Current measuring circuit based on quadrupole mass spectrometer

Abstract

The invention discloses a current measuring circuit based on a quadrupole mass spectrometer, and the circuit comprises an input module which is used for receiving a current signal from the quadrupole mass spectrometer; the current detection module is used for carrying out current measurement on the current signal; wherein the current measurement module comprises a positive current measurement unit and a negative current measurement unit, the positive current measurement unit is used for measuring the positive current of the quadrupole mass spectrometer, and the negative current measurement unit is used for measuring the negative current of the quadrupole mass spectrometer; according to the current measurement circuit, small current linear amplification and large current logarithmic amplification are realized through the current measurement module, so that a high-range measurement effect is achieved, fA level-uA level current measurement can be realized, and meanwhile, automatic range switching is realized through circuit characteristics; and the cost is about one fiftieth of that of a reed switch range switching scheme.

Description

Technical Field

[0001] The present invention relates to the technical field of current measurement circuits, and in particular to a current measurement circuit based on a quadrupole mass spectrometer. Background Art

[0002] A quadrupole mass spectrometer is an analytical instrument widely used in fields such as biology, medicine, and environmental science. Its core function is to analyze gas composition by measuring the magnitude of ion flux. The mass spectrometer's measurement principle is based on the motion characteristics of ions in the quadrupole electric field. By applying specific DC and AC electric fields, ions of different mass-to-charge ratios are steadily transported through the field, achieving separation and detection.

[0003] Currently, there are two main types of measurement circuits for quadrupole mass spectrometers. One uses a reed switch to switch between feedback resistors of varying sizes (such as 500G, 15G, 470M, and 13G) to measure current. This circuit uses Ohm's law (I = U / R) to calculate current magnitude, offering high linearity and relatively accurate current measurement. However, this circuit is prone to noise interference during switching, and the requirement for multiple high-precision resistors and reed switches leads to high costs.

[0004] Another measurement circuit uses triode logarithmic amplification to achieve multi-level measurements. This solution exploits the triode's logarithmic amplification properties, enabling measurement over a wide current range. However, this circuit is susceptible to temperature drift, resulting in reduced measurement accuracy. Especially when measuring low currents (pA and below), its accuracy cannot meet the requirements of high-precision analysis.

[0005] In summary, the existing quadrupole mass spectrometer measurement circuit has the following technical problems: Although the reed switch resistor switching circuit has high linearity, it is prone to noise interference during gear shifting, affecting measurement accuracy, and is also expensive. The transistor logarithmic amplifier circuit is easily affected by temperature drift, resulting in unstable measurement accuracy, especially when measuring small currents, and cannot achieve ideal accuracy. Summary of the Invention

[0006] Based on the problems raised by the above background technology, the purpose of the present invention is to provide a current measurement circuit based on a quadrupole mass spectrometer, which solves the problem that although the circuit of the reed switch switching resistor has high linearity, it is easy to generate noise interference during gear switching, affecting the measurement accuracy, and has a high cost; the transistor logarithmic amplifier circuit is easily affected by temperature drift, resulting in unstable measurement accuracy, especially the problem that the ideal accuracy cannot be achieved when measuring small currents.

[0007] The present invention is achieved through the following technical solutions: The present invention provides a current measurement circuit based on a quadrupole mass spectrometer, comprising: an input module, for receiving a current signal from a quadrupole mass spectrometer; A current detection module, configured to perform current measurement on the current signal; wherein the current measurement module comprises a positive current measurement unit and a negative current measurement unit, the positive current measurement unit being configured to measure the positive current of the quadrupole mass spectrometer, and the negative current measurement unit being configured to measure the negative current of the quadrupole mass spectrometer; The output module is used to output the current measurement results.

[0008] In the above technical solution, linear amplification of small currents and logarithmic amplification of large currents are realized through the current measurement module to achieve a high-range measurement effect, which can realize fA-uA current measurement. At the same time, automatic range switching is realized through circuit characteristics, and the cost is about one-fiftieth of the reed switch range switching solution.

[0009] In an optional embodiment, the positive current measurement unit includes: a resistor R3, a resistor R4, a resistor R5, a resistor R6, a field effect transistor Q1, a field effect transistor Q2, a field effect transistor Q3, a field effect transistor Q4 and a capacitor C14; The resistor R3, the resistor R4, the resistor R5 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q1 are short-circuited and connected to the first end of the resistor R3, and the gate of the field effect transistor Q1 is connected to the second end of the resistor R3; the drain and source of the field effect transistor Q2 are short-circuited and connected to the first end of the resistor R4, and the gate of the field effect transistor Q2 is connected to the second end of the resistor R4; the drain and source of the field effect transistor Q3 are short-circuited and connected to the first end of the resistor R5, and the gate of the field effect transistor Q3 is connected to the second end of the resistor R5; the drain and source of the field effect transistor Q4 are short-circuited and connected to the first end of the resistor R6, and the gate of the field effect transistor Q4 is connected to the second end of the resistor R6.

[0010] In an optional embodiment, the positive current measurement unit further includes: an operational amplifier U2; The inverting input terminal of the operational amplifier U2 is connected to the first terminal of the feedback network, and the output terminal of the operational amplifier U2 is connected to the second terminal of the feedback network; The positive power supply port of the operational amplifier U2 is connected to a capacitor C2, and the negative power supply port of the operational amplifier U2 is connected to a capacitor C3.

[0011] In an optional embodiment, the operational amplifier U2, the resistor R3, the resistor R4, the resistor R5, the resistor R6, the field effect transistor Q1, the field effect transistor Q2, the field effect transistor Q3, the field effect transistor Q4, and the capacitor C14 implement I / V conversion, wherein the formula for I / V conversion is as follows:

[0012] In the above formula, is the op amp output voltage, To convert current.

[0013] In an optional embodiment, the positive current measurement unit further includes: a current source simulating ion flow RF1, a resistor R1, a resistor R2, a capacitor C1 and a resistor R23; The resistor R1 and the capacitor C1 form an RC filter, and the RC filter is used to reduce input interference; The resistor R1 and the resistor R2 are connected in series to provide a conduction path for the Faraday collection current; The 5th port of the current source simulating ion flow RF1 is connected to the resistor R1, and the 3rd and 4th ports of the current source simulating ion flow RF1 are short-circuited and connected to the non-inverting input terminal of the operational amplifier U2 and the resistor R23.

[0014] In an optional embodiment, the operational amplifier U2 is a dual-channel LMC66 CMOS operational amplifier.

[0015] In an optional embodiment, the field effect transistor Q1, the field effect transistor Q2, the field effect transistor Q3, and the field effect transistor Q4 are N-channel JFET field effect transistors of model MMBF4117.

[0016] In an optional embodiment, the negative current measurement unit includes: a resistor R14, a resistor R15, a resistor R16, a resistor R17, a field effect transistor Q6, a field effect transistor Q7, a field effect transistor Q8, a field effect transistor Q9 and a capacitor C14; The resistor R14, the resistor R15, the resistor R16 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q6 are short-circuited and connected to the second end of the resistor R14, and the gate of the field effect transistor Q6 is connected to the first end of the resistor R14; the drain and source of the field effect transistor Q7 are short-circuited and connected to the second end of the resistor R15, and the gate of the field effect transistor Q7 is connected to the first end of the resistor R15; the drain and source of the field effect transistor Q8 are short-circuited and connected to the second end of the resistor R16, and the gate of the field effect transistor Q8 is connected to the first end of the resistor R16; the drain and source of the field effect transistor Q9 are short-circuited and connected to the second end of the resistor R17, and the gate of the field effect transistor Q9 is connected to the first end of the resistor R17.

[0017] In an optional embodiment, the input module includes: a switch H1, a low leakage current precision switch U4, a resistor R11 and a field effect transistor Q5; The switch H1 is connected to the low leakage current precision switch U4 , and the low leakage current precision switch U4 , the resistor R11 , and the field effect transistor Q5 form a current source, which is used for zeroing and calibrating the current signal.

[0018] In an optional embodiment, the output module includes: a differential amplifier U3, a resistor R7, a resistor R8, a resistor R9, a resistor R10, a capacitor C4, a capacitor C5 and a capacitor C6; Wherein, the inverting input terminal of the differential amplifier U3 is connected to the resistor R7, and the non-inverting input terminal of the differential amplifier U3 is connected to the resistor R8; The capacitor C4 and the resistor R9 are connected in parallel between the inverting input terminal and the non-inverting output terminal of the differential amplifier U3; the capacitor C5 and the resistor R10 are connected in parallel between the non-inverting input terminal and the reverse output terminal of the differential amplifier U3; The capacitor C6 is connected to the positive power supply port of the differential amplifier U3.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention realizes fA-uA current measurement and realizes automatic range switching through circuit characteristics. The cost is about one fiftieth of the reed switch range switching solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the examples. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be considered as limiting the scope. A person of ordinary skill in the art can also derive other relevant drawings based on these drawings without inventive effort. In the drawings: Figure 1 A schematic structural diagram of a current measurement circuit based on a quadrupole mass spectrometer provided in Example 1 of the present invention; Figure 2 A schematic structural diagram of a positive current measurement circuit provided in Example 1 of the present invention; Figure 3 Schematic diagram of linear coordinate I / V conversion for positive current measurement provided by Example 1 of the present invention; Figure 4 Schematic diagram of logarithmic coordinate I / V conversion for positive current measurement provided by Example 1 of the present invention; Figure 5 A schematic structural diagram of a negative current measurement circuit provided in Example 1 of the present invention; Figure 6 Schematic diagram of the linear coordinate I / V conversion for negative current measurement provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with examples and drawings. The exemplary embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0022] Example 1 provides a circuit diagram of a current measurement circuit based on a quadrupole mass spectrometer, as shown in FIG. Figure 1 As shown, the current measurement circuit based on the quadrupole mass spectrometer includes: an input module, for receiving a current signal from a quadrupole mass spectrometer; A current detection module, configured to perform current measurement on the current signal; wherein the current measurement module comprises a positive current measurement unit and a negative current measurement unit, the positive current measurement unit being configured to measure the positive current of the quadrupole mass spectrometer, and the negative current measurement unit being configured to measure the negative current of the quadrupole mass spectrometer; The output module is used to output the current measurement results.

[0023] It should be noted that the current measurement circuit provided in this embodiment achieves a high-range measurement effect by realizing linear amplification of small currents and logarithmic amplification of large currents through the current measurement module, and can realize fA-uA current measurement. At the same time, automatic range switching is realized through circuit characteristics, and the cost is about one-fiftieth of the reed switch range switching solution.

[0024] In an optional embodiment, the positive current measurement unit includes: a resistor R3, a resistor R4, a resistor R5, a resistor R6, a field effect transistor Q1, a field effect transistor Q2, a field effect transistor Q3, a field effect transistor Q4 and a capacitor C14; The resistor R3, the resistor R4, the resistor R5 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q1 are short-circuited and connected to the first end of the resistor R3, and the gate of the field effect transistor Q1 is connected to the second end of the resistor R3; the drain and source of the field effect transistor Q2 are short-circuited and connected to the first end of the resistor R4, and the gate of the field effect transistor Q2 is connected to the second end of the resistor R4; the drain and source of the field effect transistor Q3 are short-circuited and connected to the first end of the resistor R5, and the gate of the field effect transistor Q3 is connected to the second end of the resistor R5; the drain and source of the field effect transistor Q4 are short-circuited and connected to the first end of the resistor R6, and the gate of the field effect transistor Q4 is connected to the second end of the resistor R6.

[0025] It should be noted that resistors R3, R4, R5, and R6 are linear feedback resistors, forming a 1GΩ feedback resistor in series. Field-effect transistors Q1, Q2, Q3, and Q4 are N-channel JFETs. Shorting the drain and source of these devices forms a PN junction with the gate, giving them the logarithmic characteristics of a diode. Due to the JFET structure, its leakage current is much smaller than that of a diode, which improves circuit accuracy. It also has very high impedance and far superior temperature characteristics to a diode. The resulting Vth (on-state voltage) and Is (reverse saturation current) vary for different JFET models and require actual measurement.

[0026] Resistor C1 is a feedback capacitor to improve the stability of the op amp.

[0027] In an optional embodiment, the positive current measurement unit further includes: an operational amplifier U2; The inverting input terminal of the operational amplifier U2 is connected to the first terminal of the feedback network, and the output terminal of the operational amplifier U2 is connected to the second terminal of the feedback network; The positive power supply port of the operational amplifier U2 is connected to a capacitor C2, and the negative power supply port of the operational amplifier U2 is connected to a capacitor C3.

[0028] In an optional embodiment, the operational amplifier U2, the resistor R3, the resistor R4, the resistor R5, the resistor R6, the field effect transistor Q1, the field effect transistor Q2, the field effect transistor Q3, the field effect transistor Q4, and the capacitor C14 implement I / V conversion, wherein the formula for I / V conversion is as follows:

[0029] In the above formula, is the op amp output voltage, To convert current.

[0030] It should be noted that the operational amplifier U2 and the resistors R3, R4, R5, R6, field effect transistors Q1, Q2, Q3, Q4 and capacitor C14 realize transimpedance amplification.

[0031] Linear coordinate I / V conversion is shown in FIG. 2, and logarithmic coordinate I / V conversion is shown in FIG. 3. It can be seen that the circuit has linear amplification when the voltage is 0 to -1.25V, corresponding to the output current of 0-10nA, and has logarithmic amplification when the voltage exceeds -1.25V, corresponding to the current of 1uA or more, which meets the circuit design characteristics. Different JFET and feedback resistors can change the inflection point value. Figure 3 Figure 4 The principle is as follows: when the ion current passes through the resistors R3, R4, R5 and R6, the voltage across the resistors does not exceed the turn-on voltage of the field effect transistors Q1, Q2, Q3 and Q4, and the field effect transistors Q1, Q2, Q3 and Q4 are equivalent to open circuits. The feedback network is the feedback resistance of the resistors R3, R4, R5 and R6 plus the feedback capacitance of the capacitor C14. When the voltage exceeds the turn-on voltage, the field effect transistors Q1, Q2, Q3 and Q4 gradually turn on, and according to the voltage-current characteristic curve of the diode, the voltage across the diode and the current are in logarithmic relationship.

[0032] The calculation formula of the voltage across a single JFET is as follows:

[0033] The voltage-current characteristic curve of a single JFET is as follows:

[0034] In the above formula, is the forward current of the diode,

[0035] is the reverse saturation current, is the voltage drop across the two ends, is the temperature constant. The total current = the current flowing through the JEFT + the current flowing through the resistor:

[0036] +

[0037] In an alternative embodiment, the positive current measurement unit further comprises a current source, resistors R1, R2, capacitor C1 and resistor R23. The resistor R1 and the capacitor C1 constitute an RC filter, which is used to reduce input interference.​​ The resistor R1 and the resistor R2 are connected in series to provide a conduction path for the Faraday collection current; The 5th port of the current source simulating ion flow RF1 is connected to the resistor R1, and the 3rd and 4th ports of the current source simulating ion flow RF1 are short-circuited and connected to the non-inverting input terminal of the operational amplifier U2 and the resistor R23.

[0038] In an optional embodiment, the operational amplifier U2 is a dual-channel LMC66 CMOS operational amplifier.

[0039] It should be noted that the op amp used is the LMC662, a dual-channel CMOS operational amplifier. The LMC662 operates from a single power supply and is manufactured using Texas Instruments' (TI) advanced CMOS process. The device has an operating voltage range of 5V to 15V and features rail-to-rail output swing in addition to an input common-mode range (including ground). The LMC662's ultra-low input bias current of 2fA makes it ideal for use as a transimpedance amplifier and for ultra-low current sensing. In this embodiment, one channel implements positive current sensing, while the other implements negative current sensing, enabling precise current measurement.

[0040] In an optional embodiment, the field effect transistor Q1, the field effect transistor Q2, the field effect transistor Q3, and the field effect transistor Q4 are N-channel JFET field effect transistors of model MMBF4117.

[0041] It's important to note that the N-channel JFET field-effect transistor has a reverse leakage current of only 10fA and is insensitive to temperature changes, making it effective in suppressing temperature drift in logarithmic amplification applications. When the DS terminal of the transistor is shorted to form a diode, the GS turn-on threshold is 0.25V. When the current is below 1nA, it exhibits linear amplification characteristics; when the current exceeds 1nA, the diode conducts, entering logarithmic amplification mode.

[0042] In an optional embodiment, the negative current measurement unit includes: a resistor R14, a resistor R15, a resistor R16, a resistor R17, a field effect transistor Q6, a field effect transistor Q7, a field effect transistor Q8, a field effect transistor Q9 and a capacitor C14; The resistor R14, the resistor R15, the resistor R16 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q6 are short-circuited and connected to the second end of the resistor R14, and the gate of the field effect transistor Q6 is connected to the first end of the resistor R14; the drain and source of the field effect transistor Q7 are short-circuited and connected to the second end of the resistor R15, and the gate of the field effect transistor Q7 is connected to the first end of the resistor R15; the drain and source of the field effect transistor Q8 are short-circuited and connected to the second end of the resistor R16, and the gate of the field effect transistor Q8 is connected to the first end of the resistor R16; the drain and source of the field effect transistor Q9 are short-circuited and connected to the second end of the resistor R17, and the gate of the field effect transistor Q9 is connected to the first end of the resistor R17.

[0043] It should be noted that in a quadrupole mass spectrometer, there are two ways to measure ion flow: the first is a Faraday cup, which measures the current as a positive ion flow signal; the second is an electron multiplier, which converts the ion flow into an electron flow signal. The circuit structure for measuring positive current cannot measure negative current. Therefore, this embodiment provides a circuit for measuring negative current, such as Figure 5 As shown, since the measured current is negative, the diode formed by the JFET is reversed to ensure the circuit characteristics. The linear coordinate I / V conversion diagram is as follows Figure 6 shown.

[0044] In an optional embodiment, the input module includes: a switch H1, a low leakage current precision switch U4, a resistor R11 and a field effect transistor Q5; The switch H1 is connected to the low leakage current precision switch U4 , and the low leakage current precision switch U4 , the resistor R11 , and the field effect transistor Q5 form a current source, which is used for zeroing and calibrating the current signal.

[0045] It should be noted that the low-leakage current precision switch U4 and resistor R11, together with the MMBF4117N channel JFET field-effect transistor Q5, form a 100nA current source for signal zeroing and calibration; similarly, in the negative current measurement circuit, the low-leakage current precision switch U6 and resistor R22, together with the MMBF4117 N-channel JFET field-effect transistor Q10, form a 100nA current source for signal zeroing and calibration.

[0046] In an optional embodiment, the output module includes: a differential amplifier U3, a resistor R7, a resistor R8, a resistor R9, a resistor R10, a capacitor C4, a capacitor C5 and a capacitor C6; Wherein, the inverting input terminal of the differential amplifier U3 is connected to the resistor R7, and the non-inverting input terminal of the differential amplifier U3 is connected to the resistor R8; The capacitor C4 and the resistor R9 are connected in parallel between the inverting input terminal and the non-inverting output terminal of the differential amplifier U3; the capacitor C5 and the resistor R10 are connected in parallel between the non-inverting input terminal and the reverse output terminal of the differential amplifier U3; The capacitor C6 is connected to the positive power supply port of the differential amplifier U3.

[0047] It should be noted that the differential amplifier uses the THS4531 model, which can improve signal stability and achieve 16Khz low-pass signal filtering.

[0048] Similarly, the differential amplifier U5 in the negative current measurement circuit also uses the same model.

[0049] Compared to existing technologies, the transimpedance amplifier (TIA) provided in this embodiment processes weak current signals. Because the circuit requires multiple feedback networks to measure currents of varying ranges, relays are required to switch the feedback networks. However, the performance of ordinary relays often falls short of these application requirements, necessitating the use of reed switch relays. The core advantage of reed switch relays lies in their contactless mechanical structure. Their contacts are sealed within an inert gas glass tube, and a magnetic field drives the reed blades to make and break, eliminating the physical friction and arcing associated with traditional relay mechanical contacts. This design is particularly important in transimpedance amplifiers (TIAs), which often process current signals in the nanoampere range or even lower. Any noise introduced by contact wear or vibration can significantly reduce the signal-to-noise ratio and measurement accuracy. Furthermore, the reed switch's reed blades have an extremely low mass and typically operate in milliseconds, far faster than the mechanical operation of traditional relays. This enables them to respond quickly to match signal processing speeds in high-frequency or fast-switching applications, such as optical communication receivers, effectively reducing signal delay and waveform distortion. The reed switch's contacts are plated with precious metals and have no mechanical wear, so their lifespan can reach hundreds of millions of operations, while the mechanical contact lifespan of traditional relays is typically only hundreds of thousands of times. This makes the reed switch's durability in long-term monitoring or industrial automation systems significantly reduce maintenance frequency and system failure risks. The reed switch is sealed in an inert gas environment with an insulation resistance of up to 10 15ohms, with leakage current as low as femtoamperes. This characteristic is crucial in medical equipment or high-precision measurement systems, as oxidation or contamination of traditional relay contacts can cause leakage current, interfering with the accurate acquisition of weak signals. Reed switches are small and suitable for integration into compact transimpedance amplifier circuits. Their sealed structure resists humidity, dust, and corrosive gases, while the open contacts of traditional relays are susceptible to environmental factors, leading to performance degradation. Although reed switches with these advantages are priced between 50 and 200, significantly increasing the cost of TIA amplifier circuits, this circuit uses the PN junction characteristics to automatically switch ranges, eliminating the need for reed switch range switching. This saves costs while reducing delays, leakage current, and insulation, making the circuit characteristics more stable.

[0050] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A current measurement circuit based on a quadrupole mass spectrometer, characterized in that: include: an input module, for receiving a current signal from a quadrupole mass spectrometer; A current detection module, configured to perform current measurement on the current signal; wherein the current measurement module comprises a positive current measurement unit and a negative current measurement unit, the positive current measurement unit being configured to measure the positive current of the quadrupole mass spectrometer, and the negative current measurement unit being configured to measure the negative current of the quadrupole mass spectrometer; The output module is used to output the current measurement results.

2. The current measurement circuit based on a quadrupole mass spectrometer according to claim 1, characterized in that: The positive current measurement unit includes: a resistor R3, a resistor R4, a resistor R5, a resistor R6, a field effect transistor Q1, a field effect transistor Q2, a field effect transistor Q3, a field effect transistor Q4 and a capacitor C14; The resistor R3, the resistor R4, the resistor R5 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q1 are short-circuited and connected to the first end of the resistor R3, and the gate of the field effect transistor Q1 is connected to the second end of the resistor R3; the drain and source of the field effect transistor Q2 are short-circuited and connected to the first end of the resistor R4, and the gate of the field effect transistor Q2 is connected to the second end of the resistor R4; the drain and source of the field effect transistor Q3 are short-circuited and connected to the first end of the resistor R5, and the gate of the field effect transistor Q3 is connected to the second end of the resistor R5; the drain and source of the field effect transistor Q4 are short-circuited and connected to the first end of the resistor R6, and the gate of the field effect transistor Q4 is connected to the second end of the resistor R6.

3. The current measurement circuit based on a quadrupole mass spectrometer according to claim 2, characterized in that: The positive current measurement unit further includes: an operational amplifier U2; The inverting input terminal of the operational amplifier U2 is connected to the first terminal of the feedback network, and the output terminal of the operational amplifier U2 is connected to the second terminal of the feedback network; The positive power supply port of the operational amplifier U2 is connected to a capacitor C2, and the negative power supply port of the operational amplifier U2 is connected to a capacitor C3.

4. The current measurement circuit based on a quadrupole mass spectrometer according to claim 3, characterized in that: The operational amplifier U2, the resistor R3, the resistor R4, the resistor R5, the resistor R6, the field effect transistor Q1, the field effect transistor Q2, the field effect transistor Q3, the field effect transistor Q4, and the capacitor C14 implement I / V conversion, wherein the formula for I / V conversion is as follows: In the above formula, is the op amp output voltage, To convert current.

5. The current measurement circuit based on a quadrupole mass spectrometer according to claim 3, characterized in that: The positive current measurement unit further includes: a current source simulating ion flow RF1, a resistor R1, a resistor R2, a capacitor C1 and a resistor R23; The resistor R1 and the capacitor C1 form an RC filter, and the RC filter is used to reduce input interference; The resistor R1 and the resistor R2 are connected in series to provide a conduction path for the Faraday collection current; The 5th port of the current source simulating ion flow RF1 is connected to the resistor R1, and the 3rd and 4th ports of the current source simulating ion flow RF1 are short-circuited and connected to the non-inverting input terminal of the operational amplifier U2 and the resistor R23.

6. The current measurement circuit based on a quadrupole mass spectrometer according to claim 3, characterized in that: The operational amplifier U2 is a dual-channel LMC66 CMOS operational amplifier.

7. The current measurement circuit based on a quadrupole mass spectrometer according to claim 3, characterized in that: The field effect transistors Q1, Q2, Q3 and Q4 are N-channel JFET field effect transistors of model MMBF4117.

8. The current measurement circuit based on a quadrupole mass spectrometer according to claim 1, characterized in that: The negative current measurement unit includes: a resistor R14, a resistor R15, a resistor R16, a resistor R17, a field effect transistor Q6, a field effect transistor Q7, a field effect transistor Q8, a field effect transistor Q9 and a capacitor C14; The resistor R14, the resistor R15, the resistor R16 and the resistor R6 are connected in series and in parallel with the capacitor C14 to form a feedback network; The drain and source of the field effect transistor Q6 are short-circuited and connected to the second end of the resistor R14, and the gate of the field effect transistor Q6 is connected to the first end of the resistor R14; the drain and source of the field effect transistor Q7 are short-circuited and connected to the second end of the resistor R15, and the gate of the field effect transistor Q7 is connected to the first end of the resistor R15; the drain and source of the field effect transistor Q8 are short-circuited and connected to the second end of the resistor R16, and the gate of the field effect transistor Q8 is connected to the first end of the resistor R16; the drain and source of the field effect transistor Q9 are short-circuited and connected to the second end of the resistor R17, and the gate of the field effect transistor Q9 is connected to the first end of the resistor R17.

9. The current measurement circuit based on a quadrupole mass spectrometer according to claim 1, characterized in that: The input module includes: a switch H1, a low leakage current precision switch U4, a resistor R11 and a field effect transistor Q5; The switch H1 is connected to the low leakage current precision switch U4 , and the low leakage current precision switch U4 , the resistor R11 , and the field effect transistor Q5 form a current source, which is used for zeroing and calibrating the current signal.

10. The current measurement circuit based on a quadrupole mass spectrometer according to claim 1, characterized in that: The output module includes: a differential amplifier U3, a resistor R7, a resistor R8, a resistor R9, a resistor R10, a capacitor C4, a capacitor C5 and a capacitor C6; Wherein, the inverting input terminal of the differential amplifier U3 is connected to the resistor R7, and the non-inverting input terminal of the differential amplifier U3 is connected to the resistor R8; The capacitor C4 and the resistor R9 are connected in parallel between the inverting input terminal and the non-inverting output terminal of the differential amplifier U3; the capacitor C5 and the resistor R10 are connected in parallel between the non-inverting input terminal and the reverse output terminal of the differential amplifier U3; The capacitor C6 is connected to the positive power supply port of the differential amplifier U3.

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