Capacitive sensor device with EMI-robust capacitive measuring circuit

Abstract

Capacitive sensor device (31) designed to be connected between an electrical heating element (52) and a heating power supply (46) and to use the electrical heating element (52) as an antenna electrode, comprising - a common-mode choke (43) with a first and a second inductively coupled winding (44, 45), wherein the first winding (44) is designed to be connected between a first terminal (47) of the heating current supply (46) and a first terminal of the electric heating element (52), and wherein the second winding (45) is designed to be connected between a second terminal of the electric heating element and a second terminal (48) of the heating current supply (46); and - a control and evaluation circuit (32) designed to feed a periodic alternating measurement signal into the electrical heating element (52) via a measuring node (40), to measure an electrical quantity through the measuring node (40) and to derive an electrical impedance (53) between the electrical heating element (52) and a counter electrode based on the measured electrical quantity; characterized in that the control and evaluation circuit (32) comprises the following: - a third common-mode choke winding (33) which is inductively coupled to the first winding (44) and the second winding (45) of the common-mode choke (43); - a periodic signal voltage source (36) designed to apply an AC measuring voltage to an output terminal, wherein the output terminal is electrically directly connected to a first connection (34) of the third common-mode choke winding (33); - an electrical quantity measurement circuit (37) with a signal input terminal (38) and a reference input terminal (39) designed to determine the electrical quantity through the measuring node (40) using an electrical reference quantity provided to the reference input terminal (39), wherein the signal input terminal (38) is electrically connected to a second terminal (35) of the third common-mode choke winding (33) and the reference input terminal (39) is electrically connected to an earth conductor providing an AC earth potential; and - an EMI filter network (41) which is electrically connected via the signal input terminal (38) and the reference input terminal (39) of the electrical quantity measurement circuit (37).

Description

Technical field

[0001] The present invention generally relates to capacitive scanning, e.g. for detecting the presence or absence of a person on a seat (seat occupancy detection) or the presence or absence of a person's hand on the steering wheel of a car (hand touch detection).

[0002] In particular, the present invention relates to a capacitive sensor device which uses a heating element as an antenna electrode, and to a seat occupancy detection system for detecting an occupancy of the seat, in particular a vehicle seat, which includes such a capacitive sensor device. Background of the invention

[0003] Capacitive sensors and capacitive measurement and / or detection systems that employ capacitive sensors have a wide range of applications and are used, among other things, for detecting the presence and / or position of a conductive body near an antenna electrode. As used here, the term "capacitive sensor" refers to a sensor that generates a signal responding to the influence of the detected object (a person, a part of a person's body, a pet, an object, etc.) on an electric field. A capacitive sensor generally includes at least one antenna electrode to which an oscillating electrical signal is applied, and which then emits an electric field into a region of space near the antenna electrode while the sensor is operating.The sensor has at least one sensing electrode, which may be identical to or different from emission antenna electrodes, at which the influence of an object or a living being on the electric field is detected.

[0004] Various capacitive sensing mechanisms are explained, for example, in the technical document entitled "Electric Field Sensing for Graphical Interfaces" by JR Smith et al., published in IEEE Computer Graphics and Applications, 18(3): 54-60, 1998. This document describes the concept of sensing an electric field as it is used to perform non-contact three-dimensional position measurements, and in particular to sensing the position of a human hand for the purpose of inputting three-dimensional positions into a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms, which he refers to as "load mode," "parallel mode," and "transmit mode," corresponding to the different possible paths for the electric current. In "load mode," an oscillating voltage signal is applied to a transmitting electrode, which generates an oscillating electric field relative to ground.The object being sensed modifies the capacitance between the transmitting electrode and ground. In "parallel mode," also known as "coupling mode," an oscillating voltage signal is applied to the transmitting electrode, creating an electric field at a receiver electrode, and the displacement current measured at the receiver electrode is recorded. The measured displacement current depends on the sensed object. In "transmitting mode," the transmitting electrode is brought into contact with the user's body, who then becomes a transmitter relative to a receiver, either through a direct electrical connection or via capacitive coupling.

[0005] The capacitive coupling strength can be determined, for example, by applying an AC voltage signal to an antenna electrode and measuring the current flowing from this antenna electrode either to ground (in charging mode) or into a second antenna electrode (in coupling mode). This current can be measured by a transimpedance amplifier connected to the measuring electrode, which converts the current flowing into the sensor electrode into a voltage proportional to the current.

[0006] Capacitive sensors that use a heating element as an antenna electrode are known in patent literature. For example, US 2011 / 0148648 A1 discloses a capacitive occupant detection system for a vehicle seat using a seat heating element 12 as an antenna electrode. Fig. Figure 1 schematically illustrates this state of the art. The voltage source 2 provides the power supply for the heating element, for example, as a seat heating control unit. The electronic control module (ECM) 1 is designed as a capacitive measuring circuit. It comprises a common-mode choke 5, an AC voltage source 9, and capacitors 6, 7, and 8. Capacitor 8 couples the AC voltage generated by the AC voltage source 9 to node 11. The heating element 12 is represented by a complex impedance 13 to ground. The complex impedance 13 includes a capacitive component and a resistive component, which depend on the occupancy state of the vehicle seat. The complex impedance 13 is therefore also referred to below as the "unknown impedance" or "impedance to be determined." Capacitor 8, together with the unknown impedance 13, forms a voltage divider. The complex voltage U measThe connection between node 11 and ground 10 can be used to calculate the complex, unknown impedance 13. Due to its high impedance, the common-mode choke 5 decouples the AC voltage at node 11 from the AC ground. The heating element 12 can simultaneously be energized by the DC current supplied by voltage source 2 and operated with the AC voltage through the capacitive measuring circuit. Capacitors 6 and 7 ensure a defined AC ground on the side of the common-mode choke 5 connected to the DC supply of the seat heater. Ground 3 is the reference ground. The terminals of the common-mode choke 5 are numbered 5.1 to 5.4: terminal 5.1 connects the first winding to the high-voltage side of voltage source 2; terminal 5.2 connects the first winding to the high-voltage side of heating element 12; terminal 5.3 connects the second winding to the low-voltage side of the heating element 12, and terminal 5.4 connects the second winding to the low-voltage side of the voltage source 2.

[0007] Resistor 4 represents the wiring resistance of the connection between the low-voltage side of voltage source 2 and the fourth terminal 5.4 of common-mode choke 5. A similar wiring resistance exists for the upper connection between the high-voltage side of voltage source 2 and common-mode choke 5, but this can be disregarded for the following explanation. Typically, voltage source 2, which represents the seat heater control unit, is switched on and off periodically to control the heating energy of the seat heater 12 according to a pulse-width modulation schedule. A typical switching frequency would be, for example, 25 Hz.Each time the voltage source 2 is switched on, the current through the wiring resistor 4 increases from essentially 0 A to the operating current of the seat heater, which, for example, with a voltage of 12 V from the voltage source 2, a seat heater resistance of 1 Ω, and a wiring resistance of 0.1 Ω, is approximately 10.9 A. This current of 10.9 A produces a voltage drop of 1.09 V across the wiring resistor 4 each time the voltage source 2 is switched on. This implies that the voltage at the fourth terminal 5.4 of the common-mode choke 5 increases to 1.09 V, and consequently, the voltage at node 11 also increases to 1.09 V. The resistance of the second winding of the common-mode choke 5 is not considered here, but due to its finite conductance, it will also contribute to an additional voltage drop.The voltage step of 1.09 V at the sampling node can interfere with the measurement of the signal voltage at sampling node 11 because the step function has a wide frequency bandwidth. The situation worsens if the seat heating control unit connected to the electronic control module 1 interrupts the heating circuit not on the high-voltage side, but on the low-voltage side. This means that node 11 experiences a voltage drop of approximately 12 V - 1.09 V = 10.91 V, which is worse than the aforementioned 1.09 V step. This situation can occur, for example, if, for cost reasons, one type of electronic control module 1 must be usable for capacitive sampling with different types of seat heating control units.

[0008] A further challenge arises for capacitive measurement circuits intended for automotive applications, which must therefore comply with regulations and standards in the automotive sector, such as ISO 11451-4 (Road vehicles - Vehicle test methods for electrical disturbances from narrowband radiated electromagnetic energy - Part 4: Bulk current injection (BCI)). When operating with a carrier frequency > 1 MHz (for example, in the range between 4 and 6 MHz), capacitive measurement circuits are easily disturbed by injected high-frequency (HF) currents during BCI testing (BCI = Bulk Current Injection).

[0009] The international application WO 2014 / 096127 A1 or the DE 11 2013 006 074 T5 describe a capacitive sensor designed for connection between a heating element and a heating power supply. Fig. Figure 2 schematically illustrates this state of the art.

[0010] The DC voltage source 20 is designed to supply DC current to the heater 25 via the common-mode choke 22. The unknown impedance 26 is measured by the transimpedance amplifier 27, which is driven at its reference input by an AC voltage source 24. Since the voltage at the output 28 of the transimpedance amplifier 27 indicates the current into the signal input of the transimpedance amplifier 27, the voltage at the output 28 also indicates the current through the unknown impedance 26 and thus the unknown impedance value. Reference symbol 21 represents the system ground. The capacitor 23 is a simple means of filtering out high-frequency currents injected during the BCI test by short-circuiting them to the output of the AC voltage source 24. However, it should be noted that the output impedance of the AC voltage source 24 is not zero, which implies that a section of the injected high-frequency current is not short-circuited to ground.

[0011] Other capacitive sensors designed for connection between a heating element and a heating power supply are also known from publications DE 11 2011 104 407 T5 and DE 11 2013 005 610 T5. Object of the invention

[0012] Therefore, the invention is based on the objective of providing a capacitive sensor device, in particular for connection between an electrical heating element and a heating current supply, which has improved properties at least with regard to EMI robustness, in particular with regard to EMI generated during the execution of a BCI test protocol, while retaining the advantages of using a common-mode choke as described above. General description of the invention

[0013] According to one aspect of the present invention, the problem is solved by a capacitive sensor device designed to be connected between an electrical heating element and a heating power supply and to use the heating element as an antenna electrode.

[0014] The capacitive sensor device includes a common-mode choke and a control and evaluation circuit.

[0015] The common-mode choke has a first and a second winding that are inductively coupled. The first winding is designed to be connected between the first terminal of the heater power supply and the first terminal of the heater element. The second winding is designed to be connected between the second terminal of the heater element and the second terminal of the heater power supply.

[0016] The control and evaluation circuit is designed to feed a periodic alternating measurement signal into the heating element via a measuring node, and it is further designed to measure an electrical quantity through the measuring node and to derive an electrical impedance between the heating element and a counter electrode based on the measured electrical quantity.

[0017] The control and evaluation circuit includes - a third common-mode choke winding, which is inductively coupled to the first winding and the second winding of the common-mode choke; - a periodic signal voltage source designed to apply an AC measuring voltage to an output terminal, wherein the output terminal is electrically directly connected to a first terminal of the third common-mode choke winding; - an electrical quantity measurement circuit with a signal input terminal and a reference input terminal, designed to determine the electrical quantity through the measuring node using an electrical reference quantity provided to the reference input terminal, wherein the signal input terminal is electrically connected to a second terminal of the third common-mode choke winding and the reference input terminal is electrically connected to an earth conductor providing an AC earth potential; and - an EMI filter network (EMI = Electromagnetic Interference) that is electrically connected via the signal input terminal and the reference input terminal of the electrical measurement circuit.

[0018] The measuring node is operationally coupled to the third winding in order to inductively feed the periodic alternating measurement signal into the heating element.

[0019] The advantage of the capacitive sensor device lies in the fact that the EMI filter, which is electrically connected via the signal input and reference input terminals of the electrical measurement circuit, is directly connected to the AC potential, i.e., via a low-impedance electrical path, and not to the protective node potential. This allows for improved suppression of RF currents injected during the execution of a BCI test protocol.

[0020] The term “designed for” as used in this application is understood in particular to mean specially programmed, designed, set up or arranged.

[0021] It should also be noted that the terms “first”, “second”, etc. are used in this application for the purpose of differentiation only and are in no way intended to indicate or anticipate any order or priority.

[0022] Preferably, the third common-mode choke winding is inductively coupled in the same winding direction as the first and second windings. This advantageously eliminates the need to compensate for the effect of a phase reversal in the electrical quantity to be determined by the electrical measurement circuit.

[0023] In some embodiments, the first, second, and third windings of the common-mode choke are arranged within a single housing. This allows for a compact design and minimizes unwanted magnetic stray fields. Consequently, the AC signal supplied to the heating element via inductive coupling has essentially the same amplitude as the original AC measurement voltage at the measuring node.

[0024] If no 1-to-1 transformation is intended between the third winding and each of the first and second windings, the third winding may have a number of turns that differs from the number of turns of the first winding or the second winding, respectively.

[0025] In a preferred embodiment of the capacitive sensor device, the electrical measurement circuit is designed as a current measurement circuit, which is configured to determine a sensing current flowing through the third common-mode choke winding based on a reference voltage and indicates the position of an object relative to the electrical heating element. This embodiment is particularly advantageous for operating the heating element as an antenna electrode in charging mode.

[0026] Preferably, the sensing current measurement circuit includes a transimpedance amplifier (TIA), and the signal input terminal and the reference input terminal form part of the TIA. The TIA converts a current flowing in the third winding into a voltage proportional to the current. This facilitates simpler subsequent signal processing.

[0027] In some embodiments of the capacitive sensor device, the electrical measurement circuit is designed to measure a voltage at the measuring node and to derive the impedance between the heating element and a counter electrode based on the measured voltage.

[0028] Preferably, the counter electrode is connected to earth potential.

[0029] In some embodiments of the capacitive sensor device, the first and second terminals of the heating power supply are electrically coupled to the ground conductor. This AC coupling to the ground conductor ensures that the AC potential at the terminals of the first and second windings of the common-mode choke, which are electrically connected to the heating power supply, is at a defined AC potential, namely AC ground, regardless of the exact configuration of the heating power supply. This implies that the AC voltage at the second terminal of the third common-mode choke winding, which is connected to the signal input terminal of the electrical measurement circuit, for example, a transimpedance amplifier, is advantageously essentially zero volts AC.

[0030] In a preferred embodiment, the EMI filter network comprises at least one capacitor. This allows the signal input terminal and the reference input terminal of the electrical measurement circuit to be virtually short-circuited for high-frequency signals injected during the execution of a BCI test protocol. The EMI filter can also include additional inductors or ferrite beads in combination with one or more capacitors to achieve improved filtering or attenuation of the injected BCI currents.

[0031] According to a further aspect of the invention, a seat occupancy detection system is provided for detecting whether a seat, in particular a vehicle seat, is occupied. The seat occupancy detection system comprises an embodiment of the capacitive sensor device disclosed above, an electric heating element arranged on a cushion or backrest forming part of the seat and usable as an antenna electrode, and a heating current supply for providing electric current to the electric heating element.

[0032] The advantages described in connection with the capacitive sensor device according to the invention also apply to the disclosed seat occupancy detection system.

[0033] In some embodiments of the seat occupancy detection system, the heating power supply is designed to provide a direct current for the electric heating element, wherein the first winding is galvanically connected between the first terminal of the heating power supply and the first terminal of the electric heating element, and the second winding is galvanically connected between the second terminal of the heating element and the second terminal of the heating power supply. This allows for a simple design of a seat occupancy detection system, as disclosed herein, with a heating element that is arranged on a cushion or backrest to heat the seat and can be used as an antenna electrode.

[0034] According to a further aspect of the invention, a device for detecting hand contact is provided for a vehicle's steering wheel. The device for detecting hand contact comprises an embodiment of the capacitive sensor device disclosed above, an electrical heating element arranged on the steering wheel and usable as an antenna electrode, and a heating current supply for providing electrical current to the electrical heating element. Brief description of the drawings

[0035] Further details and advantages of the present invention will become apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings, wherein: Fig. 1. A schematic circuit diagram of a first capacitive scanning system of the prior art using a seat heating element as an antenna electrode is illustrated; Fig. 2 illustrates a schematic circuit diagram of a second prior art capacitive scanning system for a seat using a seat heating element as an antenna electrode; Fig. 3 illustrates a schematic circuit diagram of a seat occupancy detection system comprising a capacitive sensor device according to the invention, which uses a heating element as an antenna electrode; Fig. Figure 4 schematically shows a vehicle seat equipped with the seat occupancy detection system according to Fig. 3 is equipped; and Fig. Figure 5 schematically shows a steering wheel with a built-in capacitive system for detecting hand touches, which is equipped with the capacitive sensor device according to Fig. 3 is equipped. Description of preferred embodiments

[0036] Fig. Figure 3 illustrates a schematic circuit diagram of a seat occupancy detection system 30 comprising a capacitive sensor device 31 according to the invention.

[0037] The seat occupancy detection system 30 is designed to detect whether a seat, in particular a vehicle seat, is occupied. The seat occupancy detection system 30 comprises the capacitive sensor device 31, an electric heating element 52 arranged on a cushion or backrest that forms part of the seat, and a heating power supply 46 for providing electric current to the electric heating element 52. In particular, the heating power supply 46 is designed to provide a direct current to the electric heating element 52.

[0038] The capacitive sensor device 31 is designed to be connected between the electric heating element 52 and the heating power supply 46, using the heating element 52 as an antenna electrode. The electric heating element 52 has a complex impedance 53 with respect to ground. The complex impedance 53 comprises a capacitive component and a resistive component, which depend on the occupancy state of the vehicle seat.

[0039] The capacitive sensor device 31 comprises a common-mode choke 43 with a first and a second inductively coupled winding 44, 45. The first winding 44 is galvanically connected between a first terminal 47 of the heating power supply 46 and a first terminal of the electric heating element 52. The second winding 45 is galvanically connected between a second terminal of the electric heating element 52 and a second terminal 48 of the heating power supply 46. The resistor 49 represents a wiring resistance for the connection between the second terminal 48 of the heating power supply 46 and the second common-mode choke winding 45.The first terminal of the heating power supply 47 and the second terminal 48 of the heating power supply 46 are electrically coupled to an AC earth conductor via capacitors 50, 51 to ensure that a defined AC earth is present on the side of the common-mode choke 43, which is connected to the DC heating power supply 46. Ground 54 serves as the reference ground.

[0040] Furthermore, the capacitive sensor device 31 includes a control and evaluation circuit 32, which is designed to feed a periodic alternating measurement signal into the electric heating element 52 via a measuring node 40, to measure an electrical quantity through the measuring node 40, and to derive the complex impedance 53 between the electric heating element 52 and a counter electrode based on the measured electrical quantity. The counter electrode is connected to the potential of the reference ground 54.

[0041] For this purpose, the control and evaluation circuit comprises 32 - a third common-mode choke winding 33, - a periodic signal voltage source 36, - an electrical quantity measurement circuit 37 and - an EMI filter network 41.

[0042] The third common-mode choke winding 33 is inductively coupled to the first winding 44 and the second winding 45 of the common-mode choke 43. In contrast to the one in Fig. In the embodiment of the capacitive sensing system shown in Figure 2, the connections of the third common-mode choke winding 33 are reversed such that the third common-mode choke winding 33 is inductively coupled in the same winding direction as the first winding 44 and the second winding 45. In this particular embodiment, the third winding 33 has a number of turns equal to the number of turns in the first winding 44 and the number of turns in the second winding 45. Other embodiments in which the third winding of the common-mode choke has a different number of turns than the first and second windings are also considered. In this context, it should be noted that the first and second windings must always have the same number of turns, otherwise the common-mode choke will not function.For a compact design and low magnetic stray fields, the first winding 44, the second winding 45 and the third winding 33 of the common-mode choke 43 are arranged within a common housing 55.

[0043] The periodic signal voltage source 36 is designed to apply an alternating measurement voltage, essentially in sinusoidal form, to an output terminal. The output terminal is electrically directly connected to a first connection 34 of the third common-mode choke winding 33.

[0044] The electrical measurement circuit 37 has a signal input terminal 38 and a reference input terminal 39 and is designed to determine the electrical quantity through the measurement node 40 with respect to an electrical reference quantity provided to the reference input terminal 39. In this particular embodiment, the electrical measurement circuit 37 is implemented as a current measurement circuit that includes a transimpedance amplifier (TIA). The signal input terminal 38 and the reference input terminal 39 form part of the TIA. The current measurement circuit 37 is designed to determine a sensing current flowing through the third common-mode choke winding 33 with respect to a reference voltage provided to the reference input terminal 39, indicating the position of an object relative to the electrical heating element 52.

[0045] Experts will readily recognize that the electrical measurement circuit can alternatively be designed to measure a voltage at the measuring node and to derive the impedance between the heating element and the counter electrode based on the measured voltage.

[0046] The signal input terminal 38 of the TIA is electrically connected to a second terminal 35 of the third common-mode choke winding 33, and the reference input terminal 39 is electrically connected to the earth conductor, which provides the AC earth potential.

[0047] The AC voltage at the second terminal 35 of the third common-mode choke winding 33 is in fact essentially zero volts AC, due to the fact that the signal input terminal 38 of the TIA is held at essentially AC ground by the TIA. Since the direction of the windings 44, 45, and 33 is the same for all three windings, as indicated by the dots at the upper left edge of each of the three windings, and since an AC voltage generated by the periodic voltage source 36 is applied to the first terminal 35, the same AC voltage also appears at the measuring node 40 due to the transformer effect of the common-mode choke 43 and the defined direction of the windings.

[0048] The EMI filter network 41 includes a capacitor 42 and is electrically connected via the signal input terminal 38 and the reference input terminal 39 of the electrical measurement circuit 37. The EMI filter can also include additional inductors or ferrite beads in combination with one or more capacitors to achieve better filtering or attenuation of the injected BCI currents.

[0049] This means that capacitor 42 is electrically connected to the AC ground via a low-impedance path, namely an impedance of essentially zero, and high-frequency currents injected during the execution of a BCI test protocol are effectively short-circuited to the AC ground.

[0050] Fig. Figure 4 schematically shows a vehicle seat 56 equipped with the seat occupancy detection system 30, which includes the capacitive sensor device 31, the heating power supply 46 and the electric heating element 52, which are installed in a seat cushion of the vehicle seat 56.

[0051] Fig. Figure 5 schematically shows a steering wheel 58 with a built-in capacitive system for detecting hand touch, which is equipped with the capacitive sensor device 31, the heating power supply 46 and the electric heating element 52. List of reference symbols 1 Capacitance Measurement Circuit (ECM) 2 DC voltage source 3 Masse 4 Resistance 5 common-mode choke 6 Capacitor 7 Capacitor 8 Capacitor 9 AC voltage source 10 Connection dimensions 11 knots 12 Heating element 13 complex impedance 20 DC voltage source 21 System grounding 22 Common-mode choke 23 Capacitor 24 AC voltage source 25 Heating 26 unknown impedance 27 Transimpedance Amplifiers TIA 28 TIA output 30 Seat occupancy detection system 31 capacitive sensor device 32 Control and evaluation circuit 33 third winding 34 first connection 35 second connection 36 periodic signal voltage source 37 electrical quantity measurement circuit 38 Signal input connector 39 Reference input connector 40 measuring nodes 41 EMI filter network 42 Capacitor 43 Common-mode choke 44 first winding 45 second winding 46 Heating power supply 47 first connection 48 second connection 49 Resistance 50 Capacitor 51 Capacitor 52 electric heating element 53 complex impedance 54 Reference dimensions 55 cases 56 vehicle seats 57 seat cushions 58 Steering wheel

Claims

[1] Capacitive sensor device (31) designed to be connected between an electrical heating element (52) and a heating power supply (46) and to use the electrical heating element (52) as an antenna electrode, comprising - a common-mode choke (43) with a first and a second inductively coupled winding (44, 45), wherein the first winding (44) is designed to be connected between a first terminal (47) of the heating current supply (46) and a first terminal of the electric heating element (52), and wherein the second winding (45) is designed to be connected between a second terminal of the electric heating element and a second terminal (48) of the heating current supply (46); and - a control and evaluation circuit (32) designed to feed a periodic alternating measurement signal into the electrical heating element (52) via a measuring node (40), to measure an electrical quantity through the measuring node (40) and to derive an electrical impedance (53) between the electrical heating element (52) and a counter electrode based on the measured electrical quantity; characterized by , that the control and evaluation circuit (32) comprises the following: - a third common-mode choke winding (33) which is inductively coupled to the first winding (44) and the second winding (45) of the common-mode choke (43); - a periodic signal voltage source (36) designed to apply an AC measuring voltage to an output terminal, wherein the output terminal is electrically directly connected to a first connection (34) of the third common-mode choke winding (33); - an electrical quantity measurement circuit (37) with a signal input terminal (38) and a reference input terminal (39) designed to determine the electrical quantity through the measuring node (40) using an electrical reference quantity provided to the reference input terminal (39), wherein the signal input terminal (38) is electrically connected to a second terminal (35) of the third common-mode choke winding (33) and the reference input terminal (39) is electrically connected to an earth conductor providing an AC earth potential; and - an EMI filter network (41) which is electrically connected via the signal input terminal (38) and the reference input terminal (39) of the electrical quantity measurement circuit (37). [2] Capacitive sensor device (31) according to claim 1, wherein the third common-mode choke winding (33) is inductively coupled in the same winding direction as the first winding (44) and the second winding (45). [3] Capacitive sensor device (31) according to claim 1 or 2, wherein the first winding (44) and the second winding (45) of the common-mode choke (43) and the third common-mode choke winding (33) are arranged within a common housing (55). [4] Capacitive sensor device (31) according to one of the preceding claims, wherein the third winding (33) has a number of turns equal to the number of turns of the first winding (44) and the second winding (45). [5] Capacitive sensor device (31) according to any one of the preceding claims 1 to 3, wherein the third winding (33) has a number of turns which differs from the number of turns of the first winding (44) and the second winding (45). [6] Capacitive sensor device (31) according to one of the preceding claims, wherein the electrical quantity measurement circuit (37) is designed as a current measurement circuit which is designed to determine, on the basis of a reference voltage, a sensing current which flows through the third common-mode choke winding (33) and indicates a position of an object relative to the electrical heating element (52). [7] Capacitive sensor device (31) according to claim 6, wherein the sensing current measuring circuit (37) comprises a transimpedance amplifier and the signal input terminal (38) and the reference input terminal (39) form part of the transimpedance amplifier. [8] Capacitive sensor device (31) according to one of the preceding claims, wherein the electrical quantity measurement circuit (37) is designed to measure a voltage at the measuring node (40) and to derive the impedance (53) between the electrical heating element (52) and the counter electrode on the basis of the measured voltage. [9] Capacitive sensor device (31) according to one of the preceding claims, wherein the first connection (47) of the heating current supply (46) and the second connection (48) of the heating current supply (46) are electrically coupled to the earth conductor by alternating current. [10] Capacitive sensor device (31) according to one of the preceding claims, wherein the EMI filter network (41) comprises at least one capacitor (42). [11] Seat occupancy detection system (30) for detecting an occupancy of a seat, in particular a vehicle seat, wherein the seat occupancy detection system (30) comprises the following: - a capacitive sensor device (31) according to any one of claims 1 to 10, - an electric heating element (52) arranged on a cushion or backrest that forms part of the seat and can be used as an antenna electrode, and - a heating power supply (46) to provide electric current for the electric heating element (52). [12] Seat occupancy detection system (30) according to claim 11, wherein the heating current supply (46) is designed to provide a direct current for the electric heating element (52), wherein the first winding (44) is galvanically connected between the first terminal (47) of the heating current supply (46) and the first terminal of the electric heating element (52) and the second winding (45) is galvanically connected between the second terminal of the electric heating element (52) and the second terminal (48) of the heating current supply (46). [13] Device for detecting a hand touch for a steering wheel (56), comprising - a capacitive sensor device (31) according to any one of claims 1 to 10, - an electric heating element (52) arranged on the steering wheel (56) and usable as an antenna electrode, and - a heating power supply (46) to provide electric current for the electric heating element (52).

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