Readout circuit for a capacitive sensor

Abstract

The invention relates to a readout circuit (100) for a capacitive sensor (120) with two capacitances (CS1, CS2), said circuit comprising a capacitance / voltage converter (130) for converting two periodic sensor signals (VS1, VS2) from the sensor that are associated with the capacitances into two amplified voltage signals (Vout1, Vout2) when a periodic excitation signal (Vexc) is applied, an interference detection unit (140) for detecting interference in one of the sensor signals, and a filter unit (150) configured to generate a measurement signal (Vred) with reduced interference from an interference detection signal (Vdet) from the interference detection unit and the voltage signals, wherein the interference detection unit is configured to evaluate at least one of the sensor signals when the excitation signal is applied to the sensor by generating a filtered signal by applying a high-pass filter to the one of the sensor signals and / or a combination of the sensor signals, forming a non-negative signal from the filtered signal, applying a window function for weighting areas of the non-negative signal, evaluating the weighted signal in order to detect interference, and generating and providing the interference detection signal on the basis of the detected interference.

Description

[0001] Description

[0002] title

[0003] Readout circuit for a capacitive sensor

[0004] Technical area

[0005] The present invention relates to the field of capacitive sensors and concerns a readout circuit, a sensor system based thereon as well as a corresponding ME MS component and a method for generating a noise-reduced measurement signal using a readout circuit according to the invention and / or a sensor system according to the invention.

[0006] State of the art

[0007] The position accuracy of actuators in microelectromechanical systems depends on the performance of the corresponding position sensor system. The position sensor system is dependent, for example, on manufacturing tolerances, temperature stability in conjunction with temperature variance, and long-term stability in conjunction with the product lifetime or the time at which the device needs to be recalibrated.

[0008] The document DE 10 2015 204 874 Al relates to an actuator device for tilting a mirror element.

[0009] The document DE 10 2016 213 026 Al relates to a sensor device for detecting a tilt angle of a mirror element.

[0010] Both documents each describe an optical component comprising an actuator device for tilting a mirror element with two degrees of freedom and a sensor device for detecting a tilt angle of the mirror element. The actuator device comprises an actuator electrode structure with actuator electrodes designed as comb electrodes and forming a direct drive for pivoting the mirror element. The sensor device comprises a sensor electrode structure with sensor electrodes, which are also designed as comb electrodes. The optical component further comprises a gimbal-mounted flexure joint for supporting the mirror element, wherein the flexure joint has flexure springs and defines two tilt axes of the mirror element.

[0011] The two documents further describe that the sensor electrode structure comprises sensor stator electrodes attached to a substrate for supporting the mirror element, and sensor mirror electrodes attached to the mirror element. To calculate the tilt angle of the mirror element, the sensor mirror electrodes are configured to provide variable shielding of an electric field in the region of the sensor stator electrodes, depending on the detected tilt angle of the mirror element. The sensor mirror electrodes are subjected to the same voltage. In other words, the sensor mirror electrodes have the same potential as, for example, ground potential.

[0012] Disclosure of the invention

[0013] According to the invention, a readout circuit, a sensor system based thereon, a corresponding MEMS component and a method for generating a noise-reduced measurement signal using a readout circuit according to the invention and / or a sensor system according to the invention are proposed.

[0014] According to a first aspect of the invention, a readout circuit for a capacitive sensor with two capacitances is proposed, wherein the readout circuit comprises a capacitance-to-voltage converter for converting two periodic sensor signals of the sensor, associated with the capacitances, into two amplified voltage signals upon application of a periodic excitation signal to the sensor. The periodic excitation signal can be or comprise, for example, a sinusoidal oscillation, a triangular oscillation, a sawtooth oscillation, or a rectangular oscillation. Furthermore, the readout circuit comprises a disturbance detection unit for detecting disturbances, such as irregularities and / or interference pulses, in at least one of the sensor signals, preferably in both sensor signals, and a filter unit.The filter unit is configured to generate a noise-reduced measurement signal from a noise detection signal from the noise detection unit and the two voltage signals from the capacitance-to-voltage converter. The noise detection unit thus provides the noise detection signal to the filter unit, with the noise detection signal being a signal that contains information regarding the detected noise. The noise detection unit and / or the filter unit can be implemented, for example, as an analog circuit in a mixed-signal ASIC (application-specific integrated circuit).

[0015] In this case, the disturbance detection unit is configured to evaluate at least one of the two sensor signals by means of at least the following steps when the periodic excitation signal is applied to the sensor: generating a filtered signal by applying a high-pass filter to at least one of the two sensor signals and / or to a combination, for example averaging, of the two sensor signals, then forming a non-negative signal (which refers to a signal whose values ​​are all greater than or equal to zero) from the filtered signal, preferably by forming the magnitude, i.e. the absolute value, or the square of the amplitude of the filtered signal, applying a window function to the non-negative signal to weight areas of the non-negative signal, evaluating the weighted signal, typically using the excitation signal,for detecting disturbances and finally generating and providing the disturbance detection signal based on the detected disturbances. The magnitude or square of the amplitude of the filtered signal is calculated separately for all measurement points of the sensor signal under consideration.

[0016] The readout circuit preferably comprises a fully symmetrical operational amplifier, two integrating capacitors, and two switches, each connected in parallel to one of the two integrating capacitors and connected to one of the sensor capacitors. When closed, the switches discharge the parallel-connected integrating capacitor of the readout circuit and the connected capacitor of the sensor. Preferably, the fault detection unit is configured to detect faults in each of the sensor signals. Therefore, if the capacitive sensor provides two sensor signals, the fault detection unit is preferably configured to detect faults in both of the two sensor signals.Likewise, the fault detection unit is preferably configured to use one or more additional signals from one or more additional components to detect faults in at least one of the sensor signals, i.e., signals from components that are not identical to the capacitive sensor. For example, signals from a temperature measurement, a ground connection, and / or a component where the fault to be detected originates could be used for this purpose.

[0017] Preferably, the disturbance detection unit is further configured to detect disturbances (disturbance detection) in one of the sensor signals independently of one another, both for a high level of the excitation signal and for a low level of the excitation signal, in the case of a square wave as the excitation signal. Such separate detection by the disturbance detection unit simplifies the implementation of disturbance detection in a circuit.

[0018] The window function is preferably selected such that artifacts in the sensor signal caused by the excitation signal are reduced or removed. For example, in the case of a square wave as the excitation signal with rising and falling edges, the window function used can assume a value other than zero for the regions between adjacent rising and falling edges and a value equal to zero outside of these regions in order to remove any signal overshoot occurring in the region of the edges for the evaluation process. Alternatively, it is also conceivable that with such an excitation signal, the window function used assumes a value other than zero for the regions between adjacent falling and rising edges and a value equal to zero outside of these regions.Furthermore, a combination is also conceivable in which the window function assumes the value zero in areas around the edges and has a value other than zero elsewhere. In the case of a square wave as the excitation signal, and if the disturbance detection unit is configured to detect disturbances in one of the sensor signals independently of each other for both a high level of the excitation signal and a low level of the excitation signal, the steps of applying a window function and evaluating the thus weighted signal can be performed independently of each other for both a high level of the excitation signal and a low level of the excitation signal. The disturbance detection signal is then generated on the basis of the detected disturbances, which are detected by evaluating the two weighted signals.

[0019] According to a second aspect of the invention, a sensor system with a capacitive sensor is proposed, wherein the sensor has two capacitances. The sensor system comprises a readout circuit as described above.

[0020] The sensor system preferably comprises a device for generating the periodic excitation signal, a unit for further processing the noise-reduced measurement signal into a further-processed output signal, and an analog-to-digital converter for digitizing the further-processed output signal. The periodic excitation signal can, for example, be or comprise a sinusoidal oscillation, a triangular oscillation, a sawtooth oscillation, or a rectangular oscillation.

[0021] A particularly preferred field of application for sensor systems according to the invention is represented by MEMS-based components (MEMS components, MEMS: microelectromechanical system) that have a displacement device for tilting a component, such as a micromirror, about a tilt axis. Here, a sensor system as described above can be used as part of a MEMS component to determine a tilt angle of the displacement device. According to a third aspect of the invention, such a MEMS component with a sensor system as described above is accordingly proposed. The capacitances can be implemented, for example, by means of sensor electrodes in the form of comb electrodes.

[0022] Finally, according to a fourth aspect of the invention, a method for generating a noise-reduced measurement signal for a capacitive sensor with two capacitances using a readout circuit and / or a sensor system as described above is proposed.The method comprises applying a periodic excitation signal to the sensor, detecting two periodic sensor signals of the sensor assigned to the capacitances, generating two amplified voltage signals by converting the periodic sensor signals using a capacitance-to-voltage converter, generating a filtered signal by applying a high-pass filter to one of the sensor signals, forming a non-negative signal from the filtered signal, for example by forming the magnitude or square of the amplitude of the filtered signal, applying a window function to weight areas of the non-negative signal and finally evaluating the weighted signal to detect disturbances, generating a disturbance detection signal based on the detected disturbances and generating the disturbance-reduced measurement signal from the disturbance detection signal and the two voltage signals of the capacitance-to-voltage converter.

[0023] Advantages of the invention

[0024] The invention discloses a simple way to prevent and reduce electrical interference during measurements using capacitive sensors. In particular, it is not necessary to implement a separate interference detection unit with corresponding circuitry for each readout channel of one or more capacitive sensors. One interference detection unit can be used for multiple channels. Accordingly, the present approach is particularly advantageous with regard to the required power consumption.

[0025] A further advantage is that the supply of sensor signals to the fault detection unit can be flexibly designed and expanded. For example, signals from additional components, i.e., components that are not identical to the capacitive sensor under consideration and which are also affected by potential faults, can also be supplied to the fault detection unit to further increase the accuracy of fault detection.

[0026] Short description of the drawings

[0027] Embodiments of the invention are explained in more detail with reference to the drawings and the following description. They show:

[0028] Figure 1 is a schematic representation of a circuit of a sensor system according to the invention in an ME MS component;

[0029] Figure 2 is a flowchart illustrating the operation of an exemplary fault detection unit of a sensor system according to the invention; and

[0030] Figures 3A, 3B are graphs illustrating a method according to the invention for generating a noise-reduced measurement signal.

[0031] Embodiments of the invention

[0032] In the following description of the embodiments of the invention, identical or similar elements are designated by the same reference numerals, whereby a repeated description of these elements is omitted in individual cases. The figures only schematically illustrate the subject matter of the invention.

[0033] Figure 1 shows a schematic representation of a circuit of a sensor system 101 according to the invention in a MEMS component 102 with a capacitive sensor 120 with two capacitances Csi, Cs2 and a readout circuit 100. The MEMS component 102 comprises a displacement device 103, the tilt angle of which can be determined by means of the capacitive sensor 120. This relationship is indicated by an arrow 104.

[0034] In the illustrated readout circuit 100, a capacitance-to-voltage converter 130 is used to convert the capacitance values ​​measured by the capacitances Csi, Cs2 when a periodic excitation signal Vexc is applied to the sensor 120 into two amplified voltage signals that can be further processed. A device 110 for generating the periodic excitation signal Vexc, which is a voltage signal, is electrically connected to the sensor 120. The capacitance difference between the two capacitances Csi and Cs2 is to be determined. The applied periodic excitation signal V exc Csi and Cs2 are charged and discharged. The resulting signals Vsi and Vs2 are fed to a fully symmetrical operational amplifier 170, which converts their difference into an amplified voltage signal. Such a capacitive sensor concept enables high resolution with very small signals. For the periodic excitation signal V excA square wave is preferably selected, meaning the voltage alternates between a high and a low voltage level. The high voltage level (high level) enables measurement of the current capacitance and therefore refers to an active measurement step. The low level (low level) refers to the passive measurement step and is preferably, but not necessarily, set to zero volts.

[0035] The conversion of the current capacitances Csi and Cs2 into voltage values ​​occurs with such a circuit in two steps: In the first step, the signal Vexc drops to its low level (passive measurement step), and the switches Swi and Sw2 connected in parallel with the capacitances Csi and Cs2 are closed to discharge Csi, Cs2, and the two integrating capacitances Cu and O2. In the second step, V excto its high level (active measurement step). At the same time, Swi and Sw2 are opened, the capacitances Cu and O2 integrate the charges, and the operational amplifier 170 converts the various charges into voltage values.

[0036] According to the invention, a fault detection unit 140 is connected in parallel to the capacitance-voltage converter 130, which detects the signal Vsi of the capacitance Csi using the excitation signal V exc for disturbances. For this purpose, the disturbance detection unit 140 is configured, in the case of the application of the periodic excitation signal V excto the sensor 120 to evaluate the sensor signal Vsi by means of at least the following steps: First, a filtered signal is generated by applying a high-pass filter to the sensor signal Vsi. After forming a non-negative signal from the filtered signal and applying a window function to weight areas of the non-negative signal, the weighted signal is evaluated using the excitation signal V exc for detecting disturbances and finally generating and providing the disturbance detection signal Vdet based on the disturbances detected in this way. Based on this signal Vdet, for example based on the positions of the detected disturbances, the disturbance detection unit 140 and the outputs V ou ti, V ou t2 of the operational amplifier 170 connected filter unit 150 a noise-reduced measurement signal V red. For example, the interference detection unit 140 decides which measurement result is output as a valid value. This procedure generates the interference-reduced measurement signal V re d.

[0037] The basic functionality of the capacitance-to-voltage converter 130 described above is not affected by this procedure. The fault detection unit 140 can be used jointly by several capacitance-to-voltage converters 130. Figure 1 shows, purely as an example, a circuit that implements fault detection solely for signals Vsi coming from the capacitance Csi. However, the same detection approach can also be applied to signals Vs2 coming from Cs2. A combination, for example, averaging, of the two signals Vsi and Vs2 could also serve as fault detection using the fault detection unit 140.Likewise, additional signals influenced by disturbances to be detected and originating from components other than sensor 120, i.e., not from capacitances Csi and Cs2, can be coupled into readout circuit 100 and then fed to disturbance detection unit 140 to further improve the precision of disturbance detection. The noise-reduced measurement signal V can be used for further signal processing. re d is finally passed on to a corresponding unit 160. The thus further processed output signal Vproc can then be digitized by means of an analog-to-digital converter 180 and thus made available for digital use, for example.

[0038] Figure 2 shows a flowchart illustrating the functionality of an exemplary fault detection unit 140 of a sensor system 101 according to the invention, as shown in Figure 1. The associated Figures 3A and 3B show, as graphs, the signals after the corresponding steps from Figure 2. All graphs have identical time axes as horizontal axes (x-axes) and the vertical axes (y-axes) each represent voltage values. The specific voltage and time values ​​are to be understood purely as examples; they serve solely to illustrate the invention. The units of the plotted time and voltage values ​​are irrelevant for the following explanation; for example, the voltage values ​​could be given in volts and the time values ​​in seconds.For the plotting of sub-figures (a) to (g), identical units were chosen for the voltage values ​​so that they are comparable with each other, whereby the specified scaling factor must be observed. Sub-figures (a) to (c) of Figures 3A and 3B continue to show the same signals, i.e., the steps for evaluating the sensor signals by the fault detection unit 140 are initially identical.

[0039] The original sensor signal to be used by the disturbance detection unit 140 to detect disturbances, for example Vsi, is shown in Figures 3A and 3B, respectively, in sub-figure (a) and is provided in step 210. It contains periodic voltage shifts caused by the excitation signal V excThe peaks on the rising and falling edges are overshoots and undershoots due to the corresponding abrupt voltage changes in the excitation signal Vexc, which is shown as a dashed line in sub-figures (i). Two disturbances to be detected can be observed in the second active and sixth passive measurement steps and are highlighted by ellipses in sub-figures (a) of Figures 3A and 3B.

[0040] In step 220, a high-pass filter is applied to the original sensor signal in order to remove the portion of the excitation signal V excto achieve. The result is shown in sub-figures (b). The disturbances to be detected are still clearly visible, and the overshoots and undershoots are also still visible after step 220 of the high-pass filtering. In the next step 230, as shown in sub-figures (c), the absolute value of the signal amplitude is determined. Alternatively, the square of the amplitude can also be used. The purpose of this step 230 is to convert the bipolar signal into a non-negative form. The non-negative signal thus generated is multiplied by a window function in the time domain in the next step, namely 240a for the active measurement steps and step 240b for the passive measurement steps, whereby the window functions are selected differently between the active and passive measurement steps. Disturbance detection in the case of active measurement steps is shown in Figure 3A, the variant for the passive measurement steps in Figure 3B.The goal of using the window function is to suppress the overshoots and undershoots caused by the excitation voltage. For this purpose, the window functions can, for example, have very small coefficients at the overshoot and undershoot positions and very large coefficients for the remaining signal regions. The simplest window function could therefore be a rectangular function with the value "0" in the overshoot and undershoot regions and "1" for the rest of the signal. Corresponding signals after applying 240a, 240b such a window function are shown for the active measurement step (Figure 3A) and the passive measurement step (Figure 3B) in sub-figures (d).

[0041] The windowed signals are now further evaluated (steps in the areas 245a and 245b marked in Figure 2). In the next step 250a, 250b, an integration is performed, the result of which integration can be seen in sub-figures (e). The corresponding trigger signals, which indicate when an integration is to be performed for which signal areas, are shown as solid lines in sub-figures (i). In the case of the active measurement step (Figure 3A), the integrated value is reset at the time of the rising edge of the square-wave signal. It can be seen that the rising edge of the trigger signal comes later than that of the excitation signal Vexc. However, the falling edges of both signals are at the same positions. As can be seen in sub-figure (e) of Figure 3A, the disturbance in the second active measurement step causes a rapid rise in the integrated signal.For the passive measurement step (Figure 3B), the integrated value is reset on each rising edge of the trigger signal, as shown in sub-figure (i) of Figure 3B. The disturbance in the sixth passive measurement step also causes a rapid rise of the integrated signal.

[0042] The rapid rise of the integrated signal provides a reliable feature for detecting a disturbance. To detect the rapidly rising edge, the values ​​of the integrated signal are continuously compared in steps 270a, 270b with a previous value that precedes it by a time difference ΔT. The time difference ΔT should be small enough so that the operating conditions do not change significantly during this period. To realize such a comparison of two values, sample-and-hold operations are applied to the integrated signal at different time positions. For the active measurement step (Figure 3A), the integrated value is sampled at the falling edge of the trigger function from sub-figure (i) as shown in sub-figure (f) in step 260a and compared in step 270a with the value obtained at a previous rising edge of the excitation signal V eXc, as shown in sub-figure (g), was sampled in step 262a. If the current value is greater than the previous one, a disturbance is detected. Otherwise, no disturbance is assumed. The positions of the corresponding values ​​are illustrated by ellipses in sub-figure (e). Sub-figure (h) of Figure 3A shows the successful detection of the disturbance in the second active measurement step; the signal was generated from the signals of sub-figures (f) and (g). For the passive measurement step (Figure 3B), in step 260b the integrated value is sampled at the falling edge of the trigger function from sub-figure (i), as shown in sub-figure (f), and in step 270b it is compared with the value that, as shown in sub-figure (g), was sampled in step 262b at a previous falling edge of the excitation signal V excwas scanned. Figure (h) shows that the disturbance was successfully detected in the sixth passive measurement step. As in Figure 3A, the positions of the values ​​used for the analysis are indicated by ellipses in Figure 3B in Figure (e).

[0043] Finally, the status of the fault detection for each measurement step is checked in steps 280a, 280b. If a fault has been detected for a measurement step, i.e., either for an active measurement step or a passive measurement step, the corresponding output signal is not considered a valid value. Otherwise, the value can be stored in an output buffer, and the last valid value is released at the end of a readout period. With such a procedure, it is advantageous to average several valid measurements for each readout period in order to achieve an even better signal-to-noise ratio, for example, by averaging these values. The fault detection signal Vdet generated in this way in step 285 can finally be provided in step 290 in order to generate a noise-reduced measurement signal V re d to generate.

[0044] The invention is not limited to the embodiments described here and the aspects highlighted therein. Rather, numerous modifications are possible within the scope of the claims, which are within the scope of one skilled in the art.

Claims

Claims Readout circuit (100) for a capacitive sensor (120) with two capacitances (Csi, Cs2), the readout circuit comprising: - a capacitance-voltage converter (130) for converting two periodic sensor signals (Vsi, Vs2) of the sensor (120) associated with the capacitances (Csi, Cs2) into two amplified voltage signals (Vouti, V ou t2) when a periodic excitation signal (Vexc) is applied to the sensor (120); - a disturbance detection unit (140) for detecting disturbances in at least one of the sensor signals (Vsi, Vs2); and - a filter unit (150) which is arranged to generate a fault detection signal (Vdet) from the fault detection unit (140) and the two voltage signals (V ou ti, V ou t2) of the capacitance-voltage converter (130) a noise-reduced measuring signal (V red), wherein the disturbance detection unit (140) is arranged, in the case of application of the periodic excitation signal (V exc ) to the sensor (120) to evaluate at least one of the sensor signals (Vsi, Vs2) by means of at least the following steps: a. generating (220) a filtered signal by applying a high-pass filter to the at least one of the sensor signals (Vsi, Vs2) and / or to a combination of the sensor signals (Vsi, Vs2); b. forming (230) a non-negative signal from the filtered signal; c. applying (240a, 240b) a window function for weighting regions of the non-negative signal; d. evaluating (245a, 245b) the weighted signal for detecting disturbances; and e. generating (285) and providing (290) the disturbance detection signal (Vdet) on the basis of the detected disturbances.

2. Readout circuit (100) according to claim 1, wherein the capacitance-voltage converter (130) comprises: - a fully symmetrical operational amplifier (170); - two integrating capacitances (Cu, Ci2); and - two switches (S) each connected in parallel to one of the two integrating capacitors (Cu, O2) and connected to one of the capacitors (Csi, Cs2) of the sensor (120) w i, S W 2), which, when closed, discharge the parallel-connected integrating capacitance (Cu, Ci2) and the connected capacitance (Csi, Cs2) of the sensor (120).

3. Readout circuit (100) according to one of the preceding claims, wherein the disturbance detection unit (140) is configured to detect disturbances in each of the sensor signals (Vsi, Vs2) of the sensor (120).

4. Readout circuit (100) according to one of the preceding claims, wherein the disturbance detection unit (140) is configured to use one or more further signals from one or more further components for detecting disturbances in at least one of the sensor signals (Vsi, Vs2).

5. Readout circuit (100) according to one of the preceding claims, wherein the disturbance detection unit (140) is arranged, in the case of a square wave as the excitation signal (V exc ) detecting disturbances in one of the sensor signals (Vsi, Vs2) independently of each other both for a high level of the excitation signal (V exc ) as well as for a low level of the excitation signal (V exc ) to be carried out.

6. Readout circuit (100) according to claim 5, wherein in the case of a square wave as excitation signal (V exc ) steps c and d are carried out independently of each other for both a high level of the excitation signal (V exc) as well as for a low level of the excitation signal (V exc ) and the generation (285) of the disturbance detection signal (Vdet) is carried out on the basis of the detected disturbances which are detected by the evaluation (245a, 245b) of the two weighted signals.

7. Sensor system (101) with a capacitive sensor (120) with two capacitances (Csi, Cs2) and a readout circuit (100) according to one of the preceding claims.

8. Sensor system (101) according to claim 7, wherein the sensor system (101) further comprises a device (110) for generating the periodic excitation signal (Vexc), a unit (160) for further processing the interference-reduced measurement signal (V re d) to a further processed output signal (Vproc) and an analog-to-digital converter (180) for digitizing the further processed output signal (V prO c) includes.

9. Sensor system (101) according to claim 8, wherein the periodic excitation signal (Vexc) is or comprises a sinusoidal oscillation, a triangular oscillation, a sawtooth oscillation or a rectangular oscillation.

10. MEMS component (102) comprising a displacement device (103) for tilting a component about a tilt axis and a sensor system (101) according to one of claims 8 to 9 for determining a tilt angle of the displacement device (103).

11. Method for generating a noise-reduced measurement signal (V re d) for a capacitive sensor (120) with two capacitances (Csi, Cs2) using a readout circuit (100) according to one of claims 1 to 6 and / or a sensor system (101) according to one of claims 7 to 9 and comprising the following steps: a. Applying a periodic excitation signal (V exc) to the sensor (120); b. detecting two periodic sensor signals (Vsi, Vs2) of the sensor (120) associated with the capacitances (Csi, Cs2); c. generating two amplified voltage signals (V ou ti, V ou t2) by converting the periodic sensor signals (Vsi, Vs2) by means of a capacitance-to-voltage converter (130); d. generating (220) a filtered signal by applying a high-pass filter to one of the sensor signals (Vsi, Vs2); e. forming (230) a non-negative signal from the filtered signal; f. applying (240a, 240b) a window function to weight areas of the non-negative signal; g. evaluating (245a, 245b) the weighted signal to detect disturbances and generating (285) a disturbance detection signal (Vdet) based on the detected disturbances; and h. generating the noise-reduced measurement signal (V re d) from the fault detection signal (Vdet) and the two voltage signals (V ou ti, Vou t2) of the capacitance-voltage converter (130).

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