuit includes a photodiode D1 and an operational amplifier IC1A. The anode of the photodiode D1 and the positive input terminal of the operational amplifier IC1A are grounded together, the cathode of the photodiode D1 is connected to the reverse input terminal of the operational amplifier IC1A, and a resistor R1 and a capacitor C1 are arranged in parallel between the reverse input terminal and the output terminal of the operational amplifier IC1A; the transmitted light detection circuit includes a photodiode D2 and an operational amplifier IC1B. The anode of the photodiode D2 and the positive input terminal of the operational amplifier IC1B are grounded together, and the cathode of the photodiode D2 is connected to the reverse input terminal of the operational amplifier IC1A. The inverse input terminal of the operational amplifier IC1B is connected, and a resistor R2 and a capacitor C2 are arranged in parallel between the inverse input terminal and the output terminal of the operational amplifier IC1A; the positive power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are connected to the positive power supply, and the negative power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are connected to the analog ground; the logarithmic operation circuit includes a logarithmic amplifier IC2, a first input terminal of the logarithmic amplifier IC2 is connected to the output terminal of the operational amplifier IC1A via a resistor R3 and an adjustable resistor P1, a second input terminal of the logarithmic amplifier IC2 is connected to the output terminal of the operational amplifier IC1B via a resistor R4, and a second input terminal of the logarithmic amplifier IC2 is connected to the output terminal via a capacitor C3;

[0007] The output end of the logarithmic amplifier IC2 is connected to the signal amplification unit, and the output end of the signal amplification unit is connected to the signal compensation unit, and the absorbance value is outputted through the signal compensation unit.

[0008] Furthermore, the signal amplification unit includes a filtering circuit and a signal amplification circuit, the positive input terminal of the operational amplifier IC3 of the filtering circuit is connected to the output terminal of the logarithmic amplifier IC2 via resistors R6 and R5, and the positive input terminal of the operational amplifier IC3 is connected to the analog ground via capacitor C5, the reverse input terminal of the operational amplifier IC3 is connected to the output terminal, and the reverse input terminal of the operational amplifier IC3 is connected to the node between resistors R5 and R6 via capacitor C4, the positive power supply terminal of the operational amplifier IC3 is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply; the positive input terminal of the operational amplifier IC4 of the signal amplification circuit is connected to the output terminal of the operational amplifier IC3, an adjustable resistor P2 is connected between the reverse input terminal and the output terminal of the operational amplifier IC4, a capacitor C6 is connected in parallel at both ends of the adjustable resistor P2, and the reverse input terminal of the operational amplifier IC4 is connected to the analog ground via resistor R7, the positive power supply terminal of the operational amplifier IC4 is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply.

[0009] Furthermore, the signal compensation unit includes an operational amplifier IC8 and an adder IC7B, the positive input terminal of the operational amplifier IC8 is connected to the adjustable end of the adjustable resistor P3, one end of the adjustable resistor P3 is connected to a +5V power supply, and the other end is connected to a -5V power supply, the reverse input terminal of the operational amplifier IC8 is connected to the output terminal, the positive power supply terminal of the operational amplifier IC8 is connected to a positive power supply, and the negative power supply terminal is connected to a negative power supply, the positive input terminal of the adder IC7B is connected to the output terminal of the signal amplification unit via a resistor R10, and the positive input terminal of the adder IC7B is connected to the output terminal of the operational amplifier IC8 via a resistor R11, the reverse input terminal of the adder IC7B is connected to the output terminal via a resistor R12, and the reverse input terminal of the adder IC7B is also connected to an analog ground via a resistor R13, the positive power supply terminal of the adder IC7B is connected to a positive power supply, and the negative power supply terminal is connected to a negative power supply, and the absorbance is output through the output terminal of the adder IC7B.

[0010] Furthermore, a filtering circuit is arranged between the output end of the signal compensation unit and the resistor R10, the positive input end of the operational amplifier IC7A of the filtering circuit is connected to the output end of the signal unit via resistors R9 and R8 connected in series in sequence, and the positive input end of the operational amplifier IC7A is also connected to the analog ground via resistor R21, the reverse input end of the operational amplifier IC7A is connected to the output end, and the reverse input end of the operational amplifier IC7A is connected to the node between resistors R8 and R9 via resistor R20, the positive power supply end of the operational amplifier IC7A is connected to the positive power supply, and the negative power supply end is connected to the negative power supply.

[0011] Furthermore, the resistance values ​​of the resistor R10, the resistor R11, the resistor R12, and the resistor R13 are the same.

[0012] Furthermore, capacitors C24 and C25 are connected in parallel between the adjustable end of the adjustable resistor P3 and the positive input end of the operational amplifier IC8, and the capacitors C24 and C25 are connected to the analog ground together.

[0013] Furthermore, the output ends of the operational amplifier IC1A and the operational amplifier IC1B are respectively connected to a voltage follower circuit, wherein the first voltage follower circuit includes an operational amplifier IC5, the positive input end of the operational amplifier IC5 is connected to the output end of the operational amplifier IC1A, and the reverse input end of the operational amplifier IC5 is connected to the output end; the second voltage follower circuit includes an operational amplifier IC6, the positive input end of the operational amplifier IC6 is connected to the output end of the operational amplifier IC1B, and the reverse input end of the operational amplifier IC6 is connected to the output end; the positive power supply ends of the operational amplifier IC5 and the operational amplifier IC6 are respectively connected to the positive power supply and are respectively connected to the analog ground via capacitors; and the negative power supply ends are respectively connected to the negative power supply and are respectively connected to the analog ground via capacitors.

[0014] Furthermore, the positive power supply terminal of the logarithmic amplifier IC2 is connected to the positive power supply, and is connected to the analog ground through the parallel capacitors C12 and C13. The negative power supply terminal of the logarithmic amplifier IC2 is connected to the negative power supply, and is connected to the analog ground through the parallel capacitors C14 and C15. The capacitors C12 and C14 are both electrolytic capacitors.

[0015] Furthermore, the positive power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are respectively connected to the analog ground via capacitors.

[0016] Furthermore, the positive power supply terminal and the negative power supply terminal of the operational amplifier are respectively connected to the analog ground via capacitors.

[0017] The above technical solution is adopted: when the measuring circuit is in operation, the incident light detection circuit detects the light intensity before the light passes through the substance to be measured, the incident light passes through the photodiode D1 to form a photocurrent, the photocurrent passes through the resistor R1 in the incident light detection circuit to obtain a reference voltage Vref, and the reference voltage Vref corresponds to the light intensity of the incident light, the transmitted light circuit detects the light intensity after the light passes through the substance to be measured, the transmitted light passes through the photodiode D2 to form a photocurrent, the photocurrent passes through the resistor R2 in the transmitted light detection circuit to obtain a signal voltage Vsignal, and the reference voltage Vsig nal corresponds to the intensity of the transmitted light; then the reference voltage Vref is passed through the adjustable resistor P1 and the resistor R3 through the formula I1=Vref / (P1+R3) to obtain the reference current I1, and the signal voltage Vsignal is passed through the resistor R4 through the formula I2=Vsignal / R4 to obtain the signal current I2. The reference current I1 and the signal current I2 are respectively used as the two inputs of the logarithmic amplifier IC2. The logarithmic amplifier IC2 can calculate the logarithm of the ratio of the incident light intensity to the transmitted light intensity with a base of 10, that is, the absorbance value Vlog of the substance to be measured. And this measurement circuit can amplify the absorbance value Vlog of the substance to be measured through the signal amplification unit to meet different needs. In addition, the signal detection unit in this measurement circuit can introduce appropriate voltage compensation according to the concentration of the substance to be measured to improve the range of the detection circuit. Since the measuring circuit has an adjustable resistor P1 connected to the output end of the operational amplifier IC1A, before using the measuring circuit, the adjustable resistor P1 can be adjusted to make the signal current I2 when the transmitted light does not pass through the substance to be measured consistent with the reference current I1, thereby achieving the purpose of zero point calibration, thereby eliminating the measurement errors caused by human factors such as external conditions, optical path design or installation process, and can greatly improve the measurement accuracy of the detection circuit.

[0018] The signal amplification unit of the measurement circuit can filter the absorbance value Vlog by adopting a second-order active low-pass filter, filter out high-frequency interference in the absorbance value Vlog, and improve the signal-to-noise ratio. At the same time, the signal amplification unit can amplify the smaller absorbance value Vlog by adjusting the adjustable resistor P2 in the signal amplification circuit to meet different usage requirements and improve measurement accuracy.

[0019] When using this measuring circuit, according to the actual measurement needs, by adjusting the adjustable resistor P3 in the signal compensation unit, a voltage compensation of -5V to +5V is introduced. When the concentration of the substance to be measured is low, positive voltage compensation is selected to increase the output voltage and improve the signal-to-noise ratio. When the concentration of the substance to be measured is high, negative voltage compensation is selected to reduce the output voltage and increase the range.

[0020] This measurement circuit uses pure analog signals to measure absorbance, eliminating the A / D sampling chip, single-chip microcomputer and program design in the traditional detection circuit, and can avoid the influence of waveform distortion caused by insufficient sampling frequency and noise interference caused by the sampling circuit, making the fault tolerance and versatility of the measurement circuit higher, which can significantly save hardware costs and software development costs, and has the function of zero point calibration, which can greatly improve the measurement accuracy.

[0021] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a circuit diagram of a signal detection unit of the present invention;

[0023] Figure 2 is a circuit diagram of a signal amplifying unit of the present invention;

[0024] Figure 3 FIG. 4 is a circuit diagram of a signal compensation unit of the present invention. DETAILED DESCRIPTION

[0025] See also Figures 1 to 3 , an embodiment of a calibrable absorbance measurement circuit includes a signal detection unit, a signal amplification unit, and a signal compensation unit.

[0026] The signal detection unit includes an incident light detection circuit, a transmitted light detection circuit, and a logarithmic operation circuit. The incident light detection circuit is used to detect the light intensity before the light passes through the substance to be tested, the transmitted light detection circuit is used to detect the light intensity after the light passes through the substance to be tested, and the logarithmic operation circuit is used to calculate the absorbance value.

[0027] The incident light detection circuit includes a photodiode D1 and an operational amplifier IC1A. The anode of the photodiode D1 and the forward input terminal of the operational amplifier IC1A are grounded together, the cathode of the photodiode D1 is connected to the reverse input terminal of the operational amplifier IC1A, and a resistor R1 and a capacitor C1 are arranged in parallel between the reverse input terminal and the output terminal of the operational amplifier IC1A. The incident light forms a photocurrent through the photodiode D1, and the photocurrent obtains a reference voltage Vref through the resistor R1. The reference voltage Vref is filtered by the capacitor C1.

[0028] The transmitted light detection circuit includes a photodiode D2 and an operational amplifier IC1B. The anode of the photodiode D2 and the positive input terminal of the operational amplifier IC1B are grounded together, the cathode of the photodiode D2 is connected to the reverse input terminal of the operational amplifier IC1B, and a resistor R2 and a capacitor C2 are arranged in parallel between the reverse input terminal and the output terminal of the operational amplifier IC1A. The transmitted light forms a photocurrent through the photoelectric pole D2, and the photocurrent passes through the resistor R2 to obtain a signal voltage Vsignal, and the signal voltage Vsignal is filtered by the capacitor C2. The positive power supply terminals of the operational amplifiers IC1A and IC1B are connected to the positive power supply, and the positive power supply terminals are connected to the analog ground through the capacitor, and the ripple of the power supply is reduced by the capacitor. The negative power supply terminals of the operational amplifiers IC1A and IC1B are connected to the analog ground. The operational amplifiers IC1A and IC1B can be integrated into a dual operational amplifier chip, or independent operational amplifiers can be used.

[0029] The logarithmic operation circuit includes a logarithmic amplifier IC2, a first input end of the logarithmic amplifier IC2 is connected to the output end of the operational amplifier IC1A via a resistor R3 and an adjustable resistor P1, so that a reference voltage Vref passes through the adjustable resistors P1 and R3 to obtain a reference current I1, the reference current I1 corresponds to the light intensity of the incident light, and the value of the adjustable resistor P1 is a zero point calibration value, a second input end of the logarithmic amplifier IC2 is connected to the output end of the operational amplifier IC1B via a resistor R4, so that a signal current I2 can be obtained after a signal voltage Vsignal passes through the resistor R4, and a second input end of the logarithmic amplifier IC2 is connected to the output end via a capacitor C3, the logarithmic amplifier IC2 adopts LOG101A, and the logarithm with a base of 10 of the ratio of the reference current I1 to the signal current I2, that is, the absorbance value Vlog of the substance to be measured can be calculated by the logarithmic amplifier IC2, and the calculation formula is as follows:

[0030]

[0031] I1=Vref / (P1+R3) (2)

[0032] I2=Vsignal / R4 (3)

[0033] The positive power supply terminal of the logarithmic amplifier IC2 is connected to the positive power supply, and is connected to the analog ground through capacitors C12 and C13 connected in parallel. The negative power supply terminal of the logarithmic amplifier IC2 is connected to the negative power supply, and is connected to the analog ground through capacitors C14 and C15 connected in parallel. The capacitors C12 and C14 are both electrolytic capacitors. Since the capacitance of the electrolytic capacitor is relatively large, this embodiment achieves the purpose of filtering out the ripple of the power supply by respectively setting a large capacitor and a small capacitor in parallel at the positive power supply terminal and the negative power supply terminal of the logarithmic amplifier IC2, so as to improve the calculation accuracy of the absorbance value.

[0034] The output end of the logarithmic amplifier IC2 is connected to the signal amplification unit, and the absorbance value Vlog output by the logarithmic amplifier IC2 is amplified by the signal amplification unit, and the absorbance value of the small signal is amplified to a required multiple, thereby improving the measurement accuracy of the measurement circuit. The output end of the signal amplification unit is connected to the signal compensation unit, and appropriate voltage compensation is introduced through the signal compensation unit to output the final processed absorbance value, thereby improving the range of the measurement circuit.

[0035] In this embodiment, the signal amplification unit includes a filtering circuit and a signal amplification circuit. The positive input terminal of the operational amplifier IC3 of the filtering circuit is connected to the output terminal of the logarithmic amplifier IC2 via resistors R6 and R5, and the positive input terminal of the operational amplifier IC3 is connected to the analog ground via capacitor C5. The reverse input terminal of the operational amplifier IC3 is connected to the output terminal, and the reverse input terminal of the operational amplifier IC3 is connected to the node between the resistors R5 and R6 via capacitor C4. The positive power supply terminal of the operational amplifier IC3 is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply, and the positive power supply terminal and the negative power supply terminal are respectively connected to the analog ground via capacitors. The ripples of the positive power supply and the negative power supply are respectively filtered out by capacitors. The operational amplifier IC3, the resistor R5, the resistor R6, the capacitor C4, and the capacitor C5 constitute a second-order active low-pass filter to filter out high-frequency interference in the absorbance measurement value Vlog and improve the signal-to-noise ratio. Its transfer function is:

[0036]

[0037]

[0038]

[0039] In the formula, ω n is the cut-off angular frequency;

[0040] ξ1 is the damping coefficient of the filter circuit;

[0041] Vlog2 is the output signal of the filter circuit.

[0042] The positive input terminal of the operational amplifier IC4 of the signal amplification circuit is connected to the output terminal of the operational amplifier IC3, the adjustable resistor P2 is connected between the reverse input terminal and the output terminal of the operational amplifier IC4, a capacitor C6 is connected in parallel at both ends of the adjustable resistor P2, and the reverse input terminal of the operational amplifier IC4 is connected to the analog ground via the resistor R7, the positive power supply terminal of the operational amplifier IC4 is connected to the positive power supply, the negative power supply terminal is connected to the negative power supply, and the positive power supply terminal and the negative power supply terminal are respectively connected to the analog ground via the capacitor, the operational amplifier IC4, the resistor R7, and the adjustable resistor P2 constitute a signal amplification circuit, and the absorbance measurement value after filtering by the filtering circuit is amplified according to the following formula, and the calculation formula is as follows:

[0043]

[0044] In the formula, P2 is the resistance value of the adjustable resistor P2, which is determined according to the actual required amplification factor;

[0045] V0 is the output value of the signal amplifier circuit.

[0046] Since the absorbance measurement value output by the logarithmic amplifier IC2 is usually a small signal, this amplifier circuit can adjust the signal amplification factor by adjusting the resistance of the adjustable resistor P2 in the circuit according to actual needs, so that the absorbance measurement value can be amplified to a corresponding multiple to meet different actual needs.

[0047] In this embodiment, the signal compensation unit includes an operational amplifier IC8 and an adder IC7B. The positive input terminal of the operational amplifier IC8 is connected to the adjustable end of the adjustable resistor P3. One end of the adjustable resistor P3 is connected to a +5V power supply, and the other end is connected to a -5V power supply. The reverse input terminal of the operational amplifier IC8 is connected to the output terminal. The positive power supply terminal of the operational amplifier IC8 is connected to a positive power supply, and the negative power supply terminal is connected to a negative power supply. The positive power supply terminal and the negative power supply terminal are connected to an analog ground via capacitors, respectively. The operational amplifier IC8 acts as a voltage follower, so that the output value of the operational amplifier IC8 is equal to The voltage compensation introduced by the adjustable resistor P3, the positive input end of the adder IC7B is connected to the output end of the signal amplification unit via the resistor R10, and the positive input end of the adder IC7B is connected to the output end of the operational amplifier IC8 via the resistor R11, the reverse input end of the adder IC7B is connected to the output end via the resistor R12, and the reverse input end of the adder IC7B is also connected to the analog ground via the resistor R13, the positive power supply end of the adder IC7B is connected to the positive power supply, and the negative power supply end is connected to the negative power supply, and the final processed absorbance value is output through the output end of the adder IC7B.

[0048] The present measuring circuit is not limited to the above-mentioned embodiment. When the signal amplifying unit and the signal compensating unit are respectively arranged on two PCB circuit boards, a filtering circuit can be arranged between the output end of the signal compensating unit and the resistor R10. The positive input end of the operational amplifier IC7A of the filtering circuit is connected to the output end of the signal unit via the resistors R9 and R8 connected in series in sequence, and the positive input end of the operational amplifier IC7A is also connected to the analog ground via the capacitor C21. The reverse input end of the operational amplifier IC7A is connected to the output end, and the reverse input end of the operational amplifier IC7A is connected to the resistor R10 via the capacitor C20. R8 and the node between the resistor R9, the positive power supply terminal of the operational amplifier IC7A is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply, and the operational amplifier IC7A and the adder IC7B can be integrated in the same dual operational amplifier chip, or can be implemented by separate operational amplifiers. The operational amplifier IC7A and the adder IC7B of this embodiment are integrated in a dual operational amplifier chip, and the operational amplifier IC7A and the resistor R8, the resistor R9, the capacitor C20, and the capacitor C21 form a second-order active low-pass filter to filter the output value V0 of the signal amplifier circuit, and its transfer function is as follows:

[0049]

[0050]

[0051]

[0052] In the formula, ω m is the cut-off angular frequency of the filter circuit;

[0053] ξ2 is the damping coefficient of the filter circuit;

[0054] V3 is the output signal after filtering by the filter circuit.

[0055] When the signal compensation unit is in use, by adjusting the resistance of the adjustable resistor P3 in the circuit, a voltage in the range of -5V to +5V can be selected as the compensation voltage value V1. When the concentration of the substance to be measured is low, positive voltage compensation is selected to increase the output voltage and improve the signal-to-noise ratio. When the concentration of the substance to be measured is high, negative voltage compensation is selected to reduce the output voltage and increase the range. The compensation voltage value V1 can obtain a voltage V4 after passing through the operational amplifier IC8, and V1=V4. According to the output value V0 of the signal amplification circuit and the voltage V4, the absorbance value after the final signal compensation is calculated by the adder IC7B. The calculation formula is as follows:

[0056]

[0057] Wherein, V4 is the output signal of operational amplifier IC8, i.e., the voltage compensation value;

[0058] Vout is the absorbance value of the substance to be tested after treatment;

[0059] In this embodiment, the resistance values ​​of the resistors R10, R11, R12 and R13 are set to be the same, that is, R10=R11=R12=R13, thereby Vout=V3+V4.

[0060] Combining formulas (1) to (11), the absorbance value Vout of the substance to be tested after treatment can be obtained:

[0061]

[0062] In this embodiment, capacitors C24 and C25 are connected in parallel between the adjustable end of the adjustable resistor P3 and the positive input end of the operational amplifier IC8. The capacitors C24 and C25 are commonly connected to the analog ground. The compensation voltage value V1 is coupled to the ground through the capacitors C24 and C25 to filter out high-frequency interference of the +5V and -5V power supplies, thereby ensuring the accuracy of the measurement circuit.

[0063] In another embodiment of the measurement circuit, the output terminals of the operational amplifier IC1A and the operational amplifier IC1B are respectively connected to a voltage follower circuit, wherein the first voltage follower circuit comprises an operational amplifier IC5, the positive input terminal of the operational amplifier IC5 is connected to the output terminal of the operational amplifier IC1A, the reverse input terminal of the operational amplifier IC5 is connected to the output terminal, the voltage follower circuit is formed by the operational amplifier IC5, the output voltage signal REF is equal to the reference voltage Vref output by the operational amplifier IC1A, the intensity of the incident light can be monitored in real time by the voltage signal REF output by the operational amplifier IC5, and the attenuation of the light source can be judged by the intensity of the incident light, so that the light source can be replaced in time when the attenuation is serious, thereby ensuring the measurement accuracy; the second The voltage follower circuit includes an operational amplifier IC6, the positive input terminal of the operational amplifier IC6 is connected to the output terminal of the operational amplifier IC1B, and the reverse input terminal of the operational amplifier IC6 is connected to the output terminal. The operational amplifier IC6 constitutes another voltage follower circuit, and the voltage signal SIGNAL outputted by the operational amplifier IC6 is equal to the signal voltage Vsignal. The voltage signal SIGNAL can monitor the pollution of the optical path in real time, so that the staff can timely inspect and repair each component in the optical path according to the pollution of the optical path, and ensure the accuracy of the output transmitted light intensity. The positive power supply terminals of the operational amplifier IC5 and the operational amplifier IC6 are respectively connected to the positive power supply, and the negative power supply terminals are respectively connected to the negative power supply, and the positive power supply terminal and the negative power supply terminal are respectively connected to the analog ground through capacitors. This embodiment respectively leads to the reference voltage Vref and the signal voltage Vsignal through the voltage follower circuit, which can reduce the output impedance, thereby achieving the purpose of remotely monitoring the attenuation of the light source and the pollution of the optical path, and at the same time plays a role of buffering and isolation between the incident light detection circuit, the transmitted light detection circuit and the external system.

[0064] Before the operation of this measuring circuit, when there is no substance to be measured in the optical path, the intensity of the incident light should be the same as the intensity of the transmitted light. By detecting whether the reference current I1 is equal to the signal current I2, if I1=I2, it means that the detection circuit has been calibrated and there is no need to adjust the adjustable resistor P1. If I1≠I2, it means that there is a measurement error in the detection circuit. At this time, the resistance value of the adjustable resistor P1 in the circuit can be adjusted to make I1=I2, thereby achieving the purpose of zero point calibration, eliminating the measurement error caused by human factors such as design and installation of the detection circuit, and improving the measurement accuracy.

[0065] When the measuring circuit is in operation, the incident light detection circuit detects the light intensity before the light passes through the substance to be measured, and the incident light forms a photocurrent through the photodiode D1. The photocurrent passes through the resistor R1 in the incident light detection circuit to obtain a reference voltage Vref, and then the reference voltage Vref passes through the adjustable resistor P1 and the resistor R3 to obtain a reference current I1. The transmitted light circuit detects the light intensity after the light passes through the substance to be measured, and the transmitted light forms a photocurrent through the photodiode D2. The photocurrent passes through the resistor R2 in the transmitted light detection circuit to obtain a signal voltage Vsignal, and the signal voltage Vsignal passes through the resistor R4 to obtain a signal current I2. The reference current I1 and the signal current I2 are respectively used as the two inputs of the logarithmic amplifier IC2 The signal is obtained by calculating the logarithm of the ratio of the reference current I1 to the signal current I2 with the base 10, i.e. the absorbance measurement value Vlog of the substance to be measured, through the logarithmic amplifier IC2. Then, the absorbance value Vlog is filtered by the second-order active low-pass filter of the signal amplification unit. At the same time, according to the required amplification factor, the resistance of the adjustable resistor P2 in the signal amplification circuit is adjusted. The absorbance value Vlog of the substance to be measured is amplified by the signal amplification circuit to obtain V0, so as to meet different needs. Then, a suitable voltage compensation is introduced to V0 through the signal compensation unit to obtain the final processed absorbance value Vout. The voltage compensation can be achieved by adjusting the adjustable resistor P3 according to the concentration of the substance to be measured.

[0066] The structure of the measuring circuit is simple. It measures absorbance through pure analog signals, eliminating the A / D sampling chip, single-chip microcomputer and program design in the traditional detection circuit. It can avoid the waveform distortion caused by insufficient sampling frequency and the noise interference caused by the sampling circuit, so that the fault tolerance and versatility of the measuring circuit are higher, and the hardware cost and software development cost can be significantly saved. Moreover, the measuring circuit can realize the function of zero point calibration through the adjustable resistor P1 of the signal detection unit, thereby eliminating the measurement error caused by external conditions, optical path design or installation process and other human factors. By adjusting the adjustable resistor P2 of the signal amplification unit, the smaller absorbance value Vlog can be amplified to obtain the amplified signal V0 to meet different usage requirements, and according to the actual measurement requirements, the appropriate voltage compensation can be introduced through the adjustable resistor P2 in the signal compensation unit, thereby greatly improving the measurement accuracy of the measuring circuit. The measuring circuit can also monitor the attenuation of the light source and the pollution of the optical path in real time, so that the staff can replace the light source in time, and can timely inspect and repair the components of the optical path, to ensure the long-term stable operation of the entire detection circuit.

Claims

1. A calibratable absorbance measurement circuit, characterized in that: It includes a signal detection unit, a signal amplification unit, and a signal compensation unit; The signal detection unit includes an incident light detection circuit, a transmitted light detection circuit, and a logarithmic operation circuit. The incident light detection circuit includes a photodiode D1 and an operational amplifier IC1A. The anode of the photodiode D1 and the positive input terminal of the operational amplifier IC1A are grounded together, the cathode of the photodiode D1 is connected to the reverse input terminal of the operational amplifier IC1A, and a resistor R1 and a capacitor C1 are arranged in parallel between the reverse input terminal and the output terminal of the operational amplifier IC1A; the transmitted light detection circuit includes a photodiode D2 and an operational amplifier IC1B. The anode of the photodiode D2 and the positive input terminal of the operational amplifier IC1B are grounded together, and the cathode of the photodiode D2 is connected to the reverse input terminal of the operational amplifier IC1A. The inverse input terminal of the operational amplifier IC1B is connected, and a resistor R2 and a capacitor C2 are arranged in parallel between the inverse input terminal and the output terminal of the operational amplifier IC1A; the positive power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are connected to the positive power supply, and the negative power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are connected to the analog ground; the logarithmic operation circuit includes a logarithmic amplifier IC2, a first input terminal of the logarithmic amplifier IC2 is connected to the output terminal of the operational amplifier IC1A via a resistor R3 and an adjustable resistor P1, a second input terminal of the logarithmic amplifier IC2 is connected to the output terminal of the operational amplifier IC1B via a resistor R4, and a second input terminal of the logarithmic amplifier IC2 is connected to the output terminal via a capacitor C3; The output end of the logarithmic amplifier IC2 is connected to the signal amplification unit, and the output end of the signal amplification unit is connected to the signal compensation unit, and the absorbance value is output through the signal compensation unit; The signal amplification unit includes a filtering circuit and a signal amplification circuit. The positive input terminal of the operational amplifier IC3 of the filtering circuit is connected to the output terminal of the logarithmic amplifier IC2 via resistors R6 and R5, and the positive input terminal of the operational amplifier IC3 is connected to the analog ground via capacitor C5. The reverse input terminal of the operational amplifier IC3 is connected to the output terminal, and the reverse input terminal of the operational amplifier IC3 is connected to the node between the resistors R5 and R6 via capacitor C4. The positive power supply terminal of the operational amplifier IC3 is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply; and the positive power supply terminal and the negative power supply terminal are respectively connected to the analog ground via capacitors. The positive input terminal of the operational amplifier IC4 of the signal amplification circuit is connected to the output terminal of the operational amplifier IC3, and an adjustable resistor P2 is connected between the reverse input terminal and the output terminal of the operational amplifier IC4, and a capacitor C6 is connected in parallel at both ends of the adjustable resistor P2, and the reverse input terminal of the operational amplifier IC4 is connected to the analog ground via resistor R7, and the positive power supply terminal of the operational amplifier IC4 is connected to the positive power supply, and the negative power supply terminal is connected to the negative power supply; The signal compensation unit includes an operational amplifier IC8 and an adder IC7B. The positive input terminal of the operational amplifier IC8 is connected to the adjustable end of the adjustable resistor P3. One end of the adjustable resistor P3 is connected to a +5V power supply, and the other end is connected to a -5V power supply. The reverse input terminal of the operational amplifier IC8 is connected to the output terminal. The positive power supply terminal of the operational amplifier IC8 is connected to a positive power supply, and the negative power supply terminal is connected to a negative power supply. The positive power supply terminal and the negative power supply terminal are connected to analog grounds via capacitors, respectively. The operational amplifier IC8 acts as a voltage follower, so that the output value of the operational amplifier IC8 is equal to the adjustable resistor P3. The voltage compensation introduced, the positive input end of the adder IC7B is connected to the output end of the signal amplification unit via a resistor R10, and the positive input end of the adder IC7B is connected to the output end of the operational amplifier IC8 via a resistor R11, the reverse input end of the adder IC7B is connected to the output end via a resistor R12, and the reverse input end of the adder IC7B is also connected to the analog ground via a resistor R13, the positive power supply end of the adder IC7B is connected to the positive power supply, and the negative power supply end is connected to the negative power supply, and the final processed absorbance value is output through the output end of the adder IC7B.

2. The calibratable absorbance measurement circuit according to claim 1, characterized in that: A filter circuit is arranged between the output end of the signal compensation unit and the resistor R10, the positive input end of the operational amplifier IC7A of the filter circuit is connected to the output end of the signal unit via the series-connected resistors R9 and R8 in sequence, and the positive input end of the operational amplifier IC7A is also connected to the analog ground via the capacitor C21, the reverse input end of the operational amplifier IC7A is connected to the output end, and the reverse input end of the operational amplifier IC7A is connected to the node between the resistors R8 and R9 via the capacitor C20, the positive power supply end of the operational amplifier IC7A is connected to the positive power supply, and the negative power supply end is connected to the negative power supply.

3. The calibratable absorbance measurement circuit according to claim 1, characterized in that: The resistance values ​​of the resistors R10, R11, R12 and R13 are the same.

4. The calibratable absorbance measurement circuit according to claim 1, characterized in that: A capacitor C24 and a capacitor C25 are connected in parallel between the adjustable end of the adjustable resistor P3 and the positive input end of the operational amplifier IC8 , and the capacitors C24 and C25 are connected to the analog ground together.

5. The calibratable absorbance measurement circuit according to claim 1, characterized in that: The output ends of the operational amplifier IC1A and the operational amplifier IC1B are respectively connected to a voltage follower circuit, wherein the first voltage follower circuit includes an operational amplifier IC5, the positive input end of the operational amplifier IC5 is connected to the output end of the operational amplifier IC1A, and the reverse input end of the operational amplifier IC5 is connected to the output end; the second voltage follower circuit includes an operational amplifier IC6, the positive input end of the operational amplifier IC6 is connected to the output end of the operational amplifier IC1B, and the reverse input end of the operational amplifier IC6 is connected to the output end; the positive power supply ends of the operational amplifier IC5 and the operational amplifier IC6 are respectively connected to the positive power supply and are respectively connected to the analog ground via capacitors; and the negative power supply ends are respectively connected to the negative power supply and are respectively connected to the analog ground via capacitors.

6. The calibratable absorbance measurement circuit according to claim 1, characterized in that: The positive power supply terminal of the logarithmic amplifier IC2 is connected to the positive power supply and is connected to the analog ground through parallel capacitors C12 and C13. The negative power supply terminal of the logarithmic amplifier IC2 is connected to the negative power supply and is connected to the analog ground through parallel capacitors C14 and C15. The capacitors C12 and C14 are both electrolytic capacitors.

7. The calibratable absorbance measurement circuit according to claim 1, characterized in that: The positive power supply terminals of the operational amplifier IC1A and the operational amplifier IC1B are connected to analog grounds via capacitors respectively.

8. The calibratable absorbance measurement circuit according to any one of claims 1, 2 and 5, characterized in that: The positive power supply terminal and the negative power supply terminal of the operational amplifier are connected to the analog ground via capacitors respectively.

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