Spectrometer Amplifier Compensation

Abstract

An ion detection current conversion circuit includes: a conversion amplifier coupled to a conversion resistor assembly to convert an ion detection current generated by an ion detector into an ion detection voltage, the conversion resistor assembly including a resistor with high resistance and a capacitor compensation element; and a compensation voltage circuit for obtaining a compensation voltage from the ion detection voltage and feeding the compensation voltage to the capacitor compensation element, the compensation voltage circuit including a variable resistor for adjusting the compensation voltage. The conversion circuit may further include: a voltage reduction circuit for obtaining a reduced voltage from the ion detection voltage; and a neutralization amplifier unit for at least partially neutralizing the voltage reduction of the voltage reduction circuit. The reduced voltage allows the use of low-voltage components, such as digital variable resistors.

Description

Technical Field

[0001] This disclosure relates to conversion circuits for ion detectors in mass spectrometers and other instruments. More specifically, the invention relates to amplifiers for amplifying very small ion detection currents, said amplifiers including current-to-voltage conversion resistors with very high resistance and compensation circuitry for compensating for any non-ideal characteristics of the current-to-voltage conversion resistors. Background Technology

[0002] Mass spectrometers with ion detectors are well known. Such mass spectrometers may include mass filters for selecting ions with a specific mass charge (m / z) range and / or for spatially separating ions with different mass-to-charge ratios. Commonly used ion detectors include Faraday cups, second-electron detectors (SEDs), and others.

[0003] When a small number of ions are detected, the detection current generated by the detector can be very small, for example, less than 1 nA (nanoampere), typically less than 1 pA (picoampere), and sometimes in the range of a few fA (fenipicoampere). To convert such a small current into a voltage that can be meaningfully evaluated, a resistor with a high resistance value is required. Typically, resistors with resistances of tens or hundreds of GΩ (gigaohms), sometimes even several TΩ (megaohms) or even 10 TΩ are used. This resistor is often used as the feedback resistor in an amplifier that can be called a transimpedance amplifier.

[0004] The advantage of resistors with resistances in the hundreds of GΩ or even in the TΩ range is that their behavior is not perfectly linear, which can, of course, reduce the accuracy of ion measurements. Specifically, such resistors may suffer from parasitic capacitance and / or dielectric relaxation, also known as dielectric absorption or permeation.

[0005] This disclosure discloses a transimpedance amplifier by reference to U.S. Patent 9,431,976 (Lerche), which is incorporated herein by reference. The transimpedance amplifier includes: a resistor assembly coupled between an output terminal and an input terminal of the transimpedance amplifier; and a voltage source for applying a first voltage to a first capacitor compensation element of the resistor assembly and a second voltage to a second capacitor compensation element of the resistor assembly. The first voltage and the second voltage are each derived from the output voltage of the transimpedance amplifier. The voltage source includes a voltage controller for adjusting at least one of a first proportionality and / or a second proportionality.

[0006] The capacitance compensation element of the resistor assembly disclosed in US 9,431,976 can be a conductive ring arranged around but electrically isolated from a high-resistance resistor. This ring constitutes a capacitor to which a compensation voltage is applied. Therefore, the parasitic capacitance of the high-resistance resistor can be compensated. The compensation voltage originates from the amplifier's output signal and is a fraction of that output signal. To provide proper compensation, these fractions need to be carefully adjusted via a resistor network containing variable resistors, also known as potentiometers. However, such variable resistors are typically manually operated, and considerable time is required to manually tune their values.

[0007] US 9,431,976 mentions the option of providing digital circuitry to set the voltage applied to a capacitor compensation element, but does not describe the details of the digital circuitry. The use of digital potentiometers as variable resistors has been found advantageous because it allows for remote adjustment of the resistor value. However, the maximum voltage that can be applied to a digital potentiometer is limited. In some applications, the output voltage of an ion detection amplifier can be 50V or even 100V, which exceeds the permissible voltage of commercially available digital potentiometers, meaning that digital potentiometers cannot easily replace manually operated potentiometers. Summary of the Invention

[0008] This disclosure provides a solution to this problem. Therefore, the present invention provides a conversion circuit for converting an ion detection current generated by an ion detector into an ion detection voltage, the conversion circuit comprising:

[0009] - A conversion amplifier coupled to a conversion resistor assembly to convert an ion detection current into an ion detection voltage, the conversion resistor assembly including at least one resistor having high resistance and at least one capacitor compensation element.

[0010] - A compensation voltage circuit, the compensation voltage circuit being used to obtain at least one compensation voltage from the ion detection voltage and feed the at least one compensation voltage to the at least one capacitive compensation element, the compensation voltage circuit including at least one variable resistor for adjusting the at least one compensation voltage;

[0011] - A voltage reduction circuit, wherein the voltage reduction circuit is used to obtain a reduced voltage from the ion detection voltage, and the voltage reduction circuit is arranged to provide the reduced voltage to the at least one variable resistor.

[0012] The compensation voltage circuit includes at least one neutralizing amplifier unit coupled between the at least one capacitor compensation element and the at least one variable resistor to at least partially neutralize the voltage drop of the voltage reduction circuit.

[0013] A lower voltage can be fed to the variable resistor by providing a voltage reduction circuit for obtaining a reduced (ion detection) voltage from the ion detection voltage. Conversely, a higher ion detection voltage can be used using a commercially available variable resistor and / or other components that require a relatively low voltage. For example, while utilizing components with a maximum permissible voltage of only 20V, a maximum ion detection voltage of up to 50V or even up to 100V can be used by reducing the ion detection voltage by a factor of 0.4 or even 0.2.

[0014] The effectiveness of the capacitor compensation element is not hindered by providing at least one neutralizing amplifier to neutralize this voltage drop. At least one neutralizing amplifier for neutralizing the voltage drop of the ion detection voltage can amplify the output voltage of the digital variable resistor by a factor that is the reciprocal of the factor by which the ion detection voltage is reduced. For example, if the voltage reduction circuit applies a (first) factor of 0.2 to reduce the voltage from 100V to 20V, the compensation amplifier can apply a (second) factor of 5 for compensation. The drop or reduction of the ion detection voltage can be achieved using a voltage divider. The voltage is preferably reduced to not more than that necessary to maintain the operating range of the variable resistor and / or other low-voltage components to avoid any unnecessary amplification.

[0015] Adjustment of the compensation voltage circuit is made possible by using at least one variable resistor. Remote adjustment of the compensation voltage circuit is made possible by using, for example, a remotely controllable resistor, such as a digital variable resistor. It should be noted that the variable resistor may include a potentiometer, i.e., a resistor with three terminals.

[0016] The voltage reduction circuit may include a voltage divider and an amplifier such as an operational amplifier. The output terminal of the voltage divider may be connected to the inverting input terminal of the operational amplifier. In such embodiments, the voltage reduction factor of the voltage reduction circuit may be negative.

[0017] The voltage reduction circuit can be arranged to achieve a voltage reduction of at least 25%, preferably at least 50%, and more preferably at least 75%. In some embodiments, the voltage reduction can be 90%, corresponding to a reduction factor of 0.1 (or -0.1, if an inverting amplifier is used).

[0018] A neutralizing amplifier unit may include at least one neutralizing amplifier and at least one voltage divider, the voltage divider preferably connected to the variable resistor. The output terminal of the voltage divider, i.e., the terminal providing the distributed voltage, may be connected to the input terminal of the neutralizing amplifier, such as the inverting input terminal. Conversely, the at least one neutralizing amplifier unit may be arranged in series with the variable voltage terminal of the at least one variable resistor.

[0019] The conversion circuit may include at least two neutralizing amplifier units and at least two variable resistors. In some embodiments, the conversion circuit may include three, four, five, or six neutralizing amplifier units, each of which is coupled to an associated variable resistor.

[0020] The conversion circuit may further include at least one RC circuit coupled between the output terminal of the conversion resistor and a series arrangement of an additional neutralizing amplifier unit and an additional variable resistor. In addition to the compensation provided by the capacitive compensation element, which is primarily used to compensate for the nonlinear characteristics of the conversion resistor, this RC circuit is used to compensate for any parasitic capacitance and dielectric relaxation effects of the conversion circuit. While the at least one RC circuit is not mandatory, it improves the properties of the conversion circuit. Although the capacitive compensation element arranged near and / or around the conversion resistor is primarily used to compensate for substantially constant deviations from the desired output voltage, one or more RC circuits are primarily used to compensate for time-varying deviations from the desired output voltage.

[0021] The conversion circuit may further include an additional resistor coupled between the conversion resistor and the output of the conversion amplifier, the additional resistor having a smaller resistance than the conversion resistor. The additional resistor thus connects the output terminal of the conversion resistor to the output terminal of the conversion amplifier. The resistance of the additional resistor may be 1% smaller than the resistance of the conversion resistor, preferably 0.1%, more preferably 0.01%. In practice, the resistance of the additional resistor may be 0.001% smaller than the resistance of the conversion resistor, for example, it may be between 10kΩ and 100kΩ, while the resistance of the conversion resistor is 1TΩ or less than (100 × 10⁻⁶). 3 / 1×10 12 ) × 100% = 1.0 × 10 -5 %.

[0022] Each RC circuit corresponds to the output voltage (V) of the conversion circuit. out The effect depends on the ratio of the resistance of the additional resistor to the resistance of the resistor in the specific RC circuit.

[0023] The conversion circuit may include at least two parallel RC circuits with different time constants. That is, a single RC circuit may include components with different resistances and / or capacitances. In some embodiments, the conversion circuit may include three, four, five or more parallel RC circuits with different time constants. The time constants of the RC circuits may be approximately logarithmically distributed.

[0024] The at least one variable resistor is preferably a digital variable resistor. That is, one or more variable resistors can be digitally controllable resistors. Such resistors are capable of changing their resistance in large steps (e.g., 512 steps, 1024 steps, or 2048 steps) to allow for very fine tuning of the conversion circuit.

[0025] This disclosure also provides a conversion circuit for converting an ion detection current generated by an ion detector into an ion detection voltage, the conversion circuit comprising:

[0026] - A conversion amplifier coupled to a conversion resistor assembly to convert an ion detection current into an ion detection voltage, the conversion resistor assembly including at least one resistor having high resistance and at least one capacitor compensation element.

[0027] - A compensation voltage circuit, the compensation voltage circuit being used to obtain at least one compensation voltage from the ion detection voltage and feed the at least one compensation voltage to the at least one capacitive compensation element, the compensation voltage circuit including at least one variable resistor for adjusting the at least one compensation voltage; and

[0028] - At least one compensation RC circuit, said at least one compensation RC circuit being coupled to the switching resistor.

[0029] By providing at least one compensating RC circuit coupled to the switching resistor, additional deviations from the desired output voltage can be compensated or at least mitigated. This additional deviation can specifically be, for example, a time-varying deviation from the desired output voltage caused by dielectric relaxation effects of components of the switching circuit.

[0030] The conversion circuit may further include an additional resistor coupled between the conversion resistor and the output of the conversion amplifier. The resistance of the additional resistor is smaller than that of the conversion resistor. The resistance of the additional resistor may be less than 1% of the resistance of the conversion resistor, preferably less than 0.1%, and more preferably less than 0.01%.

[0031] The conversion circuit may include at least two parallel compensated RC circuits. The at least two parallel compensated RC circuits may have different time constants. The compensated RC circuits may be coupled to a variable resistor.

[0032] The resistance of the switching resistor can be at least 100MΩ, for example, 300MΩ, 500MΩ, 1GΩ, 10GΩ, 30GΩ, or 50GΩ. In some embodiments, switching resistors with resistances of at least 100GΩ, at least 300GΩ, at least 1TΩ, or even at least 10TΩ can also be used. Therefore, in some embodiments, resistors of 30TΩ, 50TΩ, or 100TΩ can be used.

[0033] The switching resistor assembly may include a single capacitance compensation element. In other embodiments, the switching resistor assembly may include more than one capacitance compensation element, such as three capacitance compensation elements, but embodiments with two, four, five, six or more capacitance compensation elements may also be used. The switching resistor assembly preferably includes a single switching resistor, but embodiments with two or more switching resistors connected in series and / or parallel may also be used.

[0034] This disclosure also provides a mass spectrometer including the conversion circuit described above. This mass spectrometer may include magnetic sector units and / or electric sector units and / or Faraday cup arrays. Other detectors, such as SEM (secondary ion multiplier), may also be used. The mass spectrometer may also include an ion source.

[0035] It should be noted that Japanese patent application JP 2013-148372 discloses a mercury atomic absorption spectrometer, which includes a gain-adjusting digital potentiometer used to adjust the amplifier gain, thereby adjusting a standard voltage to a predetermined set voltage. This Japanese patent application does not mention a compensation circuit for compensating for the non-ideal characteristics of a high-impedance feedback resistor.

[0036] It is further noted that UK patent application GB 2,424,330 discloses a transimpedance amplifier with a shielded feedback resistor. The screen circuit allows for more precise definition and limitation of the capacitive effect of the feedback resistor. Therefore, the screen is driven at half the output voltage. Although this known arrangement can mitigate some of the resistive effect in the capacitive effect of the feedback resistor, the arrangement is not adjustable and therefore cannot be adjusted according to the characteristics of a single high-value feedback resistor. Attached Figure Description

[0037] Figure 1 An exemplary embodiment of a mass spectrometer to which the present disclosure can be applied is illustrated schematically.

[0038] Figure 2 schematically shown Figure 1 A more detailed embodiment of a part of a mass spectrometer.

[0039] Figure 3 schematically illustrates a first embodiment of a prior art conversion amplifier unit.

[0040] Figure 4 schematically illustrates a second embodiment of a prior art conversion amplifier unit.

[0041] Figure 5 A first embodiment of the conversion amplifier unit of this disclosure is schematically shown.

[0042] Figure 6 A second embodiment of the conversion amplifier unit of this disclosure is schematically shown.

[0043] Figure 7 A third embodiment of the conversion amplifier unit of this disclosure is schematically shown.

[0044] Figure 8 A fourth embodiment of the conversion amplifier unit of this disclosure is schematically shown.

[0045] Figure 9 A fifth embodiment of the conversion amplifier unit of this disclosure is schematically shown. Detailed Implementation

[0046] The mass spectrometer to which the present invention can be applied is illustrated by way of example. Figure 1 The mass spectrometer 100 shown includes an ion source 110, a beam focusing unit 120, a magnetic sector unit 130, a detector unit 140, and a detector signal processing unit 150. The ion source 110 can be a plasma source such as an inductively coupled plasma (ICP) source or a non-ICP source such as a filament source. The ion source 110 is arranged to generate a raw ion beam 101, which is focused by the beam focusing unit 120 into a focused ion beam 102. The beam focusing unit 120 may include suitable ion optics known per se. A mass filter (not shown) may optionally be arranged between the beam focusing unit 120 and the magnetic sector unit 130, such as that described in, for example, U.S. Patent 10,867,780, which is incorporated herein by reference.

[0047] In the magnetic sector unit 130, ions contained in the ion beam 102 can be separated according to their respective masses. Therefore, a single focused ion beam 102 entering the magnetic sector unit 130 is split into multiple ion beams 103 that can reach different detectors in the detector unit 140, thereby allowing the detection of ions with different masses separately. The detector unit 140 generates an ion detection signal IS, which can be amplified and further processed in the signal processing unit 150 to generate an output signal OS, which may contain the average detection frequency of each ion detector and therefore the average detection frequency for each ion mass range.

[0048] In some embodiments, the magnetic sector unit may be replaced by or combined with an electric sector unit. Instead of sector units such as the magnetic sector unit 130, or in addition to the aforementioned sector units, filter units such as multi-pole units (e.g., quadrupole or hexapole units) may be used.

[0049] Figure 1 A portion of the mass spectrometer 100 is shown in more detail. Figure 2 middle. Specifically, Figure 2 A portion of the detector unit 140 and a portion of the signal processing unit 150 are schematically shown.

[0050] The detector unit 140 shown includes Faraday cups (FC) 141, 143, and 145 and additional ion detectors (ID) 142, 144, and 146. Detector unit 140 may include more (or fewer) Faraday cups and / or more (or fewer) additional ion detectors than shown herein. In the illustrated example, the additional ion detectors are each arranged immediately following a Faraday cup, but this is not required. The additional ion detectors may include compact discrete multiplier electrodes (CDD) and / or secondary electron multiplier tubes (SEM).

[0051] As is well known, Faraday cups and similar ion detectors generate a small current proportional to the number of impacting ions. These small currents, typically in the range of 0.1 fA to 1 nA, are converted into a voltage, which is then amplified to generate an ion detection voltage. For this purpose, the signal processing unit 150 includes an input resistor 151 with a very large resistance, typically in the range of 1 GΩ to 1 TΩ. This large resistance value is necessary to generate a suitable voltage for further processing: 1 TΩ (10) +12 A resistor with a current of 1pA (10Ω) -12 A) The current passing through a resistor of only 1V generates a voltage.

[0052] exist Figure 2 In the schematic diagram, the detector current i flowing out of the Faraday cup 141D The input resistor 151 is fed to the input terminals of the input or switching resistor 151 and the amplifier 152. It should be noted that the amplifier 152 is only schematically shown here, and the amplifier 152 may, for example, comprise an operational amplifier with inverting and non-inverting inputs, in which the input resistor 151 is arranged in its negative feedback loop. If the non-inverting input is connected to ground, the resistor 151 is effectively connected to ground through the inverting input (virtual ground) of the operational amplifier, and the input current of the amplifier can be ignored. The switching resistor 151 and the amplifier 152 together constitute amplifier circuit 153, more specifically, transimpedance amplifier circuit 153.

[0053] It is worth noting that, for the sake of simplicity, only a single input resistor 151 and a single amplifier 152 are shown here, but it should be understood that each Faraday cup and / or similar detector 141, 143, and 145 of detector unit 140 can be connected to a separate input resistor 151 and a separate amplifier 152. In some embodiments, two or more Faraday cups can share an input resistor and an amplifier. Ion detectors 142, 144, and 146 can each be connected to a discriminator, which in turn can be connected to a counter ( Figure 2 (Not shown in the image).

[0054] The output voltage V generated by amplifier 152 out It can be equal to or greater than the input voltage V in For example, 10 times or 100 times larger. If the output voltage V out The magnitude and input voltage V in Essentially the same, amplifier 152 can be used solely as a buffer. This is because its representation of the number of detected ions can also be referred to as the detection voltage V. D Output voltage V out The voltage is fed to a voltage-to-frequency converter (VFC) 154, which generates a voltage V with a frequency response characteristic of the voltage V. out A proportional frequency pulse P. The VFC has been found to have high linearity, which allows for accurate detection. It is worth noting that detector signals from certain types of detectors, such as compact discrete multiplier electrodes (CDD) and / or secondary electron multiplier tubes (SEM), are typically not fed into the VFC. It should also be noted that the invention is not limited to ion detection signals generated by ion detectors, but can also be used in other fields, such as optical signals.

[0055] The pulse P generated by VFC 154 can be fed to an optional pulse processing circuit 155, which, for example, can determine the average frequency of the pulse generated by VFC 154 during a certain time period, which may be referred to as the measurement interval. For each time period, the pulse processing circuit 155 can generate an average frequency f. A This average frequency can represent the number of ions impacting the corresponding detector during the specific time period. To determine the average frequency, the pulse processing circuit can count the number of pulses within the time period, determine the duration of the time period, and divide the number of pulses by the duration. In some embodiments, the optional pulse processing circuit 155 may additionally or alternatively generate other data, such as the variance of the pulses and / or the duration of the pulse interval. In some embodiments, the signal processing unit 150 may not output an average frequency or similar data, but instead output the actual pulse P generated by the VFC 154.

[0056] exist Figure 2 In the diagram, amplifier 152 is shown as having a single input. As mentioned above, in a practical implementation, amplifier 152 may include two inputs, such as an inverting input and a non-inverting input. This is shown in Figure 3, where the inverting input of differential amplifier 152 provides a virtual ground, and the switching resistor 151 is part of a negative feedback loop.

[0057] As mentioned above, resistors with very high or even ultra-high resistance values, such as 1 TΩ, often suffer from nonlinear behavior, which may be at least in part due to parasitic capacitance and / or parasitic impedance. As disclosed in US 9,431,976, this nonlinear behavior can be compensated for using compensating elements.

[0058] Figure 4 illustrates a conversion circuit according to the prior art, which may also be referred to as a transimpedance amplifier circuit. The conversion circuit 153 of Figure 4 includes an operational amplifier 152 and a resistor assembly comprising an ultra-high resistance feedback (conversion) resistor 151 coupled between the output of the operational amplifier 152 and its inverting input. To compensate for the nonlinearity of the feedback resistor 151, a compensation element 157 comprises conductive (e.g., metallic) compensation cylinders C1, C2, and C3 surrounding an insulator of the feedback resistor 151. Each conductive cylinder has a length along the longitudinal direction of the feedback resistor 151, and the lengths of each conductive cylinder C1, C2, and C3 may be the same or different. As shown in Figure 4, each cylinder C1, C2, and C3 surrounds a different segment of the feedback resistor 151 along its length, which may also be referred to as R1.

[0059] Each of the cylinders C1, C2, and C3 of the compensation element 157 may have a gap between it and the outer layer of the feedback resistor 151. Each cylinder C1, C2, and C3 is electrically isolated from the feedback resistor 151 and the other cylinders, allowing different voltages to be applied to each cylinder. Each of the cylinders C1, C2, and C3 may also function as part of a capacitor.

[0060] To apply a suitable voltage to the compensation element 157, a compensation circuit 158 ​​is provided, comprising a first voltage divider and a second voltage divider. The first voltage divider includes resistors R81, R82, R83, and R84, while the second voltage divider includes resistors R85, R86, R87, and R88. Resistors R81, R83, and R86 are variable resistors, i.e., potentiometers with two main terminals and one branch terminal, wherein the resistance between the branch terminal on one side and the main terminal on the other side can be changed. An additional resistor R89 ​​is provided to receive an optional adjustment voltage Vx.

[0061] It has been found that dielectric charging and discharging effects may occur in the arrangement shown in Figure 4. These effects are at least in part due to parasitic capacitance in resistor 151 (R1), for example, caused by the inherent capacitance of the windings of the resistor wire, such high-ohmic resistance is typically caused by the resistor wire. It has been found that compensating for this parasitic capacitance by using compensation circuit 158 ​​is possible only to a limited extent. According to one aspect of this disclosure, a solution to this problem is provided by a compensating RC circuit, which will refer to Figure 5 and 6 To explain in more detail.

[0062] While the known arrangement shown in Figure 4 effectively compensates for parasitic capacitance, its drawback is the need for manual tuning. That is, the variable resistors R81, R83, and R86 must be adjusted by a field engineer, for example, which is labor-intensive. Preferably, the variable resistors can be adjusted electronically, thereby allowing remote control. Digital variable resistors are available, but typically have lower maximum voltages, such as 20V. In many ion detection applications, ion detection voltages of 50V or even 100V can be used. This makes it impossible to use conventionally available commercially available digital variable resistors (digital potentiometers). Another aspect of this disclosure improves the solution to this problem by feeding only a fraction of the ion detection voltage to the digital variable resistor and then amplifying the output voltage of the digital variable resistor by approximately the reciprocal of this fraction to obtain the desired compensation voltage. This will be referenced later. Figure 7 and 8 Further explanation is needed.

[0063] Figure 5A first embodiment of the conversion unit according to this disclosure is schematically shown. The input terminals of the conversion unit 153 are shown connected to the inverting input of the conversion amplifier 152, while the non-inverting input is connected to ground, optionally via a network of compensating resistors (not shown).

[0064] The output of the conversion amplifier 152 is connected to the output terminal of the conversion unit 153, and is connected to the inverting input terminal of the conversion amplifier 152 through an additional resistor R2 and a conversion resistor 151 (also referred to as R1). Figure 5 In some embodiments, the switching resistor 151 is provided with a compensation element 157, which may consist of a single component, such as a conductive tube that at least partially surrounds but is electrically isolated from the switching resistor 151. The resistance of the switching resistor 151 can be very high, for example, 10 GΩ, 100 GΩ, 1 TΩ or higher. The resistance of the additional resistor R2 can be lower, for example, between 10 kΩ and 100 kΩ. It should be understood that the contribution of the additional resistor R2 to the series resistance of the switching resistor 151 and the additional resistor R2 will be very small. In some embodiments, specifically in embodiments where the value of the switching resistor is less than approximately 500 GΩ, such as 100 GΩ, the compensation element 157 does not need to include a structure surrounding the switching resistor but can be constituted by a conventional capacitor.

[0065] The conversion unit 153 is shown to include a compensation circuit 158. The compensation circuit 158 ​​provides two types of compensation:

[0066] - By providing a fraction of the output voltage of the conversion circuit to the capacitor compensation element 157, thereby directly affecting the conversion resistor 151, any non-ideal properties of the conversion resistor 151 are compensated; and

[0067] - The components of the switching unit 153 are compensated by an RC circuit with an appropriate time constant, including any dielectric relaxation effect of the switching resistor 151.

[0068] These two types of compensation are preferably used in combination, but they can also be used independently of each other. It should be noted that the components of the conversion unit 153 typically have relatively large tolerances, not only the high-impedance feedback resistor but also other components. Furthermore, many components including the PCB (printed circuit board) exhibit dielectric relaxation effects, which can have a relatively strong impact on the output voltage, considering the extremely small current conversion to voltage.

[0069] Therefore, in Figure 5In the embodiments shown, the compensation circuit 158 ​​can be divided into two parts. The first part is connected to the compensation element 157 and, in the illustrated embodiment, includes only a first variable resistor 201, which is used to apply a fraction of the output voltage to the compensation element 157. Therefore, using the first variable resistor 201, a voltage between 0% and 100% of the output voltage can be supplied to the compensation element 157, typically in the range of 30% to 70%, for example, 50%. This is similar to the prior art arrangement shown in Figure 4. However, in embodiments of this disclosure, the variable resistor can be digitally variable, thereby allowing remote control of the variable resistor.

[0070] The second part of the compensation circuit 158 ​​includes a parallel RC circuit coupled between the switching resistor 151 and the common rail 300 via corresponding variable resistors 202-205. The common rail 300 can be connected to ground.

[0071] For example, the first RC circuit includes a series arrangement of a first capacitor C1 and a resistor R3 connected to a variable resistor 202. It should be noted that in the illustrated embodiment, variable resistors 201-205 are all arranged between the output of amplifier 152 and a common rail, which can then be connected to ground. Parallel RC circuits preferably have different time constants, achieved through different resistor and capacitor values. Therefore, the values ​​of C1 and R3 are typically different from the values ​​of C2 and R4, thus providing different time constants. In some embodiments, the time constants of the RC circuit are logarithmically distributed. While the duration of some time constants may be several seconds (e.g., 2 seconds), the duration of other time constants may be several minutes (e.g., 2.5 minutes or 150 seconds).

[0072] Therefore, in the illustrated embodiment, each of the four compensation RC circuits is directly connected to the output terminal of the switching resistor 151, and consists of a series arrangement of free capacitors (e.g., C2), resistors (e.g., R4), and variable resistors (e.g., 203). The maximum compensation effect of each RC circuit can be determined by the ratio of R2 to the corresponding resistance of the RC circuit, and thus in the illustrated example, by the ratios R2 / R3, R2 / R4, R2 / R4, and R2 / R6, respectively. The maximum compensation effect of the RC circuit can be expressed as Vmaxcomp = (R2 / RX) × ΔV, where RX is the corresponding resistance of the RC circuit, and ΔV is the output terminal of the switching resistor R1, i.e., the voltage step at the connection between R1 and R2. In practice, the resistance of R2 is much smaller than that of R3, R4, R5, or R6.

[0073] The variable resistor can be a remotely controllable digital potentiometer, thereby enabling remote setting of the desired resistor value. This also allows for automatic setting of the desired value. In some embodiments, digital potentiometers that consume very little power, such as less than 100 μW, preferably less than 10 μW, are chosen to maintain a constant temperature of circuit 153. Each digital potentiometer preferably has a large number of resistance steps, such as 1024 steps, to allow for precise setting of the desired resistor value. Alternatively or additionally, a manually settable linear trimmer can be used.

[0074] In the illustrated example, four parallel RC circuits are provided, but this disclosure is not limited to four RC circuits; rather, one, two, three, five, six, or more parallel RC circuits can be provided. Preferably, each RC circuit has a different time constant. In one embodiment, the capacitor and resistor can be selected such that the time constants of the RC circuits are approximately 2 seconds, 8 seconds, 15 seconds, and 70 seconds, respectively. It should be noted that the time constant is affected by the value of resistor R2, whose resistance can, for example, be between 10 kΩ and 100 kΩ.

[0075] Figure 6 A second embodiment of the conversion unit according to this disclosure is illustrated schematically. Figure 6 The conversion unit 153 is similar to Figure 5 The conversion unit in the middle, but the compensation element 157 is composed of three parts 157a, 157c and 157c, instead of as Figure 5 The circuit comprises individual parts. To this end, a circuit is provided for providing different compensation voltages to the three parts 157a, 157b, and 157c to provide better compensation. A first voltage divider includes resistors R7 and R8 arranged in series with a first variable resistor 201 and is used to provide voltage to the first compensation element part 157a. A second voltage divider includes resistors R9 and R10 arranged in series with a second variable resistor 202 and is used to provide voltage to the second compensation element part 157b. A third voltage divider includes resistors R12 and R13 arranged in series with a variable resistor 203. The principle of this circuit is described in the aforementioned patent US9,431,976.

[0076] exist Figure 6 In this embodiment, only two RC circuits are provided: a first RC circuit comprising C1 and R3 coupled to the variable resistor 204; and a second RC circuit comprising C2 and R4 coupled to the variable resistor 205. It should be understood that fewer than two RC circuits may be provided, such as only one RC circuit, or perhaps no RC circuits at all. Conversely, more than two RC circuits may be provided, such as three or four.

[0077] Figure 7 A third embodiment of the conversion unit according to this disclosure is schematically shown. The input terminals of the conversion unit are shown connected to the inverting input of the conversion amplifier 152, while the non-inverting input is connected to ground (common rail 300 in the illustrated example), optionally via a network of compensating resistors (not shown).

[0078] Figure 7 The embodiments are similar to Figure 5 and 6 In this embodiment, the output of the conversion amplifier 152 is connected to the output terminal of the conversion unit 153, and is connected to its inverting input via an additional resistor R2 and a conversion resistor R1 to provide feedback, as shown below. Figure 5 As shown. In Figure 7 In one embodiment, the switching resistor 151 is provided with a single-component compensation element 157, which may be a conductive tube that partially surrounds the switching resistor 151 but is electrically isolated from it. The switching resistor R1 (151) can have a very high resistance relative to the additional resistor R2.

[0079] like Figure 5 and 6 In this embodiment, compensation circuit 158 ​​is arranged to provide compensation voltage to compensation element 157 and to provide a time constant by using RC circuits. In the illustrated embodiment, four compensation RC circuits are directly connected to the terminals of switching resistor 151, and each consists of a series arrangement of a capacitor (e.g., C1) and a resistor (e.g., R3) connected to the respective variable resistor 200.

[0080] However, Figure 7 The embodiment is designed for a switching resistor 151 with a value of approximately 1 TΩ. In this embodiment, the voltage at the output of the switching unit 153 can reach a value of 50V or even 100V. This would make it impossible to use a digital potentiometer for the variable resistors 201-205, as digital potentiometers capable of handling such voltages are unavailable. Therefore, another aspect of the invention reduces the voltage applied to the variable resistor by using voltage division. For example, the voltage applied to the variable resistor can be divided by a factor of 10, but other factors, such as 5, 15, or 20, can also be used, as appropriate. The factors mentioned herein are absolute values, because the division factor can be negative, such as -10, resulting in a change in the sign of the voltage. It should be understood that the voltage applied to the variable resistor can be considered as multiplied by a factor less than 1, such as a factor of 0.1 (multiplication).

[0081] exist Figure 7In this embodiment, the (inverting) voltage divider stage consists of a voltage divider amplifier 220 and a voltage divider including resistors R15 and R16 connected between the output of amplifier 152 and an additional rail 400. The output of voltage divider amplifier 220 is also connected to the additional rail. The connection of resistors R15 and R16 is connected to the inverting input of voltage divider amplifier 220, while the non-inverting input of voltage divider amplifier 220 is connected to a common rail 300, which can then be connected to ground. The ratio of resistors R15 and R16 determines the distribution ratio, the sign of which is inverted by voltage divider amplifier 220. Variable resistors 201-205 are connected between common rail 300 and additional rail 400 and therefore receive a reduced voltage V from voltage divider amplifier 220. red The voltage assigned to this voltage is neutralized by voltage multiplier amplifier units 211-215 coupled to the corresponding variable resistors 201-205. Therefore, the reciprocal of the amplified voltage division may result in the original voltage being supplied to, for example, compensation element 157. Alternatively, voltage multiplier amplifier units 211-215 may provide a suitable voltage for further processing, which may differ from the original unassigned voltage. In the illustrated embodiment, each voltage multiplier amplifier unit (e.g., 21) includes an amplifier (e.g., A1) and a resistor network (e.g., R21, R22). The amplifier (e.g., A1) may be an operational amplifier and may have inverting and non-inverting inputs, the inverting inputs being connected to the resistor network (e.g., R21 and R22), and the non-inverting inputs being connected to a common rail 300.

[0082] The non-inverting input of amplifier A1 in the first voltage multiplier amplifier unit 211 is connected to an additional rail 400 via resistor R31 to reduce the (output) voltage V. red The signal is fed to the first voltage multiplier amplifier unit 211. It should be noted that the first voltage multiplier amplifier unit 211 is connected to the compensation element 157, and... Figure 7 In this embodiment, the other voltage multiplier amplifier units 212-215 are not connected to the compensation element 157 but are connected to the RC circuit. Therefore, with a reduced voltage V... red Instead of a much higher output voltage V out Use all variable resistors.

[0083] The variable resistor is preferably, but not necessarily, digitally controlled, as a manually controlled variable resistor can also be used. By using variable resistors 202-205, the RC network can be precisely tuned to provide the desired compensation effect.

[0084] Figure 8 A fourth embodiment of the conversion circuit of this disclosure is schematically shown. Figure 8 The embodiments are similar to Figure 7This embodiment differs from the previous one in that the switching resistor 151 has three compensation element portions 157 instead of a single compensation element. Furthermore, only two RC networks are used instead of four. The operating principle of this embodiment is similar to... Figure 6 and 7 Examples of implementations.

[0085] The three compensation elements 157 are configured with different voltages using a resistor network (voltage divider), which includes variable resistors, preferably digital variable resistors. Figure 8 In the illustrated embodiment, the first portion 157a of the compensation element 157 receives voltage from a resistor network connected between the output voltage and ground, and this resistor network does not utilize the reduced voltage V present at the additional rail 400. red Therefore, this resistor network does not include a voltage multiplier amplifier. Figure 7 (211 in the example). In this embodiment, the voltage applied to the first variable resistor 201 is due to the values ​​of R7 and R8 being so small that the voltage division and multiplication are omitted.

[0086] The second and third compensation elements 157 receive voltages from resistor networks including variable resistor 202 and voltage multiplier amplifier unit 212, and variable resistor 203 and voltage multiplier amplifier unit 213, respectively. In this embodiment, voltage multiplier amplifier units 212 and 213 can not only amplify the voltage of the variable resistors, but also shift these voltages to a desired level.

[0087] In this embodiment, similar to Figure 6 and 7 In one embodiment, voltage multiplier amplifiers 214 and 215, together with their respective variable resistors 204 and 205, are connected to an RC network.

[0088] As mentioned above, voltage multiplier amplifiers 212-215 provide to invert or substantially neutralize the voltage division caused by voltage divider amplifier 220 and resistors R15 and R16. Therefore, the amplification factor is approximately the reciprocal of the fraction of voltage that is distributed. For example, if the voltage is distributed to result in a fraction of -0.2, the amplification factor of the correction amplifier can be approximately -5, for example, in the range between -4 and -6.

[0089] Figure 9 A fifth embodiment of the conversion circuit of this disclosure is schematically shown. Figure 9 The embodiments are similar to Figure 8The embodiment differs in that the voltage dividers R9 / R10 and R12 / R39 have been replaced by resistors R31 and R32, respectively. Therefore, the non-inverting input of amplifier A1 in the first corrective amplifier unit 211 is connected to the additional rail 400 via resistor R31. Similarly, the non-inverting input of amplifier A2 in the second corrective amplifier unit 212 is connected to the additional rail 400 via optional resistor R32. Because resistors R31 and R32 are connected to the additional rail 400, they receive a reduced voltage V. red And as a result, only a very small current will flow through R31 and R32, causing very little heat. Instead, resistors R9 / R10 and R12 / R39 are connected to the full output voltage V. out The full output voltage can be greater than V. red For example, 10 times higher. Therefore, the current flowing through resistors R9 / R10 and R12 / R39 will be higher, resulting in more heat. To minimize heat generation in the switching circuit, you can use... Figure 9 Examples of implementations.

[0090] Although embodiments have been described with reference to mass spectrometers, this disclosure can also be applied to spectral determination or other fields of spectroscopy, such as spectroscopic determination.

[0091] Therefore, those skilled in the art will understand that this disclosure is not limited to the embodiments shown, and that many additions and modifications can be made without departing from the scope of this disclosure as defined in the appended claims.

Claims

1. A conversion circuit for converting an ion detection current generated by an ion detector into an ion detection voltage, the conversion circuit comprising: - A conversion amplifier coupled to a conversion resistor assembly to convert an ion detection current into an ion detection voltage, the conversion resistor assembly including at least one resistor having high resistance and at least one capacitor compensation element. - A compensation voltage circuit, the compensation voltage circuit being used to obtain at least one compensation voltage from the ion detection voltage and feed the at least one compensation voltage to the at least one capacitive compensation element, the compensation voltage circuit including at least one variable resistor for adjusting the at least one compensation voltage; - A voltage reduction circuit, wherein the voltage reduction circuit is used to obtain a reduced voltage from the ion detection voltage, and the voltage reduction circuit is arranged to provide the reduced voltage to the at least one variable resistor. The compensation voltage circuit includes at least one neutralizing amplifier unit coupled between the at least one capacitor compensation element and the at least one variable resistor to at least partially neutralize the voltage drop of the voltage reduction circuit.

2. The conversion circuit according to claim 1, wherein the voltage reduction circuit includes a voltage divider and an amplifier.

3. The conversion circuit according to claim 1 or 2, wherein the voltage reduction circuit is arranged to achieve a voltage reduction of at least 25%.

4. The conversion circuit according to claim 1, wherein the at least one neutralizing amplifier unit comprises an amplifier and a voltage divider.

5. The conversion circuit according to claim 4, wherein the at least one neutralizing amplifier unit is arranged in series with the variable voltage terminal of the at least one variable resistor.

6. The conversion circuit according to any one of claims 1 to 2, 4 to 5, comprising at least two neutralizing amplifier units and at least two variable resistors.

7. The conversion circuit of claim 6, further comprising at least one RC circuit coupled between the output terminal of the conversion resistor and a series arrangement of a further neutralizing amplifier unit and a further variable resistor.

8. The conversion circuit of claim 7, further comprising an additional resistor coupled between the conversion resistor and the output of the conversion amplifier, wherein the resistance of the additional resistor is smaller than that of the conversion resistor.

9. The conversion circuit of claim 8, wherein the resistance of the additional resistor is less than 1% of the resistance of the conversion resistor.

10. The conversion circuit according to any one of claims 7 to 9, comprising at least two parallel RC circuits with different time constants.

11. The conversion circuit according to claim 10, wherein the time constant is approximately logarithmically distributed.

12. The conversion circuit according to any one of claims 1 to 2, 4 to 5, 7 to 9, and 11, wherein at least one variable resistor is a digital variable resistor.

13. A conversion circuit for converting an ion detection current generated by an ion detector into an ion detection voltage, the conversion circuit comprising: - A conversion amplifier coupled to a conversion resistor assembly to convert an ion detection current into an ion detection voltage, the conversion resistor assembly including at least one resistor having high resistance and at least one capacitor compensation element. - A compensation voltage circuit, the compensation voltage circuit being used to obtain at least one compensation voltage from the ion detection voltage and feed the at least one compensation voltage to the at least one capacitive compensation element, the compensation voltage circuit including at least one variable resistor for adjusting the at least one compensation voltage; as well as - At least one RC circuit, said at least one RC circuit being coupled to the switching resistor.

14. The switching circuit of claim 13, wherein the at least one RC circuit is coupled between the switching resistor and another variable resistor.

15. The conversion circuit of claim 14, further comprising an additional resistor coupled between the conversion resistor and the output of the conversion amplifier, the additional resistor having a smaller resistance relative to the resistance of the conversion resistor.

16. The conversion circuit of claim 15, wherein the resistance of the additional resistor is less than 1% of the resistance of the conversion resistor.

17. The conversion circuit according to any one of claims 13 to 16, comprising at least two parallel RC circuits with different time constants.

18. The conversion circuit according to claim 17, wherein the time constant is approximately logarithmically distributed.

19. The switching circuit according to any one of claims 1 to 2, 4 to 5, 7 to 9, 11, 13 to 16, 18, wherein the switching resistor assembly includes a capacitor compensation element.

20. The switching circuit according to any one of claims 1 to 2, 4 to 5, 7 to 9, 11, 13 to 16, 18, wherein the switching resistor assembly comprises three capacitor compensation elements.

21. The switching circuit according to any one of claims 1 to 2, 4 to 5, 7 to 9, 11, 13 to 16, 18, wherein the resistance of the switching resistor is at least 100 GΩ.

22. A mass spectrometer comprising a conversion circuit according to any one of claims 1 to 21.

23. The mass spectrometer according to claim 22, further comprising a magnetic sector unit and / or an electric sector unit.

24. The mass spectrometer according to claim 22 or 23, further comprising an array of Faraday cups.

25. The mass spectrometer according to claim 22 or 23, further comprising an ion source.

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