A signal detection circuit for an electron multiplier

Abstract

The present invention relates to a signal detection circuit of an electron multiplier, comprising: an inductive coupling circuit, a signal conversion circuit, a signal amplifying circuit, a background noise subtraction circuit and a digital-to-analog conversion chip, wherein the signal output end of the electron multiplier is connected to the inductive coupling circuit, the inductive coupling circuit is connected to the signal conversion circuit, the signal conversion circuit is connected to the signal amplifying circuit, the signal amplifying circuit is connected to the background noise subtraction circuit, and the background noise subtraction circuit is connected to the digital-to-analog conversion chip. The present invention adopts inductive coupling to perform signal coupling, which can effectively avoid signal attenuation, improve the sensitivity of the mass spectrometer, ensure that the coupling inductor always works below the resonant frequency, avoid the circuit response drop caused by capacitive coupling, ensure the circuit response speed, and the authenticity of sample detection; adopt the digital-to-analog conversion chip to generate the background noise threshold voltage, ensure the accuracy of the threshold voltage and the accuracy of mass spectrometer signal detection.

Description

Technical Field

[0001] The invention relates to the technical field of signal detection of mass spectrometers, and in particular to a signal detection circuit of an electron multiplier. Background Art

[0002] A mass spectrometer is usually composed of several main parts, including an ion source system, an ion optical system, a radio frequency power supply system, and a signal detection system. The signal detection system of a mass spectrometer generally uses an electron multiplier as a detector. When the signal detection system is working, a DC voltage V is applied across the electron multiplier. CEM , a uniform axial electric field is established. After charged particles such as ions enter the low potential end of the electron multiplier, they collide with the inner surface of the tube and stimulate secondary electrons. CEM The generated axial electric field accelerates the generation of enough secondary electrons, which, after repeated collisions, produce a gain of 10 at the high potential end of the electron multiplier. 8 The electron beam is a pulse signal. The signal is coupled to the signal detection circuit through the coupling circuit. The signal detection circuit amplifies and compares the signal and then transmits it to the signal counting unit, and finally calculates the mass spectrometry signal intensity of the sample.

[0003] At present, the current signal output by the electron multiplier is generally in the form of capacitive coupling, and the DC blocking effect of the coupling capacitor is used to couple the pulsed current signal to the signal detection circuit for counting. However, when the capacitance of the coupling capacitor is too small, the low-frequency signal will be severely attenuated when passing through the coupling capacitor. When the concentration of the sample to be tested is low, the number of ion pulses that can be counted is small, the frequency is low, and there is signal attenuation in the case of capacitive coupling.

[0004] Usually, the signal processing circuit will set a threshold voltage to deduct the noise of the circuit itself and the noise generated by the dark current of the electron multiplier. The circuit will deduct the attenuated signal as noise, which will directly cause the instrument to be unable to detect the sample and affect the instrument's detection limit, that is, sensitivity; and when the coupling capacitor capacity is too large, the circuit will be delayed, affecting the response time of the detection circuit and unable to display the signal in real time. At the same time, when the capacity is too large, it will also generate a larger leakage current, and the noise of the subsequent signal detection circuit will also increase. The setting of the threshold voltage generally adopts the method of resistor voltage division. The accuracy of the threshold voltage generated by the resistor voltage division method is affected by the accuracy of the voltage divider resistor and the reference power supply. The threshold voltage is generally rough, affecting the accuracy of mass spectrometry signal detection. Summary of the invention

[0005] In order to achieve the above-mentioned purpose and other advantages according to the present invention, the purpose of the present invention is to provide a signal detection circuit of an electron multiplier, including: an inductive coupling circuit, a signal conversion circuit, a signal amplification circuit, a background noise subtraction circuit and a digital-to-analog conversion chip, the signal output end of the electron multiplier is connected to the inductive coupling circuit, the inductive coupling circuit is connected to the signal conversion circuit, the signal conversion circuit is connected to the signal amplification circuit, the signal amplification circuit is connected to the background noise subtraction circuit, and the background noise subtraction circuit is connected to the digital-to-analog conversion chip.

[0006] Furthermore, the inductive coupling circuit includes a coupling inductor and a first resistor, and the signal output end of the electron multiplier, the first resistor, and the signal conversion circuit are connected to the coupling inductor.

[0007] Furthermore, the coupling inductor adopts a first transformer, the signal output end of the electron multiplier is connected to the primary coil of the first transformer, the first resistor is connected between two terminals of the primary coil of the first transformer, and the signal conversion circuit is connected to the secondary coil of the first transformer.

[0008] Furthermore, the signal conversion circuit includes a second resistor and a third resistor, the second resistor is connected to the first terminal of the secondary coil of the first transformer, and the third resistor is connected to the second terminal of the secondary coil of the first transformer.

[0009] Furthermore, the signal amplifying circuit includes two paths, and each path of the signal amplifying circuit is a two-stage signal amplifying circuit.

[0010] Further, the two-stage signal amplification circuit includes a first operational amplifier and a second operational amplifier, wherein the non-inverting input terminal of the first operational amplifier in one signal amplification circuit is connected to the second resistor and the first terminal of the secondary coil of the first transformer, the non-inverting input terminal of the first operational amplifier in the other signal amplification circuit is connected to the third resistor and the second terminal of the secondary coil of the first transformer, the inverting input terminal of the first operational amplifier is connected to the resistor, the output terminal of the first operational amplifier is connected to the inverting input terminal of the first operational amplifier via the resistor, the output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier, the inverting input terminal of the second operational amplifier is connected to the resistor, the output terminal of the second operational amplifier is connected to the inverting input terminal of the second operational amplifier via the resistor, and the output terminal of the second operational amplifier is connected to the background noise subtraction circuit.

[0011] Furthermore, the background noise subtraction circuit includes a DC bias isolation sub-circuit, an inductive coupling circuit sub-circuit, a signal conversion sub-circuit, and a comparison sub-circuit. The DC bias isolation sub-circuit is connected to the output end of the second operational amplifier, the inductive coupling circuit sub-circuit is connected to the DC bias isolation sub-circuit, the DC bias isolation sub-circuit is connected to the comparison sub-circuit, and the comparison sub-circuit is connected to the digital-to-analog conversion chip.

[0012] Furthermore, the DC bias isolation subcircuit includes a first capacitor and a second capacitor, the first capacitor is connected to the output end of the second operational amplifier in one of the signal amplification circuits, and the second capacitor is connected to the output end of the second operational amplifier in another signal amplification circuit.

[0013] Furthermore, the inductive coupling circuit subcircuit includes a second transformer, the signal conversion subcircuit includes a fourth resistor, the first terminal of the primary coil of the second transformer is connected to the first capacitor, the second terminal of the primary coil of the second transformer is connected to the second capacitor, and the fourth resistor is connected between two terminals of the secondary coil of the second transformer.

[0014] Furthermore, the comparison subcircuit includes a comparator, a negative input terminal of the comparator is connected to the first terminal of the primary coil of the second transformer, a positive input terminal of the comparator is connected to the digital-to-analog conversion chip, and an output terminal of the comparator and the digital-to-analog conversion chip are connected to a digital integrated circuit chip.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The invention provides a signal detection circuit for an electron multiplier. The inductive coupling method is adopted for signal coupling, which can effectively avoid signal attenuation and improve the sensitivity of a mass spectrometer. The inductive coupling method is adopted for signal coupling, which ensures that the coupling inductor always works below the resonance frequency, avoids the circuit response drop caused by the capacitive coupling, ensures the circuit response speed and the authenticity of the sample detection. The digital-to-analog conversion chip is adopted to generate a background noise threshold voltage, which ensures the accuracy of the threshold voltage and the accuracy of the mass spectrometer signal detection. The two-stage signal amplification form is adopted, which can effectively increase the bandwidth of the signal detection, improve the signal detection capability, and ensure that there is no signal loss during the sample detection.

[0017] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. The specific implementation of the present invention is given in detail by the following embodiments and their accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0019] Figure 1 It is a schematic diagram of the capacitive coupling principle;

[0020] Figure 2 The figure is a principle block diagram of a signal detection circuit of an electron multiplier;

[0021] Figure 3 The figure is a signal detection circuit diagram of an electron multiplier. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form a new embodiment.

[0023] Capacitive coupling principle Figure 1 As shown, the output signal S of the electron multiplier in It is the coupling of the electron beam signal (pulse signal) and the DC high voltage bias. If the DC high voltage bias is directly coupled to the subsequent signal detection circuit, it will inevitably burn out the components of the subsequent signal detection circuit. in Using coupling capacitor C 1 The DC isolation function blocks the DC high voltage bias and couples the electron beam signal (pulse signal) to the subsequent signal detection circuit. In this way, the pulse signal without DC high voltage bias is converted into a voltage signal S through resistor R1. out Used in signal detection circuits.

[0024] However, when using capacitive coupling, when the capacity of the coupling capacitor is too small, the low-frequency signal will be severely attenuated when passing through the coupling capacitor. When the concentration of the sample to be tested is low, the number of countable ion pulses is small and the frequency is low. In the case of capacitive coupling, there is signal attenuation, and the general signal processing circuit has a threshold voltage setting (noise deduction). The circuit will consider the attenuated signal as noise and deduct it, which directly causes the instrument to be unable to detect the sample and affects the instrument's detection limit, that is, sensitivity. When the capacity of the coupling capacitor is too large, the circuit will be delayed, affecting the response time of the detection circuit and making it impossible to display the signal in real time. At the same time, when the capacity is too large, it will also generate a larger leakage current, and the noise of the subsequent signal detection circuit will also increase.

[0025] In order to overcome the shortcomings of the above technical solutions, the present invention adopts the method of inductive coupling and digital-to-analog conversion chip to generate threshold voltage. Inductive coupling also has the function of isolating the pre-stage DC high voltage bias. In the ideal inductor, the impedance increases linearly with the increase of frequency, but in the actual inductor, there is an equivalent parallel capacitor, which forms a parallel resonant circuit with the inductor, and there is a resonant frequency. When it is less than the resonant frequency, the inductor shows inductive reactance, and when it exceeds the resonant frequency, it shows capacitive reactance. Generally, the maximum signal frequency of the mass spectrometer electron multiplier is about tens of megahertz, and the corresponding inductance value can be calculated to ensure that the resonant frequency is greater than the output signal frequency of the electron multiplier, so that the coupled inductor always shows inductive characteristics. In the case of inductive characteristics, the circuit will not have a situation similar to capacitive coupling where the signal is attenuated or responds slowly.

[0026] A signal detection circuit for an electron multiplier, such as Figure 2 As shown, it includes: an inductive coupling circuit, a signal conversion circuit, a signal amplifying circuit, a background noise subtraction circuit and a digital-to-analog conversion chip. The signal output end of the electron multiplier is connected to the inductive coupling circuit, the inductive coupling circuit is connected to the signal conversion circuit, the signal conversion circuit is connected to the signal amplifying circuit, the signal amplifying circuit is connected to the background noise subtraction circuit, the background noise subtraction circuit is connected to the digital-to-analog conversion chip, and the background noise subtraction circuit and the digital-to-analog conversion chip are connected to a digital integrated circuit chip such as an FPGA. The signal output by the electron multiplier is isolated from the DC high voltage bias through the inductive coupling circuit, and the pulse signal to be detected is coupled to the signal conversion circuit. The signal conversion circuit converts the current signal into a voltage signal, and the voltage signal is amplified to a processable amplitude through the signal amplifying circuit; the signal generated by the signal amplifying circuit is compared with the background noise threshold voltage generated by the digital-to-analog conversion chip, and the background noise in the signal is subtracted. The comparator outputs high and low level signals to the FPGA signal counting unit, and finally forms a mass spectrometry signal according to the counting. The present invention adopts inductive coupling to couple signals, which can effectively avoid signal attenuation, improve the sensitivity of mass spectrometer, ensure that the coupled inductor always works below the resonant frequency, avoid the circuit response drop caused by capacitive coupling, ensure the circuit response speed, and the authenticity of sample detection. The present invention adopts a digital-to-analog conversion chip to generate a background noise threshold voltage, ensuring the accuracy of the threshold voltage and the accuracy of mass spectrometer signal detection.

[0027] like Figure 3 As shown, the inductive coupling circuit includes a coupling inductor and a first resistor R12, and the signal output end of the electron multiplier, the first resistor R12, and the signal conversion circuit are connected to the coupling inductor.

[0028] In one embodiment, the coupled inductor adopts the first transformer T1, the signal output end of the electron multiplier is connected to the primary coil of the first transformer T1, the first resistor R12 is connected between the two terminals of the primary coil of the first transformer T1, and the signal conversion circuit is connected to the secondary coil of the first transformer T1.

[0029] The signal conversion circuit includes a second resistor R1 and a third resistor R6. The second resistor R1 is connected to a first terminal of the secondary coil of the first transformer T1, and the third resistor R6 is connected to a second terminal of the secondary coil of the first transformer T1.

[0030] The signal amplification circuit includes two paths, and each path of the signal amplification circuit is a two-stage signal amplification circuit. The two-stage signal amplification form can effectively increase the bandwidth of signal detection, improve the signal detection capability, and ensure that there is no signal loss during sample detection.

[0031] One of the two-stage signal amplification circuits includes a first operational amplifier U1 and a second operational amplifier U2, the non-inverting input terminal of the first operational amplifier U1 is connected to the second resistor R1 and the first terminal of the secondary coil of the first transformer T1, the inverting input terminal of the first operational amplifier U1 is connected to the resistor R2, the output terminal of the first operational amplifier U1 is connected to the inverting input terminal of the first operational amplifier U1 via the resistor R3, the output terminal of the first operational amplifier U1 is connected to the non-inverting input terminal of the second operational amplifier U2, the inverting input terminal of the second operational amplifier U2 is connected to the resistor R4, the output terminal of the second operational amplifier U2 is connected to the inverting input terminal of the second operational amplifier U2 via the resistor R5, and the output terminal of the second operational amplifier U2 is connected to the background noise subtraction circuit.

[0032] The other two-stage signal amplification circuit includes a first operational amplifier U3 and a second operational amplifier U4, wherein the non-inverting input terminal of the first operational amplifier U3 is connected to the third resistor R6 and the second terminal of the secondary coil of the first transformer T1, the inverting input terminal of the first operational amplifier U3 is connected to the resistor R7, the output terminal of the first operational amplifier U3 is connected to the inverting input terminal of the first operational amplifier U3 via the resistor R8, the output terminal of the first operational amplifier U3 is connected to the non-inverting input terminal of the second operational amplifier U4, the inverting input terminal of the second operational amplifier U4 is connected to the resistor R9, the output terminal of the second operational amplifier U4 is connected to the inverting input terminal of the second operational amplifier U4 via the resistor R10, and the output terminal of the second operational amplifier U4 is connected to the background noise subtraction circuit.

[0033] The background noise subtraction circuit includes a DC bias isolation subcircuit, an inductive coupling circuit subcircuit, a signal conversion subcircuit, and a comparison subcircuit. The DC bias isolation subcircuit is connected to the output end of the second operational amplifier, the inductive coupling circuit subcircuit is connected to the DC bias isolation subcircuit, the DC bias isolation subcircuit is connected to the comparison subcircuit, and the comparison subcircuit is connected to the digital-to-analog conversion chip.

[0034] The DC bias isolation subcircuit includes a first capacitor C1 and a second capacitor C2. The first capacitor C1 is connected to the output end of the second operational amplifier U2 in one of the signal amplification circuits, and the second capacitor C2 is connected to the output end of the second operational amplifier U4 in the other signal amplification circuit.

[0035] The inductive coupling circuit subcircuit includes a second transformer T2, the signal conversion subcircuit includes a fourth resistor R11, a first terminal of the primary coil of the second transformer T2 is connected to the first capacitor C1, a second terminal of the primary coil of the second transformer T2 is connected to the second capacitor C2, and the fourth resistor R11 is connected between two terminals of the secondary coil of the second transformer T2.

[0036] The comparison subcircuit includes a comparator U5, a negative input terminal of the comparator U5 is connected to the first terminal of the primary coil of the second transformer T2, a positive input terminal of the comparator U5 is connected to the digital-to-analog conversion chip U6, and an output terminal of the comparator U5 and the digital-to-analog conversion chip U6 are connected to a digital integrated circuit chip such as an FPGA.

[0037] The working principle of the signal detection circuit of the electron multiplier is:

[0038] The output signal of the electron multiplier forms a signal loop with the primary coil of the transformer T1 through the resistor R12, isolates the DC high voltage bias, and is coupled to the secondary coil of the transformer T1 through the transformer T1. The secondary coil of the transformer T1 is converted into voltage signals with opposite phases through the resistors R1 and R6 respectively. The two voltage signals are amplified by the two-stage signal amplification circuit respectively. The two-stage signal amplification can increase the bandwidth of the amplification circuit to ensure that the high-frequency signal is not distorted; the signal amplified by the signal amplification circuit isolates the DC bias output by the operational amplifier through C1 and C2 respectively, and couples the pulse signal to the primary coil of the transformer T2. After passing through the transformer T2, the signal is converted into a voltage signal through the resistor R11, and the signal is compared with the background noise threshold voltage. The noise generated by the circuit itself and the noise signal generated by the dark current of the electron multiplier are compared and subtracted at the comparator U5. The comparator outputs high and low level signals to the FPGA signal counting unit, and finally the FPGA calculates the mass spectrum signal according to the count. At the same time, the threshold voltage is generated by the FPGA-controlled digital-to-analog conversion chip AD5321, which has a precision of 12 bits and can ensure the accuracy of the threshold voltage.

[0039] In view of the fact that the signal frequency detected by the electron multiplier is generally around tens of megahertz, the signal amplification circuit uses the high-speed operational amplifier THS3202, whose bandwidth can reach 2GHz; the comparator uses ADCMP553, whose minimum detectable pulse width is 700ps.

[0040] The above description is only an embodiment of this specification and is not intended to limit one or more embodiments of this specification. For those skilled in the art, one or more embodiments of this specification may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of one or more embodiments of this specification shall be included in the scope of the claims of one or more embodiments of this specification. One or more embodiments of this specification One or more embodiments of this specification One or more embodiments of this specification One or more embodiments of this specification.

Claims

1. A signal detection circuit for an electron multiplier, It is characterized in that include: An inductive coupling circuit, a signal conversion circuit, a signal amplifying circuit, a background noise subtraction circuit and a digital-to-analog conversion chip, wherein the signal output end of the electron multiplier is connected to the inductive coupling circuit, the inductive coupling circuit is connected to the signal conversion circuit, the signal conversion circuit is connected to the signal amplifying circuit, the signal amplifying circuit is connected to the background noise subtraction circuit, the background noise subtraction circuit is connected to the digital-to-analog conversion chip, and the background noise subtraction circuit and the digital-to-analog conversion chip are connected to a digital integrated circuit chip; The signal output by the electron multiplier is isolated from the DC high voltage bias by an inductive coupling circuit, and the pulse signal to be detected is coupled to the signal conversion circuit. The signal conversion circuit converts the current signal into a voltage signal, and the voltage signal is amplified to a processable amplitude by the signal amplification circuit; the signal generated by the signal amplification circuit is compared with the background noise threshold voltage generated by the digital-to-analog conversion chip, and the background noise in the signal is subtracted. The background noise subtraction circuit outputs high and low level signals to the signal counting unit of the digital integrated circuit chip, and finally forms a mass spectrometry signal according to the count; The noise floor threshold voltage is generated by the FPGA-controlled digital-to-analog conversion chip AD5321, and the accuracy of the digital-to-analog conversion chip AD5321 is 12 bits; The signal amplification circuit adopts high-speed operational amplifier THS3202, and its bandwidth can reach 2GHz; The signal amplifying circuit includes two paths, and each path of the signal amplifying circuit is a two-stage signal amplifying circuit.

2. The signal detection circuit of an electron multiplier according to claim 1, Features: The inductive coupling circuit includes a coupling inductor and a first resistor. The signal output end of the electron multiplier, the first resistor, and the signal conversion circuit are connected to the coupling inductor.

3. The signal detection circuit of an electron multiplier according to claim 2, Features: The coupling inductor adopts a first transformer, the signal output end of the electron multiplier is connected to the primary coil of the first transformer, the first resistor is connected between two terminals of the primary coil of the first transformer, and the signal conversion circuit is connected to the secondary coil of the first transformer.

4. The signal detection circuit of an electron multiplier according to claim 3, Features: The signal conversion circuit includes a second resistor and a third resistor, the second resistor is connected to the first terminal of the secondary coil of the first transformer, and the third resistor is connected to the second terminal of the secondary coil of the first transformer.

5. The signal detection circuit of an electron multiplier according to claim 4, Features: The two-stage signal amplification circuit includes a first operational amplifier and a second operational amplifier, wherein the non-inverting input terminal of the first operational amplifier in one signal amplification circuit is connected to the second resistor and the first terminal of the secondary coil of the first transformer, the non-inverting input terminal of the first operational amplifier in the other signal amplification circuit is connected to the third resistor and the second terminal of the secondary coil of the first transformer, the inverting input terminal of the first operational amplifier is connected to the resistor, the output terminal of the first operational amplifier is connected to the inverting input terminal of the first operational amplifier via the resistor, the output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier, the inverting input terminal of the second operational amplifier is connected to the resistor, the output terminal of the second operational amplifier is connected to the inverting input terminal of the second operational amplifier via the resistor, and the output terminal of the second operational amplifier is connected to the background noise subtraction circuit.

6. The signal detection circuit of an electron multiplier according to claim 5, Features: The background noise subtraction circuit includes a DC bias isolation subcircuit, an inductive coupling circuit subcircuit, a signal conversion subcircuit, and a comparison subcircuit. The DC bias isolation subcircuit is connected to the output end of the second operational amplifier, the inductive coupling circuit subcircuit is connected to the DC bias isolation subcircuit, the DC bias isolation subcircuit is connected to the comparison subcircuit, and the comparison subcircuit is connected to the digital-to-analog conversion chip.

7. The signal detection circuit of an electron multiplier according to claim 6, Features: The DC bias isolation subcircuit includes a first capacitor and a second capacitor. The first capacitor is connected to the output end of the second operational amplifier in one of the signal amplification circuits, and the second capacitor is connected to the output end of the second operational amplifier in another signal amplification circuit.

8. The signal detection circuit of an electron multiplier according to claim 7, Features: The inductive coupling circuit subcircuit includes a second transformer, the signal conversion subcircuit includes a fourth resistor, a first terminal of the primary coil of the second transformer is connected to the first capacitor, a second terminal of the primary coil of the second transformer is connected to the second capacitor, and the fourth resistor is connected between two terminals of the secondary coil of the second transformer.

9. The signal detection circuit of an electron multiplier according to claim 8, Features: The comparison subcircuit includes a comparator, a negative input terminal of the comparator is connected to the first terminal of the primary coil of the second transformer, a positive input terminal of the comparator is connected to the digital-to-analog conversion chip, and an output terminal of the comparator and the digital-to-analog conversion chip are connected to a digital integrated circuit chip.

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