Method for operating electrochemical measurement point and electrochemical measurement point

Abstract

The invention relates to a method for operating an electrochemical measurement point and an electrochemical measurement point. The method comprises the following steps: providing an electrochemical measurement point having a sensor circuit, a first cable, and at least one first electrochemical half-cell, the sensor circuit having a control unit, the control unit having a first digital-to-analog converter, a first analog-to-digital converter, and a second digital-to-analog converter, wherein the sensor circuit further has a first terminal, a second terminal, at which a first electrode signal of the first electrochemical half-cell is measured, and a first input filter, which is arranged between the second digital-to-analog converter and the second terminal, at which a superposition of a first DC voltage signal and a first AC voltage signal is output by the first digital-to-analog converter at the first terminal, the first electrode signal at the first analog-to-digital converter is evaluated such that an AC voltage amplitude offset and / or phase shift between the AC voltage signal and the first electrode signal is determined.

Description

Technical Field

[0001] The invention relates to a method for operating an electrochemical measuring point and to an electrochemical measuring point. Background Art

[0002] In analytical measurement technology, in particular in the field of water management, in the field of environmental analysis, in industry (for example in food technology, biotechnology and pharmaceuticals) and for various laboratory applications, measured variables such as pH value, conductivity or even concentration of analytes (such as ions or dissolved gases in gaseous or liquid measurement media) are of great importance. These measured variables can be detected and / or monitored, for example, by means of electrochemical sensors. Electrochemical sensors for measuring the pH of a measurement medium usually consist of two electrochemical half-cells.

[0003] A half-cell consists, for example, of a so-called reference cell with an electrically contacted electrolyte which is electrically conductively connected to the measured medium via a membrane. Another possible half-cell consists of an electrically contacted buffer solution with a defined pH value, for example for measuring pH values, which is separated from the measured medium by a thin glass barrier. Furthermore, the half-cell can be made via metal contacts (e.g. noble metal contacts) which are directly electrically connected to the measured medium or via an enamel membrane.

[0004] In the following, electrochemical half-cells will be dealt with, but the invention and the associated advantages can be used by a person skilled in the art for any sensor configuration, in particular also for a one-piece configuration. Figure 1 and Figure 2 The equivalent circuit diagrams of describe various configurations. In this case, for example, the above-mentioned subassemblies are not configured in so-called individual electrodes, but in so-called combined electrodes. The voltage generated in the electrochemical half-cell enables conclusions to be drawn about the chemical properties of the measured medium.

[0005] In this document, the term "half-cell" is used to describe all subcomponents of the sensor that are in direct contact with the measured medium or analyte. The term "electrochemical measuring point" is used for the entire system consisting of the electronics, possible cable connections and half-cell.

[0006] Electrochemical half-cells usually have very high impedances, such as 5 gigaohms or more. Therefore, the measurement of the voltage generated by the electrochemical half-cell is achieved by wiring and evaluation electronics with a sensor circuit, the components of which are designed to have particularly high impedance. Otherwise, measurement errors are generated, for example, due to leakage currents. The design of the sensor cable is particularly important here, because it is usually exposed to moisture or steam and can usually reach a large length, such as 50m. For the same reason, the corresponding electronic components for signal evaluation usually have to be time-consuming to clean, cast or paint in order to prevent leakage currents from being generated by flux residues or solder residues. A method known from the prior art for protecting leakage currents with high-impedance input signals is to use a so-called guard potential or protection potential, i.e. a low-impedance drive signal set to the same voltage as the critical high-impedance input signal. In this case, the low-impedance signal is routed in a so-called protection line in a ring around the high-impedance input line with the input signal. This means that no leakage current can flow between the input line and the protection line. In the process, a certain amount of feedback capacitance is also inevitably applied between the input line and the protection line. In practice, therefore, the principle can only be used within certain limits, namely if the feedback capacitance assumes a sufficiently small amount, since otherwise the feedback coupling mediated by the capacitance would cause the system to tend to oscillate.

[0007] In patent application DE 10 2021 107 754 A1, the applicant proposes a solution to make the guard potential or protection potential less dependent on the feedback capacitance. A starting point in this case is not to change the guard potential or protection potential in stages or rapidly, but to adjust them slowly using digital methods. However, under certain conditions, such a gain in the stability of the circuit can require an increased delay in the stabilization of the measured value, in particular in the case of rapid changes in the measured medium. Such a delay of the measured signal (e.g. voltage) compared to the ideal voltage curve (full curve) is particularly Figure 4 The overshoot is visible from the dashed curve in the form of an overshoot. This overshoot is caused by the fact that even in the case of gentle slope tracking of the guard potential, the charging / discharging current flows via the filter capacitor of the circuit and via the connected cable capacitance. This aspect is particularly important when the charging / discharging current flows via the sensor half-cell, which has a high internal resistance. Summary of the invention

[0008] It is therefore an object of the invention to provide a circuit having optimal characteristics with regard to robustness to variations of the measurement signal and response time.

[0009] This object is achieved according to the invention by a method for operating an electrochemical measuring point.

[0010] The method according to the invention comprises the following steps:

[0011] - providing an electrochemical measuring point having a sensor circuit, a first cable, at least one first electrochemical half-cell, wherein the sensor circuit has a control unit having a first digital-to-analog converter, a first analog-to-digital converter and a second digital-to-analog converter, wherein the sensor circuit also has a first terminal, a second terminal and a first input filter arranged between the second digital-to-analog converter and the second terminal,

[0012] - measuring a first electrode signal of a first electrochemical half-cell at a second terminal,

[0013] - outputting, by the first digital-to-analog converter, a superposition of the first DC voltage signal and the first AC voltage signal at the first terminal,

[0014] - evaluating the first electrode signal at the first analog-to-digital converter such that an AC voltage amplitude offset and / or a phase shift between the AC voltage signal and the first electrode signal is determined,

[0015] - determining a first half-cell impedance of the first electrochemical half-cell based on the AC voltage amplitude offset and / or phase shift,

[0016] - determining a filter capacitance of a first input filter and / or a cable capacitance of a first cable,

[0017] - determining the time derivative of the first electrode signal and the time derivative of the DC voltage signal GS1,

[0018] - determining a first correction value based on the time derivative of the first electrode signal and the time derivative of the first DC voltage signal and the first half-cell impedance,

[0019] - determining a measurement value based on the first electrode signal,

[0020] - determining a display value based on the measured value and the first correction value,

[0021] - Outputs the displayed value.

[0022] By using the method according to the invention, measured value distortions, so-called voltage overshoots, can be corrected in the output display value. This allows a more precise measured value display with an optimal response time in the measured value display when the measured value changes.

[0023] According to one embodiment of the present invention, a protection voltage is outputted to the second terminal by a digital-to-analog converter, and a displacement current is determined based on a voltage difference between the first terminal and the second terminal and a cable impedance, wherein the displacement current is considered in the step of determining the correction value.

[0024] According to one embodiment of the invention, when determining the time derivative of the first electrode signal, a time period of less than 3 seconds is used.

[0025] According to one embodiment of the invention, the cable length of the first cable input by the user is taken into account when determining the cable capacitance.

[0026] According to one embodiment of the invention, when determining the half-cell impedance, a median filtering is applied to the determined values ​​of the AC voltage amplitude offset and / or the determined values ​​of the phase shift, in particular a median filtering over a period exceeding 3 seconds.

[0027] The abovementioned object is also achieved by a method for operating an electrochemical measuring point.

[0028] The method according to the invention comprises the following steps:

[0029] - providing an electrochemical measuring point having a sensor circuit, a first cable, a second cable, a third cable, a first electrochemical half-cell and a second electrochemical half-cell, wherein the sensor circuit has a control unit having a first digital-to-analog converter, a first analog-to-digital converter and a second digital-to-analog converter, wherein the sensor circuit also has a first terminal, a second terminal and a first input filter arranged between the second digital-to-analog converter and the second terminal,

[0030] - measuring a first electrode signal of a first electrochemical half-cell at a second terminal,

[0031] - determining a time derivative of the first electrode signal and a time derivative of the second electrode signal at a fourth terminal,

[0032] - outputting, by the first digital-to-analog converter, a superposition of the first DC voltage signal and the first AC voltage signal at the first terminal,

[0033] - evaluating the first electrode signal at the first analog-to-digital converter and evaluating the second electrode signal at the second analog-to-digital converter such that in each case a DC voltage signal or an AC voltage signal and an AC voltage amplitude offset and / or a phase shift between the first electrode signal and the second electrode signal is determined,

[0034] - determining a first half-cell impedance of the first electrochemical half-cell and a second half-cell impedance of the second electrochemical half-cell based on the AC voltage amplitude offset and / or phase shift,

[0035] - determining a first correction value based on the time derivative of the first electrode signal and the first half-cell impedance of the first half-cell,

[0036] - determining a second correction value based on the time derivative of the second electrode signal and the second half-cell impedance of the second half-cell,

[0037] - determining a measurement value based on the first electrode signal and the second electrode signal,

[0038] - determining a display value based on the measured value and the first correction value and the second correction value,

[0039] - Outputs the displayed value.

[0040] The abovementioned objects are also achieved by means of an electrochemical measuring point.

[0041] The electrochemical measuring point according to the present invention comprises:

[0042] - a sensor circuit having a control unit suitable for carrying out the method according to the invention, wherein the sensor circuit further comprises a first terminal and a second terminal,

[0043] - a first cable having an inner conductor and a shield, wherein the shield is connected to the first terminal and the inner conductor is connected to the second terminal,

[0044] - a first electrochemical half-cell having an input and an output, a second electrochemical half-cell having an input and an output,

[0045] - wherein the input of the first electrochemical half-cell is suitable for connection to a measured medium and the output of the first electrochemical half-cell is connected to the shielding,

[0046] Therein, the input of the second electrochemical half-cell is suitable for connection to a measured medium and the output of the second electrochemical half-cell is connected to the inner conductor.

[0047] The abovementioned objects are also achieved by means of an electrochemical measuring point.

[0048] The electrochemical measuring point according to the present invention comprises:

[0049] - a sensor circuit having a control unit suitable for carrying out the method according to the invention, wherein the sensor circuit further comprises a first terminal, a second terminal, a third terminal, a fourth terminal and a fifth terminal,

[0050] - a first electrochemical half-cell having an input and an output, a second electrochemical half-cell having an input and an output,

[0051] - a first cable having an inner conductor and a shield, wherein the inner conductor of the first cable is connected to the second terminal and the output of the second half-cell and the third terminal is connected to the shield of the first cable,

[0052] - a second cable having an inner conductor and a shield, wherein the inner conductor of the second cable is connected to the fourth terminal and the output of the first half-cell and the shield of the second cable is connected to the fifth terminal,

[0053] - A third cable, wherein the third cable is connected to the first terminal and to an input of the first half-cell and to an input of the second half-cell.

[0054] The abovementioned objects are also achieved by means of an electrochemical measuring point.

[0055] The electrochemical measuring point according to the present invention comprises:

[0056] - a sensor circuit having a control unit suitable for carrying out the method according to the invention, wherein the sensor circuit comprises a first terminal, a second terminal, a third terminal and a fourth terminal,

[0057] - a first electrochemical half-cell having an input and an output, a second electrochemical half-cell having an input and an output,

[0058] - a first cable having an inner conductor and a shield, wherein the inner conductor of the first cable is connected to the second terminal and the output of the second half-cell, and the fourth terminal is connected to the shield of the first cable,

[0059] Wherein the shield is also connected to the output of the first half cell,

[0060] A third cable, wherein the third cable is connected to the first terminal and to an input of the first half cell and an input of the second half cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The present invention is explained in more detail based on the following description of the accompanying drawings. In the accompanying drawings:

[0062] - Figure 1 : shows a schematic representation of an electrochemical measuring point according to the invention with a sensor circuit according to the invention,

[0063] - Figure 2 : Shown is an alternative sensor circuit Figure 1 An alternative embodiment of the electrochemical measurement point shown,

[0064] - Figure 3 : Shown with an alternative sensor circuit Figure 1 An alternative embodiment of the electrochemical measurement point shown,

[0065] - Figure 4 : Schematic diagram showing ideal, real and corrected display measurement signal curves. DETAILED DESCRIPTION

[0066] Figure 1A first embodiment of an electrochemical measuring point 2 according to the invention is shown, having a sensor circuit 1 according to the invention, a first cable 70, a first electrochemical half-cell 100 and a second electrochemical half-cell 200. In one embodiment (not shown), the electrochemical measuring point 2 has only the first electrochemical half-cell 100. In this embodiment (not shown), the second electrochemical half-cell 200 is replaced by a wire.

[0067] The first cable 70 is preferably a shielded cable. The shielded cable has at least one inner line and an outer shield, which protects the inner line from electromagnetic radiation. The measuring medium with the analyte can also be optionally connected via an additional potential equalization line (see Figure 2 The third cable 76 in the equivalent circuit diagram of the electrochemical half-cell, which is connected to the terminals 202 and 102 of the equivalent circuit diagram of the electrochemical half-cell, is in direct contact. In general, it should be noted at this point that the shielded cable resembles a capacitor in its electrostatic behavior in certain areas. Therefore, a shielded cable, i.e., for example a coaxial cable, has a capacitance between the inner conductor and the outer conductor. In a first approximation, such a shielded cable can therefore be considered as a capacitor with a capacitance C_coax. Therefore, the change over time (d U_coax / dt) of the voltage U_coax measured between the inner line and the outer shielding can be used to infer the "capacitor", i.e. the effective charging / discharging current I_coax of the shielded cable as follows: I_coax=C_coax(d U_coax / dt). If this charge / discharge current I_coax flows via a sensor half-cell with a high impedance R_sens, it generates a voltage drop U_Rsens at the resistor R_sens which can be calculated as follows: U_Rsens=R_sens*I_coax=R_sens*C_coax*(d U_coax / dt).

[0068] The sensor circuit 1 has a voltage source 10, a control unit 20, a first impedance converter 30, a first input filter 40, a first output filter 50, a first terminal 60 and a second terminal 61. The voltage source 10 is connected to the control unit 20 and is suitable for providing a first voltage potential VCC and a second voltage potential GND to the control unit 20. For example, the voltage source 10 provides the voltage potentials VCC and GND via two separate lines of the control unit 20. The voltage source 10 preferably provides a unipolar operating voltage. The first voltage potential VCC is, for example, 2.5V greater than the second voltage potential GND. The first voltage potential VCC is, for example, between 1.8V and 5V.

[0069] The control unit 20 has a first digital-to-analog converter 21, a first analog-to-digital converter 22 and a second digital-to-analog converter 23. The second digital-to-analog converter 23 can be designed as a pulse width modulator. One advantage of using pulse width modulators is that they are usually installed in conventional microcontrollers in larger quantities and require less current than analog-to-digital converters with continuous output levels. The control unit 20 is, for example, a microcontroller. The task of the second digital-to-analog converter 23 is to output a DC voltage signal and supply it to a filter circuit (first output filter 50).

[0070] The first impedance converter 30 has an input 31 and an output 32. The input 31 of the first impedance converter 30 is connected to the input AA of the first input filter 40 and to the second terminal 61. The output 32 of the first impedance converter 30 is connected to the first analog-to-digital converter 22.

[0071] The first input filter 40 has an input AA and an output BB. The first input filter 40 comprises at least one first capacitor 41. The first capacitor 41 has a capacitance of at least 220 pF, preferably at least 1 nF.

[0072] The first output filter 50 has an input A and an output B and connects the output of the second digital-to-analog converter 23 to the first input filter 40 and, if necessary, alternately decouples the first input filter 40 and the second digital-to-analog converter 23 from possibly present internal and external interference signals. If interference signals are not expected at either the digital-to-analog converter 23 or the input, a direct connection of the input A and the output B is also possible, i.e. without the output filter 50.

[0073] The output filter 50 advantageously has a first capacitor 51 and a first resistor 52. Thus, the first output filter 50 forms a first RC element RC1. The RC element is preferably low-pass. The first capacitor 51 of the first RC element RC1 preferably has a capacitance that is significantly greater than the capacitance of the first capacitor 41 of the first input filter 40. For example, the first capacitor 41 of the first input filter 40 has a capacitance of 1 nF, and the first capacitor 51 of the first output filter 50 has a capacitance that is, for example, ten times higher, i.e., for example, 10 nF. The second digital-to-analog converter 23 is connected to the input A of the first output filter 50. The output B of the first output filter 50 is connected to the output BB of the first input filter 40. The first capacitor 51 is connected to a first terminal between the resistor 52 and the output B, and to a second terminal having a voltage potential (e.g., ground potential).

[0074] The first terminal 60 is suitable for connecting the first cable 70 or another cable. For example, the shield 72 of the first cable is connected to the first terminal 60. The second terminal 61 is also suitable for connecting the first cable 70 or another cable. For example, the inner conductor 71 of the first cable 70 is connected to the second terminal 61 (see Figure 1 ).

[0075] The first terminal 60 is connected to the first digital-to-analog converter 21. The second terminal 61 is connected to the input 31 of the first impedance converter 30 and the input AA of the first input filter 40.

[0076] The first electrochemical half-cell 100 has an intrinsic resistance 101, an input 102 and an output 103. The second electrochemical half-cell 200 has an intrinsic resistance 201, an input 202 and an output 203. The intrinsic resistance is also called series impedance. The first electrochemical half-cell 100 is suitable for forming a DC voltage potential and providing the DC voltage potential at the output 103 of the first electrochemical half-cell 100. The second electrochemical half-cell 200 is suitable for forming a DC voltage potential and providing the DC voltage potential at the output 203 of the second electrochemical half-cell 200.

[0077] and Figure 1 As in the first embodiment of the electrochemical measuring point 2 shown in , the outlet 103 of the first electrochemical half-cell 100 is connected to the shield 72 of the first cable 70, and the output 203 of the second electrochemical half-cell 200 is connected to the inner conductor 71 of the first cable 70. The input 202 of the second electrochemical half-cell 200 is connected to the input 102 of the first electrochemical half-cell 100 via a measurement medium (not shown).

[0078] exist Figure 1 In the embodiment of , there is no direct additional connection of the evaluation electronics to the measured medium via so-called potential equalization lines (exposed connections to inputs 202 and 102 of the equivalent circuit diagram). Figure 2 As shown in , for example, this can alternatively take place via an additional cable connection (third cable 76 ), for example in order to be able to reliably prevent a current flow via one of the two half-cells in applications such as electroplating cells, in which the measured medium is energized with the analyte.

[0079] like Figure 1 As shown in , the third impedance converter 36 can be connected via its input to the first digital-to-analog converter 21 and via its output to the first terminal 60 .

[0080] Figure 2An alternative second embodiment of an electrochemical measuring point 2 is shown with an alternative embodiment of a sensor circuit 1 and three cables 70, 73, 76. In addition to the first cable 70, there is a second cable 73 and a third cable 76 with an inner conductor 74 and a shield 75. The third cable 76 can also have a shield.

[0081] exist Figure 2 In an alternative second embodiment of the sensor circuit 1 shown in , the control unit 20 further has a second analog-to-digital converter 24 and a third digital-to-analog converter 25. The sensor circuit 1 further has a third terminal 62, a fourth terminal 63, a fifth terminal 64, a second input filter 45 having an input AA' and an output BB', a second output filter 55 having an input C and an output D, and a second impedance converter 33 having an input 34 and an output 35. The output 35 is connected to the second analog-to-digital converter 24, and the input 34 is connected to the fourth terminal 63. The input AA' of the second input filter 45 is connected to the input 34 of the second impedance converter 33, and the input C of the second output filter 55 is connected to the third digital-to-analog converter 25, and the output D of the second output filter 55 is connected to the output BB' of the second input filter 45 and the fifth terminal 64. The second input filter 45 is constructed similarly to the first input filter 40 and has a capacitor 46. The second output filter 55 is constructed similarly to the first output filter 50 and has a resistor 57 and a capacitor 56.

[0082] exist Figure 2 In the second embodiment of the electrochemical measurement point 2 shown in , the third cable 76 is connected to the first terminal 60 and the input 202 of the second half-cell 200. The inner conductor 71 of the first cable 70 is connected to the second terminal 61 and the output 203 of the second half-cell 200, and the third terminal 62 is connected to the shield 72 of the first cable 70. The inner conductor 74 of the second cable 73 is connected to the fourth terminal 63 and the output 103 of the first half-cell 100. The fifth terminal 64 is connected to the shield 75 of the third cable 73. The above-mentioned capacitance of the coaxial cables (i.e. the first cable 70 and the second cable 73) is connected in parallel to the filter capacitors of the first input filter 40 and the second input filter 45, whereby the same voltage drop as across the first input filter 40 or the second input filter 45 is generated between the inner conductor 71 or 74 of the first cable 70 or the second cable 73 and the coaxial shield 72, 75 of the first cable 70 or the second cable 73.

[0083] exist Figure 3 In the third embodiment of the electrochemical measurement point 2 shown in FIG. Figure 2 The embodiment shown in is different in that the second cable 73 is unnecessary. Figure 3In the embodiment shown in , the coaxial shield 72 of the first cable 70 is connected to the fourth terminal 63 and the output of the first electrochemical half-cell 103 .

[0084] The sensor circuit 1 enables a signal (e.g., a voltage or a current) to be outputted at one or more signal outputs of the control unit 20 (i.e., at the first digital-to-analog converter 21, at the second digital-to-analog converter 23, and at the third digital-to-analog converter 25) in order to control the electrochemical half-cell 100, 200, and the signal can be connected to a circuit network consisting of capacitors and resistors with known impedance or known resistance. Via the first terminal 60, the second terminal 61, the third terminal 62, the fourth terminal 63, and the fifth terminal 64, at least one electrochemical half-cell 100, 200 can be connected to the circuit network. At the signal input of the control unit 20, i.e., in the first analog-to-digital converter 22 and the second analog-to-digital converter 24, the first electrode signal ES1 or the second electrode signal ES2, i.e., the voltage or the current, can be read in.

[0085] Furthermore, an AC voltage signal at at least one signal output of the control unit 20 can be modulated into an output DC voltage signal. Furthermore, the resistance of the electrochemical half-cell 100, 200 can be inferred via the frequency-dependent attenuation of the signal response, wherein the attenuation results from interconnecting the electrochemical half-cell 100, 200 and the circuit network, and wherein the signal response is formed by the AC voltage amplitude measured by the control unit 20. Thus, a numerical approximation of the resistance of the electrochemical half-cell 100 can be obtained by means of the control unit 20.

[0086] This is achieved, for example, via a model-based approach. To this end, it is possible to calculate which input signals will be generated in the control unit 20 for a given AC voltage signal, i.e., which first electrode signal ES1 or which second electrode signal ES2 will be generated if the impedance of the electrochemical half-cell 100, 200 can assume a given value. In other words, under the assumption that the unknown circuit components of the electrochemical half-cell 100, 200 will have an assumed resistance R, for example, it is possible to calculate with the aid of the known capacitance and resistor of the circuit network which voltages will be generated at the signal input of the control unit 20 with the analog-to-digital converter 22, 24 under this assumption. The determined deviation between the calculated value under the assumption of the cell impedance R of the electrochemical half-cell 100, 200 and the actually measured signal curve of the first electrode signal ES1 and the second electrode signal ES2 is a measure of the correctness of the assumption. Therefore, the method for estimating the impedance of the electrochemical half-cell 100, 200 is to accurately search for the value of the cell impedance R via an algorithm that produces the minimum deviation between the actually measured electrode signals ES1, ES2 and the modeled signals in the model. In this case, one or more unknown parameters of the system may be determined, such as both the cable capacitance of the connecting cable and the ohmic source impedance of the electrochemical half-cell 100, 200, since both may be unknown to the evaluation logic. Alternatively, it is also conceivable to let the possibly unknown cable length be set by the user, for example via an operating mask on the display, and thus only estimate the impedance of the sensor cell. Furthermore, the user may also input an estimated value for the sensor cell impedance.

[0087] The method according to the invention for operating an electrochemical measuring point 2 is described below.

[0088] In a first step, an electrochemical measuring point 2 is provided according to the first, second or third embodiment described above. However, for the sake of simplicity, the first embodiment of the sensor circuit 1 will first be used (see Figure 1 ) to describe the method. Then, the first embodiment and the second embodiment of the electrochemical measurement point 2 (see Figure 2 ) or the third embodiment (see Figure 3 ). The provision of the electrochemical measuring point 2 includes that it is naturally ready for operation. This means that the first electrochemical half-cell 100 and the second electrochemical half-cell 200 are in contact with the measured medium. This Figure 1 1 and 2. This is shown by the connection between the input 102 of the first electrochemical half-cell 100 and the input 202 of the second electrochemical half-cell 200.

[0089] Subsequently, the first electrode signal ES1 is measured at the second terminal 61. The first electrode signal ES1 is generated by the first electrochemical half-cell 100 or the second electrochemical half-cell 200 and is provided at the output 103 or 203 thereof.

[0090] Then, the first electrode signal ES1 at the first analog-to-digital converter 22 is evaluated such that an AC voltage amplitude offset and / or a phase shift between the AC voltage signal WS1 and the first electrode signal ES1 is determined.

[0091] A first half-cell impedance of first electrochemical half-cell 100 is then determined based on the AC voltage amplitude offset and / or phase shift.

[0092] The filter capacitance of the first input filter 40 and the cable capacitance of the first cable 70 are then determined.

[0093] Then a time derivative ES1 ′ of the first electrode signal ES1 and a time derivative GS1 ′ of the DC voltage signal GS1 are determined.

[0094] A first correction value is then determined based on the time derivative ES1 ′ of the first electrode signal and the time derivative GS1 ′ of the first DC voltage signal and the first half-cell impedance.

[0095] Then, a measurement value is determined based on the first electrode signal ES1.

[0096] Then, a display value is determined based on the measured value and the first correction value.

[0097] Then, the display value is output.

[0098] An alternative embodiment of the method is described below.

[0099] The first electrode signal ES1 ′ is preferably a DC voltage signal having a modulated AC voltage component, wherein the AC voltage amplitude and phase of the signal depends on the sensor cell impedance 201 , 101 , ie the intrinsic resistance of the first and second electrochemical half-cells 100 , 200 .

[0100] Subsequently, for example as described above, a step of estimating the sensor cell impedance based on the AC voltage component is preferably performed in the controller 20, i.e. based on the model by an adaptive algorithm, so that as a result of this step of estimation in the controller 20, there are values ​​for unknown components of the equivalent circuit diagram, such as estimates of the resistance of the electrochemical half-cell 201, 101 and / or the cable capacitance of the connecting line 76, 70, 73. As described above, parts of the unknown parameters can also be made available by user input, for example by an operator of the device entering the length L of the connecting cable via an input mask, and determining the cable capacitance by multiplying the length L by a typical capacitive coating of the cable (e.g. 100 pF capacitance per meter of cable length).

[0101] The DC voltage value of the first electrode signal ES1 depends, for example, on the analyte concentration present in the measurement medium or the pH value of the solution. The first electrode signal ES1 is in principle a digitally determined differential voltage between the signal at the input of the first analog-to-digital converter 22 and the output of the first digital-to-analog converter 21, i.e., the potential difference between the cable connections 60 and 61. Thus, the first electrode signal ES1 is the electrochemical voltage of the electrochemical measurement point 2. If the electrochemical measurement point 2 is a pH sensor, the pH of the measurement medium can be calculated based on the first electrode signal ES1.

[0102] In the step of measuring the first electrode signal ES1, the first electrode signal ES1 is preferably subjected to analog filtering simultaneously by the first input filter 40 and optionally by the first output filter 50. For example, the first electrode signal ES1 is smoothed by the first capacitor 41 of the first input filter 40. The filtering by the first output filter 50 comprises generating a filter signal by the second digital-to-analog converter 23 and feeding it into the first output filter 50. The filter signal affects the first electrode signal ES1 via the first input filter 40 connected to the first output filter 50. For filtering, the filter capacitor preferably has a very large capacitance, i.e. greater than 100 nF. The output filter 50 is particularly advantageous if a low current pulse width modulated digital signal is used as a filter signal for the digital-to-analog conversion in the second digital-to-analog converter 23.

[0103] Preferably, the voltage of the first electrode signal ES1 is measured and averaged over a time period dt1. This means, for example, that a mean filter with a predetermined duration dt1 is used. The duration of the average is, for example, 1 to 3 seconds.

[0104] In a next step, a time derivative ES1' of the first electrode signal ES1 is preferably determined by the control unit 20. The time derivative ES1' of the first electrode signal ES1 is preferably stored in the control unit 20 for later evaluation in a memory.

[0105] This can be achieved, for example, by detecting in a first step the voltage of the signal ES1 averaged over a past duration dt1 of, for example, 3 seconds using a shorter time frame DeltaT than dt1, for example 100 ms, so that a new mean value is calculated at time intervals DeltaT. Thus, a time series of values ​​of ES1 is generated in the control unit. The time derivative is then calculated from the difference between two consecutive values ​​ES1_t0 and ES1_t1 via the relation (ES1_t1–ES1_t0) / DeltaT.

[0106] Alternatively, the time derivative ES1 ′ can also be determined using the same time duration dt1 that is also used for averaging the DC voltage value, or a longer duration.

[0107] As an alternative to the above-described method based on a time series of an averaged electrode signal, it is also possible to detect the time derivative in such a way that the signal curve of the electrode signal ES1 is continuously detected and numerically integrated.

[0108] This can be done, for example, by first averaging the voltage values ​​of the output and measured voltage over the time period dt1, for example by integrating them by averaging (e.g. from dt1=DeltaT*32), and determining the time derivative UPoint(t) of the voltage t such that UPoint(t)=(integral of U over the time interval [t-DeltaT-dt1, t-DeltaT]"-"integral of U over the time interval [t-dt1, t]") / (DeltaT*dt1).

[0109] Advantageously, the time dt1 is selected such that it is an exact integer multiple of all time periods of the AC voltage frequency of those outputs at the digital-to-analog converter for the purpose of impedance estimation at the sensor electrodes. For example, if the lowest frequency for impedance determination is 1 / 3 Hz, a time of 3 s or an integer multiple of 3 s is advantageous as the average time dt1. Thus, the calculation of the time derivative is not falsified by the AC voltage signal modulated onto the output signal. For the duration DeltaT, the period is suitably selected for which the measuring device displays a new measurement value on a display or a new measurement value is sent to an external control unit, i.e., for example, a time of 100 ms.

[0110] As a result of the preceding steps, on the one hand, time derivatives are known for all output signals at the signal outputs 21 , 23 , 25 and, on the other hand, measured values ​​of the time derivatives of the input signals ES1 , ES2 are also available.

[0111] According to one embodiment, a displacement current is determined for all capacitance values ​​(and possibly inductance values) of the filter circuits 40, 50 and the cable 70, the displacement current being determined from the time derivative of the voltage reduction across the effective impedance and the known or estimated inductance or capacitance. For example, a coaxial cable 70 with capacitance CCable, which is connected via its outer conductor 72 to a voltage VGuard outputted by a voltage source with a time derivative VGuardPoint(t), and via its inner conductor 71 to a measured voltage VInput(t) with a time derivative VInputPoint(t), has a charge / discharge current ICharge / discharge(t) = CCable * (VInputPoint(t) - VGuardPoint(t)).

[0112] According to one embodiment of the invention, the charging / discharging current resulting from the tracking of the voltage source of the circuit is derived from the known component values ​​of the filter capacitor and its voltage.

[0113] By determining the charge / discharge current, it can be achieved that the voltage error generated by the charge / discharge current in the cable can be calculated and thus compensated. The voltage error can thus be determined as "voltage error" = "impedance of the sensor" * "sum of charge / discharge current".

[0114] Compared to the ideal voltage curve (full curve), this "voltage error" is especially Figure 4 If the determined voltage error is now used as a correction value for correcting the measured value (i.e. the voltage measured at the half-cell or first electrode signal ES1), the overshoot can be avoided and even the response time of the displayed measurement signal (display signal) can be improved (see Figure 4 point curve in the figure).

[0115] This is preferably followed by a step of outputting, by the first digital-to-analog converter 21, at the first terminal 60 a superposition of the first DC voltage signal GS1 and the first AC voltage signal WS1. The DC voltage signal is advantageously tracked during operation to achieve an optimal modulation of the analog-to-digital converter during operation, so that it is in fact a time-varying variable, which varies only with a very low frequency compared to the AC voltage signal. In this sense, compared to the AC voltage, one can effectively speak of a DC voltage source which has only a very small amount of time derivative GS1' compared to the AC voltage signal. For example, in order to readjust the DC voltage GS1, it is tracked with a slowly varying ramp so that, for example, a new DC voltage set point is reached within a period of 30 seconds.

[0116] The AC voltage signal WS1 advantageously comprises more than one oscillation frequency, for example two or three different frequencies.

[0117] The AC voltage signal WS1 has, for example, a frequency component smaller than 50 Hz and / or a frequency component larger than 500 Hz.

[0118] For example, it is possible to perform a superposition of two sinusoidal signals with different frequencies. Thus, the voltage V_DAC at the output of the first digital-to-analog converter 21 is generated, for example, according to the following formula:

[0119] U1=V_DAC_DC+V_DAC_F1*sin(2*pi*F1)+V_DAC_F2*sin(2*pi*F2),

[0120] wherein U1 is the first output voltage U1 of the first digital-to-analog converter 21, wherein V_DAC_DC is the DC voltage component of the first output voltage U1, wherein V_DAC_F1 is the AC voltage component of the first output voltage U1 of the first frequency F1, and wherein V_DAC_F2 is the AC voltage part of the first output voltage U1 of the second frequency F2.

[0121] Preferably, this is followed by the following step of evaluating the first electrode signal ES1 at the first analog-to-digital converter 22 such that an AC voltage amplitude offset and / or phase shift between the DC voltage signal GS1 and / or the AC voltage signal WS1 (i.e. the output signal of the first analog-to-digital converter 21) and the first electrode signal ES1 (i.e. the input signal at the first analog-to-digital converter 22) is determined, from which the cable capacitance of the first cable 70 (and, if present, the second cable 73) and the source impedance or inherent resistance 101, 201 of the first electrochemical half-cell 100 or the second electrochemical half-cell 200 can be inferred.

[0122] The half-cell impedances of first electrochemical half-cell 100 and second electrochemical half-cell 200 are preferably determined based on an AC voltage amplitude offset and / or phase shift.

[0123] This is preferably followed by a step of determining the sensor impedance based on the cable impedance and the half-cell impedance.

[0124] In a further step, a correction value is preferably determined based on the time derivative ES1 ′, the known capacitance in the circuit, and the sensor impedance.

[0125] Subsequently, a measured value is preferably determined based on the first electrode signal ES1.

[0126] Furthermore, there is preferably a step of determining the display value based on the measured value and the correction value. This means that the measured DC voltage value, i.e. the first electrode signal ES1 of the first electrochemical half-cell 100 and the second electrochemical half-cell 200, is corrected before being output, for example on a display or on a fieldbus interface to a control center, in order to correct the estimated voltage error value, i.e. the correction value.

[0127] Advantageously, after subtracting the estimated voltage error signal, i.e. after correcting the measured value by the correction value, there is a moving average filtering with an adaptively controlled averaging time. This means that a short averaging time is used for rapidly variable voltages and a long averaging time is used for almost constant voltages.

[0128] Finally, the display value, i.e. the corrected measured value, is preferably output. The display value is output by the control unit 20, for example, to an external device to be displayed, which is connected to the electrochemical sensor 2 wirelessly or via a cable, or directly on a display, for example integrated in the sensor circuit 1 to be displayed.

[0129] This ensures that the displayed values ​​of the outputs reflect the actual measured values ​​more quickly than would be possible without the correction values ​​(see Figure 4 point curve in the figure).

[0130] Specifically, it is possible to use Figure 3 The correction value is determined by an example of a sensor wiring of, for example, as indicated below. On the one hand, the relevant capacitance in the connecting line 61 is formed by the capacitance of the cable 70 between the inner conductor 71 and the outer conductor 72. On the other hand, the capacitance value of the filter circuit 40 is decisive. For determining the correction value, the filter circuit 40 can be considered as a capacitor 41 connecting the first electrode signal ES1 to the signal B driven by the digital-to-analog converter 23. On the other hand, the cable 70 acts as a capacitor with a cable capacitance C_Coax_70 between the second electrode signal ES2 and the first electrode signal ES1.

[0131] If the first electrode signal ES1, the second electrode signal ES2 and the signal at the output B of the first output filter 50 have a static curve that does not change with time, no displacement current flows via the capacitor. However, if the signal ES1, ES2 or B changes with time, that is, its time derivative ES1', ES2' or B' is not equal to zero, the displacement current becomes effective. Then, the charging current via the capacitor 41 is calculated from the time derivatives of the signal ES1 and the signal B as I_C41=C*(ES1'-B'). The time derivative of the signal B can thus be determined via the time characteristics of the output control signal of the second digital-to-analog converter 23. The signal curves of the signals ES1 and ES2 are obtained from the measurements at the first analog-to-digital converter 22 and the second analog-to-digital converter 24. Therefore, the displacement current is obtained via the cable capacitance as I_Coax_70=C_Coax_70*(ES1'-ES2'). Therefore, the total capacitor current I_C41+I_Coax_70 is effective at the signal node ES1. Since the amplifier 31 operating as an impedance converter has a very high impedance at the input, this total charge / discharge current must flow via the effective resistor R_201 of the connected half-cell and generate there a voltage drop R_201 *(I_C41 + I_Coax_70) to be considered as correction signal.

[0132] Given that the DC voltage value of the first electrode signal ES1 and the voltage drop across the resistor 201 are considered as the correction signal, a display value corresponding to the voltage value of the voltage source 202 of the half-cell connected to the terminal 61 can be determined.

[0133] Therefore, the correction value for the voltage drop across resistor 101 is calculated as a correction signal for voltage source 102 of half-cell 100 , which correction signal comes from the charging / discharging current of capacitor 46 across filter 45 and the negative charging / discharging current I_Coax_70 effective for signal ES2 , and the value of resistor 101 .

[0134] Also applies to Figure 2 , where the time derivative of the voltage curve of signal B and ES1 is decisive for the charging / discharging current of the capacitance of coaxial cable 70 , and the time derivative of signal D and ES2 is decisive for the charging / discharging current of the capacitance of cable 73 .

[0135] Figure 4 The result after subtracting the correction signal is shown by way of example, such as in the method according to the invention according to Figure 2 The solid line therein outlines the actual curve of the voltage of the signal source 201 over time. The dashed line outlines the associated curve of the signal ES1 measured at the ADC. During the course of time, the tracking protection signal B causes the protection signal to be tracked to the value of the electrode signal ES1 with a gentle slope in order to avoid leakage current between the signal B and ES1 via the cable 70 and the filter 40. This tracking is represented in the charging / discharging current via the capacitor 41 and the capacitance of the cable 70, and causes overshoot in the form of a dashed line. Figure 4 The dotted line in shows the resulting output value, where the calculated voltage drop at the half-cell impedance of ES1 is subtracted in order to compensate for the effect of the recharging current. Advantageously, as described above, in order to determine the time derivative of the signal used for the correction calculation, an integration is first performed over the time period of the AC voltage signal output to the DAC, which AC voltage signal is used to determine the half-cell impedances 201 and 101.

[0136] Reference numerals list

[0137] 1 Sensor circuit

[0138] 2 Electrochemical measurement points

[0139] 10 Voltage Source

[0140] 20 Control unit

[0141] 21 First Digital-to-Analog Converter

[0142] 22 First analog-to-digital converter

[0143] 23 Second DAC

[0144] 24 Second analog-to-digital converter

[0145] 25 Third DAC

[0146] 30 First impedance converter

[0147] 31 Input of the first impedance converter

[0148] 32 Output of the first impedance converter

[0149] 33 Second impedance converter

[0150] 34 Input of the second impedance converter

[0151] 35 Output of the second impedance converter

[0152] 36 Third Impedance Converter

[0153] 40 First input filter

[0154] 41 First capacitor of the first input filter

[0155] 41' Second capacitor of the first input filter

[0156] 41'' Third capacitor for the first input filter

[0157] 42 First resistor of the first input filter

[0158] 42 Second resistor of the first input filter

[0159] 45 Second input filter

[0160] 46 Capacitor for the second input filter

[0161] 50 First output filter

[0162] 51 First capacitor of the first output filter

[0163] 51' Second capacitor of the first output filter

[0164] 51'' Third capacitor of the first output filter

[0165] 51''' The fourth capacitor of the first output filter

[0166] 52 The first resistor of the first output filter

[0167] 52' Second resistor of the first output filter

[0168] 52'' Third resistor of the first output filter

[0169] 52''' The fourth resistor of the first output filter

[0170] 55 Second output filter 55

[0171] 56 Capacitor for the second output filter

[0172] 57 Resistor of the second output filter

[0173] 60 First terminal

[0174] 61 Second terminal

[0175] 62 Third terminal

[0176] 63 Fourth terminal

[0177] 64 Fifth terminal

[0178] 70 First Cable

[0179] 71 Inner conductor of first cable

[0180] 72 Shielding member of first cable

[0181] 73 Second Cable

[0182] 74 Inner conductor of the second cable

[0183] 75 Shielding of the second cable

[0184] 76 Third Cable

[0185] 100 First electrochemical half-cell

[0186] 101 Intrinsic resistance of the first electrochemical half-cell

[0187] 102 Input of the first electrochemical half-cell

[0188] 103 Output of the first electrochemical half-cell

[0189] 200 Second electrochemical half-cell

[0190] 201 Intrinsic resistance of the second electrochemical half-cell

[0191] 202 Input of the second electrochemical half-cell

[0192] 203 Output of the second electrochemical half-cell

[0193] A Input of the first output filter 50

[0194] A' The input of the second output filter 55

[0195] AA Input of the first input filter 40

[0196] AA' The input of the second input filter 45

[0197] B The output of the first output filter 50

[0198] B' The output of the second output filter 55

[0199] BB The output of the first input filter 40

[0200] BB' The output of the second input filter 45

[0201] ES1 First electrode signal

[0202] ES1' Derivative of the first electrode signal

[0203] ES2 Second Electrode Signal

[0204] ES2' The derivative of the second electrode signal

[0205] GS1 First DC voltage signal

[0206] GS1' The derivative of the first DC voltage signal

[0207] GND Second voltage potential

[0208] K1 The first protection signal of the second digital-to-analog converter 23

[0209] K2 The second protection signal of the third digital-to-analog converter 25

[0210] RC First RC element

[0211] RC' Second RC element

[0212] RC'' Third RC element

[0213] RC''' Fourth RC element

[0214] U1 first output voltage

[0215] US output signal

[0216] VCC first voltage potential

Claims

1. A method for operating an electrochemical measuring point (2), comprising the following steps: - providing an electrochemical measuring point (2), the electrochemical measuring point (2) having a sensor circuit (1), a first cable (70), at least one first electrochemical half-cell (100), wherein the sensor circuit (1) has a control unit (20), the control unit (20) having a first digital-to-analog converter (21), a first analog-to-digital converter (22) and a second digital-to-analog converter (23), wherein the sensor circuit (1) also has a first terminal (60), a second terminal (61) and a first input filter (40) arranged between the second digital-to-analog converter (23) and the second terminal (61), - measuring a first electrode signal (ES1) of the first electrochemical half-cell (100) at the second terminal (61), - the first digital-to-analog converter (21) outputs a superposition of a first DC voltage signal (GS1) and a first AC voltage signal (WS1) at the first terminal (60), - evaluating the first electrode signal (ES1) at the first analog-to-digital converter (22) such that an AC voltage amplitude offset and / or a phase shift between the AC voltage signal (WS1) and the first electrode signal (ES1) is determined, - determining a first half-cell impedance of the first electrochemical half-cell (100) based on the AC voltage amplitude offset and / or the phase shift, - determining a filter capacitance of the first input filter (40) and / or a cable capacitance of the first cable (70), - determining a time derivative (ES1') of said first electrode signal (ES1) and a time derivative (GS1') of said DC voltage signal GS1, - determining a first correction value based on the time derivative of the first electrode signal (ES1') and the time derivative of the first DC voltage signal (GS1') and the first half-cell impedance, - determining a measurement value based on said first electrode signal (ES1), - determining a display value based on said measured value and said first correction value, - Outputs the displayed value.

2. The method according to claim 1, wherein: The digital-to-analog converter (23) outputs a protection voltage to the second terminal (61), and a displacement current is determined based on a voltage difference between the first terminal (60) and the second terminal (61) and a cable impedance, wherein the displacement current is taken into account in the step of determining the correction value.

3. A method according to any one of the preceding claims, wherein: When determining the time derivative (ES1') of the first electrode signal (ES1), a time period of less than 3 seconds is used.

4. A method according to any one of the preceding claims, wherein: The cable length of the first cable (70) is input by the user and is taken into account when determining the cable capacitance.

5. A method according to any one of the preceding claims, wherein: When determining the half-cell impedance, a median filter or a mean filter is applied to the determined value of the AC voltage amplitude offset and / or the determined value of the phase shift relative to the determined value of the half-cell impedance, in particular a median filter or a mean filter over a period of more than 3 seconds.

6. A method for operating an electrochemical measuring point (2), comprising the following steps: - providing an electrochemical measuring point (2), the electrochemical measuring point (2) comprising a sensor circuit (1), a first cable (70), a second cable (73), a third cable (76), a first electrochemical half-cell (100) and a second electrochemical half-cell (200), wherein the sensor circuit (1) comprises a control unit (20), the control unit (20) comprises a first digital-to-analog converter (21), a first analog-to-digital converter (22) and a second digital-to-analog converter (23), wherein the sensor circuit (1) further comprises a first terminal (60), a second terminal (61) and a first input filter (40) arranged between the second digital-to-analog converter (23) and the second terminal (61), - measuring a first electrode signal (ES1) of the first electrochemical half-cell (100) at the second terminal (61), - determining a time derivative (ES1') of the first electrode signal (ES1) and a time derivative (ES2') of the second electrode signal (ES2) at the fourth terminal (63), - the first digital-to-analog converter (21) outputs a superposition of a first DC voltage signal (GS1) and a first AC voltage signal (WS1) at the first terminal (60), - evaluating the first electrode signal (ES1) at the first analog-to-digital converter (22) and evaluating the second electrode signal (ES2) at the second analog-to-digital converter (24) such that in each case an AC voltage amplitude offset and / or a phase shift between the DC voltage signal (GS1) or the AC voltage signal (WS1) and the first electrode signal (ES1) and the second electrode signal (ES2) is determined, - determining a first half-cell impedance of the first electrochemical half-cell (100) and a second half-cell impedance of the second electrochemical half-cell (200) based on the AC voltage amplitude offset and / or the phase shift, - determining a first correction value based on the time derivative (ES2') of the first electrode signal and the first half-cell impedance of the first half-cell (100), - determining a second correction value based on the time derivative (ES1') of the second electrode signal and the second half-cell impedance of the second half-cell (200), - determining a measurement value based on the first electrode signal (ES1) and the second electrode signal (ES2), - determining a display value based on the measurement value and the first correction value and the second correction value, - Outputs the displayed value.

7. An electrochemical measurement point (2), comprising: - a sensor circuit (1) having a control unit (20) adapted to carry out the method according to claims 1 to 5, wherein the sensor circuit (1) further comprises a first terminal (60) and a second terminal (61), - a first cable (70) having an inner conductor (71) and a shield (72), wherein the shield (72) is connected to the first terminal (60) and the inner conductor (71) is connected to the second terminal (61), - a first electrochemical half-cell (100) having an input (102) and an output (103), a second electrochemical half-cell (200) having an input (202) and an output (203), - wherein the input (102) of the first electrochemical half-cell (100) is suitable for connection to the measured medium and the output (103) of the first electrochemical half-cell (100) is connected to the shield (72), - wherein the input (202) of the second electrochemical half-cell (200) is suitable for connection to the measured medium and the output (203) of the second electrochemical half-cell (200) is connected to the inner conductor (71).

8. An electrochemical measurement point (2), comprising: - a sensor circuit (1) having a control unit (20) adapted to carry out the method according to claim 6, wherein the sensor circuit (1) further comprises a first terminal (60), a second terminal (61), a third terminal (62), a fourth terminal (63) and a fifth terminal (64), - a first electrochemical half-cell (100) having an input (102) and an output (103), a second electrochemical half-cell (200) having an input (202) and an output (203), - a first cable (70) having an inner conductor (71) and a shield (72), wherein the inner conductor (71) of the first cable (70) is connected to the second terminal (61) and the output (203) of the second half-cell (200), and the third terminal (62) is connected to the shield (72) of the first cable (70), - a second cable (73), the second cable (73) having an inner conductor (74) and a shield (75), wherein the inner conductor (74) of the second cable (73) is connected to the fourth terminal (63) and the output (103) of the first half-cell (100), and the shield (75) of the second cable (73) is connected to the fifth terminal (64), - a third cable (76), wherein the third cable (76) is connected to the first terminal (60) and to the input (102) of the first half-cell (100) and to the input (202) of the second half-cell (200).

9. An electrochemical measurement point (2), comprising: - a sensor circuit (1) having a control unit (20) suitable for carrying out the method according to claims 1 to 5, wherein the sensor circuit (1) comprises a first terminal (60), a second terminal (61), a third terminal (62) and a fourth terminal (63), - a first electrochemical half-cell (100) having an input (102) and an output (103), a second electrochemical half-cell (200) having an input (202) and an output (203), - a first cable (70) having an inner conductor (71) and a shield (72), wherein the inner conductor (71) of the first cable (70) is connected to the second terminal (61) and the output (203) of the second half-cell (200), and the fourth terminal (63) is connected to the shield (72) of the first cable (70), wherein the shield (72) is also connected to the output (103) of the first half cell (100), - a third cable (76), wherein the third cable (76) is connected to the first terminal (60) and to the input (102) of the first half-cell (100) and to the input (202) of the second half-cell (200).

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