Calibration of an evaluation circuit

Abstract

Method for calibrating an evaluation circuit (200) for determining a resonant frequency (f0) of an electrical resonant circuit (100), wherein the evaluation circuit (200) comprises an evaluation coil (220), a transconductance amplifier (210) and a compensation circuit (230), wherein the evaluation coil (220) can be inductively coupled to the electrical resonant circuit (100), wherein an output of the transconductance amplifier (210) is connected via a feedback branch (260) to an input of the transconductance amplifier (210), wherein the evaluation coil (220) is arranged in the feedback branch (260) of the transconductance amplifier (210), wherein the compensation circuit (230) is arranged in parallel to the transconductance amplifier (210) and evaluation coil (220), the procedure comprises the following steps: - Interrupting the feedback branch (260) of the evaluation circuit (200); - Applying an alternating voltage (425) to the input of the transconductance amplifier (210); - Determining an output voltage (415) present at the output of the voltage amplifier (240); - Adjusting the gain factor (G) of the transconductance amplifier (210) to a value chosen such that the output voltage (415) at the output of the voltage amplifier (240) has a minimum amplitude; - Closing the feedback branch (260) of the evaluation circuit (200).

Description

[0001] The invention relates to a method for calibrating an evaluation circuit according to claim 1 and an evaluation circuit according to claim 6.

[0002] Document US 2006 / 0061351A1 describes a sensor and a method with noise compensation.

[0003] Document US 2005 / 0062484A1 discloses a method and device for detecting metal targets at close range over long distances.

[0004] It is known to use LC resonant circuits to detect physical quantities such as pressure, force, humidity, and temperature. The resonant frequency of these circuits changes depending on the measured quantity. Such passive sensor circuits do not require their own power supply and allow for contactless readout. The dependence of the resonant frequency on the physical quantity can be achieved, for example, by using capacitive, inductive, or resistive electronic components in the LC resonant circuit, whose electrical capacitance, inductance, or resistance changes under the influence of the measured quantity. Such sensors were described, for example, by J.C. Butler et al. (John C. Butler, Anthony J. Vigliotti, Fred W. Verdi and Shawn M. Walsh: Wireless, passive, resonant-circuit, inductively coupled, inductive strain sensor. Sensors and Actuators A: Physical, Vol. 102, Issues 1-2, pp. 100-100).61-66, 2002) and by PJ Chen et al. (Po-Jui Chen, Damien C. Rodger, Salomeh Saati, Mark S. Humayun, and Yu-Chong Tai: Implantable Parylene-Based Wireless Intraocular Pressure Sensor. 21st IEEE Int. Conf. On MEMS 2008, pp. 58-61, 2008).

[0005] To read out such LC resonant circuits, the resonant frequency of the LC circuit must be determined. Changes in this resonant frequency can then be used to infer changes in the associated physical quantity. It is known to determine the resonant frequency of LC resonant circuits using dip meters. Such dip meters have a tunable oscillator and an externally accessible coil. The coil is brought close to the LC resonant circuit to be read out in order to establish inductive coupling. The frequency of the oscillator driving the coil is varied over a predefined range.If the oscillation frequency of the dip meter matches the resonant frequency of the sensor's resonant circuit, the sensor's resonant circuit absorbs vibrational energy from the dip meter's oscillator. This results in a measurable drop in the oscillator's energy, thus enabling the determination of the resonant frequency of the LC sensor's resonant circuit. A disadvantage of such dip meters is the need to sweep through the frequency range, which limits the maximum possible measurement speed. Another disadvantage is that the dip meter's coil must be coupled to the LC resonant circuit with only a weak inductive coupling, otherwise the circuit's natural frequency will be distorted.

[0006] M. Nowak et al. (M. Nowak, N. Delorme, F. Conseil, G. Jacquemod: A novel architecture for remote interrogation of wireless battery-free capacitive sensors. 13th IEEE Conf. on Electronics, Circuits and Systems 2006, pp. 1236-1239, 2006) proposed the use of an oscillator circuit that utilizes the inductively coupled LC sensor resonant circuit as the frequency-determining resonator element in the feedback loop of an amplifier. One advantage of this circuit is the direct conversion of the measured quantity into the frequency of an electrical oscillation. The frequency can then be determined simply and precisely using a known reference oscillation, such as a crystal oscillator. However, such an evaluation system requires strong inductive coupling between the evaluation system and the LC resonant circuit.If the inductive coupling is too weak, the effective impedance of the feedback loop is dominated by the frequency response of the evaluation coil, preventing oscillation. Nowak et al. therefore proposed the use of an active compensation circuit to compensate for the frequency response of the evaluation coil. Precise dimensioning of the compensation circuit is essential for its correct operation. Consequently, manufacturing variations and parameter changes due to lifetime and temperature can significantly impair the circuit's functionality. Disclosure of the invention

[0007] The object of the present invention is to provide a method for calibrating an evaluation circuit. This object is achieved by a method with the features of claim 1. Furthermore, it is an object of the present invention to provide an improved evaluation circuit for determining the resonant frequency of an electrical resonant circuit. This object is achieved by an evaluation circuit with the features of claim 6. Preferred embodiments are specified in the dependent claims.

[0008] A method according to the invention for calibrating an evaluation circuit for determining the resonant frequency of an electrical resonant circuit relates to an evaluation circuit with an evaluation coil, a transconductance amplifier, and a compensation circuit. The evaluation coil can be inductively coupled to the electrical resonant circuit, an output of the transconductance amplifier is connected to an input of the transconductance amplifier via a feedback loop, the evaluation coil is arranged in the feedback loop of the transconductance amplifier, and the compensation circuit is arranged in parallel with the transconductance amplifier and the evaluation coil.The procedure comprises steps for interrupting the feedback loop of the evaluation circuit, applying an AC voltage to the input of the transconductance amplifier, determining the output voltage at the output of the voltage amplifier, adjusting the gain of the transconductance amplifier to a value chosen such that the output voltage at the output of the voltage amplifier has a minimal amplitude, and closing the feedback loop of the evaluation circuit. Advantageously, this procedure eliminates the need for complex calibration of the compensation circuit before commissioning the evaluation circuit. A further advantage is that sufficient stability of the evaluation system is ensured with regard to temperature- and lifetime-related changes in component values.

[0009] Preferably, the gain of the transconductance amplifier is adjusted by a control loop such that the output voltage at the output of the voltage amplifier is below a predetermined threshold, in particular a minimum value. Advantageously, the adjustment of the gain can be automated by using a control loop. Furthermore, the use of a control loop increases the robustness of the proposed method.

[0010] In a further development of the method, the evaluation circuit also includes a voltage amplifier arranged in series with the evaluation coil in the feedback branch of the transconductance amplifier. The method further includes a step, performed according to the aforementioned steps, to adjust the gain of the voltage amplifier to a specific value. This value is chosen such that an oscillation with a constant amplitude is established in the evaluation circuit. Advantageously, this step ensures that the evaluation circuit fulfills the Barkhausen criterion, which states that the loop gain of the open oscillator loop must be equal to one at the sensor resonant frequency. This ensures that a stable oscillation at the sensor resonant frequency is established in the evaluation circuit.

[0011] Preferably, the gain of the voltage amplifier is adjusted using a control loop so that an oscillation with a constant amplitude is established in the evaluation circuit. Using a control loop offers the advantage that the gain adjustment can be automated and is insensitive to interference.

[0012] Advantageously, the frequency of the alternating voltage should deviate from the resonant frequency of the electrical resonant circuit by at least three times the resonant frequency of the electrical resonant circuit divided by the quality factor of the electrical resonant circuit. This has the advantage that the influence of the electrical resonant circuit on the frequency response of the transconductance amplifier can then be neglected.

[0013] An evaluation circuit according to the invention for determining a resonant frequency of an electrical resonant circuit comprises an evaluation coil, a transconductance amplifier, a compensation circuit and a voltage amplifier.The evaluation coil can be inductively coupled to the electrical resonant circuit. An output of the transconductance amplifier is connected to an input of the transconductance amplifier via a feedback loop. The evaluation coil is arranged in the feedback loop of the transconductance amplifier. The compensation circuit is arranged in parallel with the transconductance amplifier and the evaluation coil. The voltage amplifier is arranged in series with the evaluation coil in the feedback loop of the transconductance amplifier. A switch for disconnecting the feedback loop is provided in the feedback loop of the transconductance amplifier. The input of the transconductance amplifier can be connected to an AC voltage source, and the output of the voltage amplifier can be connected to a voltmeter. Advantageously, this evaluation circuit allows calibration to compensate for aging-related and temperature-dependent effects.

[0014] In a preferred embodiment of the evaluation circuit, the voltmeter and the transconductance amplifier can be connected to a control device configured to adjust the gain of the transconductance amplifier such that an output voltage detected by the voltmeter has an amplitude below a predetermined threshold, in particular a minimum amplitude. An advantage of such a control device is that it enables automatic and noise-insensitive adjustment of the transconductance amplifier's gain.

[0015] Advantageously, the compensation circuit exhibits a negated frequency response of a coil with ohmic winding resistance. The compensation circuit then advantageously compensates the frequency response of the evaluation coil, causing the effective load impedance of the transconductance amplifier to have a phase zero and a magnitude maximum at the resonant frequency of the electrical resonant circuit.

[0016] According to one embodiment of the evaluation circuit, the compensation circuit comprises an operational amplifier with an inverting input, a non-inverting input and an output connected to an output of the compensation circuit, a compensation resistor arranged between an input of the compensation circuit and the inverting input of the operational amplifier, a compensation capacitor connected in parallel to the compensation resistor and a negative feedback resistor arranged between the output of the compensation circuit and the inverting input of the operational amplifier.

[0017] According to another embodiment of the evaluation circuit, the compensation circuit has an operational amplifier with an inverting input, a non-inverting input and an output connected to an output of the compensation circuit, and a compensation resistor arranged between an input of the compensation circuit and the inverting input of the operational amplifier, wherein a negative feedback resistor and a negative feedback coil are provided in series between the output of the compensation circuit and the inverting input of the operational amplifier.

[0018] It is advantageous to provide circuit-related means in the evaluation circuit to determine the amplitude of an oscillation in the evaluation circuit.

[0019] Preferably, the evaluation circuit also includes a digital counter for determining the frequency of an oscillation in the evaluation circuit.

[0020] The invention will now be explained in more detail with reference to the attached figures. They show: Fig. 1 a schematic view of an electrical resonant circuit; Fig. 2 a schematic representation of an evaluation circuit; Fig. 3 a block diagram of a compensation circuit and Fig. 4 A schematic representation of an evaluation circuit during a calibration process. Embodiments of the invention

[0021] Fig. Figure 1 shows a schematic block diagram of a resonant circuit 100. The resonant circuit 100 is suitable for use as a passive sensor for physical quantities such as pressure, force, humidity or temperature and is also referred to below as an LC or sensor resonant circuit.

[0022] The resonant circuit 100 comprises a sensor inductance 110, a sensor capacitance 120, and a sensor resistance 130, which are arranged in series within the resonant circuit 100. The sensor inductance 110 can be an inductor. The sensor capacitance 120 can be a capacitor. The sensor resistance 130 can, for example, be the electrical resistance of the inductor forming the sensor inductance 110 and the resistance of the leads connecting components 110, 120, and 130. The sensor inductance 110 and / or the sensor capacitance 120 can change depending on an external physical quantity. For example, the sensor capacitance 120 can be a capacitor whose capacitance changes depending on an external pressure.

[0023] The resonant circuit 100 has a resonant frequency f0, which depends on the values ​​of the sensor inductance 110 and the sensor capacitance 120. The resonant frequency f0 is also referred to as the natural frequency. If the sensor inductance 110 and / or the sensor capacitance 120 depend on an external physical quantity, then the resonant frequency f0 also depends on the external physical quantity.

[0024] Fig. Figure 2 shows a schematic block diagram of an evaluation circuit 200 suitable for determining the resonant frequency f0 of the resonant circuit 100. The evaluation circuit 200 comprises a transconductance amplifier 210, an evaluation coil 220, a compensation circuit 230, and a voltage amplifier 240. The transconductance amplifier 210 can be a voltage-controlled current source. An output of the transconductance amplifier 210 is connected to an input of the transconductance amplifier 210 via a feedback loop 260. The evaluation coil 220 and the voltage amplifier 240 are connected in series in the feedback loop 260. The compensation circuit 230 is connected in parallel with the transconductance amplifier 210 and the evaluation coil 220.The input of the compensation circuit 230 is thus connected to the output of the voltage amplifier 240, while the output of the compensation circuit 230 is connected to an addition point 250, which is located in the feedback branch 260 between the evaluation coil 220 and the voltage amplifier 240.

[0025] The evaluation coil 220 can be inductively or transformer-coupled to the sensor inductance 110 of the sensor resonant circuit 100. For this purpose, the sensor inductance 110 and the evaluation coil 220 are brought close to each other such that the sensor inductance 110 is located in the near field of an electromagnetic field generated by the evaluation coil 220. The evaluation circuit 200 then forms an oscillator circuit that uses the transformer-coupled sensor resonant circuit 100 as the frequency-determining resonator element in the feedback 260 of the transconductance amplifier 210. One advantage of this circuit is the direct conversion of the physical quantity detected by the sensor resonant circuit 100 into a frequency of an oscillation in the evaluation circuit 200. This frequency can then be easily and precisely determined using a known reference oscillation to ascertain the measured quantity.

[0026] The inductive coupling of the evaluation coil 220 to the sensor inductance 110 results in an effective impedance of the feedback branch 260, the magnitude of which depends on the sensor inductance 110, the sensor capacitance 120, and the sensor resistance 130. However, with only weak inductive coupling between the evaluation coil 220 and the sensor inductance 110, the effective impedance of the feedback branch 260 is dominated solely by the frequency response of the evaluation coil 220. In this case, the input voltage of the transconductance amplifier 210 leads the output current of the transconductance amplifier 210 by almost 90° for all frequencies, so that no oscillation develops in the evaluation circuit 200. In order for an oscillation with the natural frequency f0 of the sensor resonant circuit 100 to occur in the evaluation circuit 200, the effective impedance of the feedback branch 260 must have a phase of 0° at the resonance frequency f0.Therefore, the evaluation circuit 200 includes the compensation circuit 230, which compensates for the frequency response of the evaluation coil 220, so that the effective load impedance of the transconductance amplifier 210 exhibits a phase zero and a magnitude maximum at the resonant frequency f0 of the resonant circuit 100. The effective load impedance of the transconductance amplifier 210 then qualitatively corresponds to the frequency response of an LC parallel resonant circuit. In principle, any circuit that exhibits an inverse frequency response of a coil is suitable as the compensation circuit 230. For example, the compensation circuit 230 can be implemented as a negative impedance converter.

[0027] In Fig. Figure 3 schematically illustrates a possible embodiment of a compensation circuit 230. The compensation circuit 230 of Fig. The circuit comprises an operational amplifier 300 with an inverting input 310 and a non-inverting input 320. One output of the operational amplifier 300 is connected to an output 370 of the compensation circuit 230. The output of the operational amplifier 300 is also connected to the inverting input 310 of the operational amplifier 300 via a negative feedback resistor 350. The non-inverting input 320 of the operational amplifier 300 is connected to a ground terminal. The inverting input 310 of the operational amplifier 300 is connected to an input 360 of the compensation circuit 230 via a compensation resistor 330. A compensation capacitor 340 is connected in parallel to the compensation resistor 330.

[0028] In an alternative embodiment not shown, the compensation circuit 230 comprises an operational amplifier with an inverting input, a non-inverting input, and an output connected to an output of the compensation circuit 230. Furthermore, in this embodiment, a compensation resistor is provided between an input of the compensation circuit 230 and the inverting input of the operational amplifier. Additionally, a negative feedback resistor and a negative feedback coil are arranged in series between the output of the compensation circuit 230 and the inverting input of the operational amplifier.

[0029] The compensation circuit 230 is designed to compensate for the frequency response of the evaluation coil 220. However, this requires precise dimensioning of the compensation circuit 230. In the Fig. In the embodiment of the compensation circuit 230 shown in Figure 3, for example, precise dimensioning of the compensation resistor 330, the compensation capacitor 340, and the negative feedback resistor 350 is necessary, whereby the aforementioned components must be dimensioned according to the electrical characteristics of the evaluation coil 220. If the compensation circuit 230 is not dimensioned with sufficient accuracy, the coupled system consisting of the evaluation circuit 200 and the resonant circuit 100 will either exhibit an additional series resonant frequency, in which case the frequency of the parallel resonance will change, or it will even disappear completely. The precise dimensioning of the compensation circuit 230 is made more difficult by manufacturing variations and the service life and temperature dependencies of the components used in the evaluation circuit 200.However, it was discovered that the compensation circuit 230 can also be calibrated by changing the gain factor G of the transconductance amplifier 210. This is explained below.

[0030] An analysis of the frequency-dependent impedance of the evaluation coil 220 with coupled sensor resonant circuit 100 reveals that the in Fig. The embodiment of the compensation circuit 230 shown in Figure 3 is optimally adapted when the ratio of the negative feedback resistor 350 to the compensation resistor 330 is equal to the product of the gain G of the transconductance amplifier 210 and the winding resistance of the evaluation coil 220. Furthermore, the product of the negative feedback resistor 350 and the capacitance of the compensation capacitor 240 must be equal to the product of the gain G of the transconductance amplifier 210 and the inductance of the evaluation coil 220. Fulfilling the second condition is particularly important for the correct functioning of the compensation circuit 230. Therefore, the gain G of the transconductance amplifier 210 must be set to the value of the negative feedback resistor 350 times the capacitance of the compensation capacitor 340 divided by the inductance of the evaluation coil 220.A method for finding a suitable value for the amplification factor G is described below using the following example. Fig. 4 explained.

[0031] In addition to compensating for the frequency response of the evaluation coil 220, the oscillator formed by the evaluation circuit 200 must satisfy the so-called Barkhausen criterion in order for stable oscillation to be achieved in the evaluation circuit 200. The Barkhausen criterion states that the loop gain of the open oscillator loop must have a value of 1 at the resonant frequency f0 of the resonant circuit 100. If the loop gain is too low, only a damped, decaying oscillation amplitude will occur, while if the loop gain is too high, the oscillation amplitude will continuously increase. The voltage amplifier 240 has a variable gain factor g and serves to adjust the loop gain of the oscillator loop of the evaluation circuit 200 so that the Barkhausen criterion is met.

[0032] Fig. Figure 4 shows a view of an evaluation circuit 400 during a calibration process. In contrast to the evaluation circuit 200, the Fig. 2 was in Fig. 4 The feedback branch 260 is interrupted by means of a switch (not shown). This results in Fig. 4, unlike Fig.2. The output of the voltage amplifier 240 is not connected to the input of the transconductance amplifier 210. Instead, the output of the voltage amplifier 240 is connected to a voltmeter 410, which measures an output voltage 415 provided by the voltage amplifier 240. The input of the transconductance amplifier 210 is connected to an AC voltage source 420, by means of which an AC voltage 425 can be applied to the input of the transconductance amplifier 210. The frequency of the AC voltage 425 is chosen to be as far removed as possible from the resonant frequency f0 of the sensor resonant circuit 100. It has proven advantageous if the frequency of the AC voltage 425 deviates from the resonant frequency f0 of the resonant circuit 100 by at least three times the resonant frequency f0 of the resonant circuit 100 divided by a quality factor Q of the resonant circuit 100.For example, the frequency of the AC voltage 425 can be chosen to be half the resonant frequency f0 of the resonant circuit 100. This ensures that the influence of the resonant circuit 100 coupled to the evaluation coil 220 on the frequency response of the transfer function of the open oscillator loop of the evaluation circuit 200 can be neglected.

[0033] In the next process step, the gain factor G of the transconductance amplifier 210 is adjusted so that the AC voltage 425 measured at the output of the voltage amplifier 240 has an amplitude below a predetermined threshold or the smallest possible amplitude, ideally a zero amplitude. The gain factor G is preferably adjusted using a control loop. The control loop adjusts the gain factor G, for example, so that the amplitude of the measured AC voltage 425 reaches a minimum, or that the amplitude lies within a threshold range around a minimum. This has the advantage that no human intervention is required.

[0034] After the gain factor G of the transconductance amplifier 210 has been set, the control loop of the evaluation circuit 200 is closed again. For this purpose, the voltmeter 410 is disconnected from the output of the voltage amplifier 240 and the AC voltage source 420 is disconnected from the input of the transconductance amplifier 210. Subsequently, the output of the voltage amplifier 240 is reconnected to the input of the transconductance amplifier 210 by means of the switch (not shown).

[0035] The gain factor G of the transconductance amplifier 210 and the compensation circuit 230 are now calibrated such that the compensation circuit 230 compensates for the frequency response of the evaluation coil 220. To ensure compliance with the Barkhausen criterion, only an adjustment of the gain factor g of the voltage amplifier 240 is necessary. For this purpose, the gain factor g of the voltage amplifier 240 is set so that an oscillation with a constant amplitude is established in the evaluation circuit 200. The evaluation circuit 200 may, for this purpose, include a means (not shown in the figures) for determining the amplitude of the oscillation. It is particularly preferred that the gain factor g of the voltage amplifier 240 be adjusted by means of a control loop.This has the advantage that no human intervention is necessary to adjust the amplification factor g of the voltage amplifier 240.

[0036] The evaluation circuit 200 can also include a means for determining the frequency of an oscillation within the evaluation circuit 200, which is not shown in the figures. This means for determining the frequency of the oscillation can, for example, be a digital counter. In this case, a time or frequency reference (for example, a crystal oscillator) is also required.

[0037] The resonance frequency f0 of the resonant circuit 100 determined by the described evaluation circuit 200 can, for example, be in the range of a few kilohertz to a few tens of megahertz.

Claims

[1] Method for calibrating an evaluation circuit (200) for determining a resonant frequency (f0) of an electrical resonant circuit (100), wherein the evaluation circuit (200) comprises an evaluation coil (220), a transconductance amplifier (210) and a compensation circuit (230), wherein the evaluation coil (220) can be inductively coupled to the electrical resonant circuit (100), wherein an output of the transconductance amplifier (210) is connected via a feedback branch (260) to an input of the transconductance amplifier (210), wherein the evaluation coil (220) is arranged in the feedback branch (260) of the transconductance amplifier (210), wherein the compensation circuit (230) is arranged in parallel to the transconductance amplifier (210) and evaluation coil (220), the procedure comprises the following steps: - Interrupting the feedback branch (260) of the evaluation circuit (200); - Applying an alternating voltage (425) to the input of the transconductance amplifier (210); - Determining an output voltage (415) present at the output of the voltage amplifier (240); - Adjusting the gain factor (G) of the transconductance amplifier (210) to a value chosen such that the output voltage (415) at the output of the voltage amplifier (240) has a minimum amplitude; - Closing the feedback branch (260) of the evaluation circuit (200). [2] Method according to claim 1, wherein the gain factor (G) of the transconductance amplifier (210) is adjusted by a control loop such that the output voltage (415) applied to the output of the voltage amplifier (240) has an amount below a predetermined threshold, in particular a minimum amount. [3] Method according to one of claims 1 or 2, wherein the evaluation circuit also includes a voltage amplifier (240) which is arranged in series with the evaluation coil (220) in the feedback branch (260) of the transconductance amplifier (210), wherein the method comprises the following further method step, which is carried out according to the steps mentioned in the preceding claims: - Setting a gain factor (g) of the voltage amplifier (240) to a value such that the value is chosen so that an oscillation with a time constant amplitude is established in the evaluation circuit (200). [4] Method according to claim 3, wherein the gain factor (g) of the voltage amplifier (240) is adjusted by a control loop such that an oscillation with a time constant amplitude is established in the evaluation circuit (200). [5] Method according to one of the preceding claims, wherein the frequency of the alternating voltage (425) deviates from the resonance frequency (f0) of the electrical resonant circuit (100) by at least three times the resonance frequency (f0) of the electrical resonant circuit (100) divided by the quality factor (Q) of the electrical resonant circuit (100). [6] Evaluation circuit (200) for determining a resonant frequency (f0) of an electrical resonant circuit (100), wherein the evaluation circuit (200) comprises an evaluation coil (220), a transconductance amplifier (210), a compensation circuit (230) and a voltage amplifier (240), wherein the evaluation coil (220) can be inductively coupled to the electrical resonant circuit (100), wherein an output of the transconductance amplifier (210) is connected via a feedback branch (260) to an input of the transconductance amplifier (210), wherein the evaluation coil (220) is arranged in the feedback branch (260) of the transconductance amplifier (210), wherein the compensation circuit (230) is arranged in parallel to the transconductance amplifier (210) and evaluation coil (220), wherein the voltage amplifier (240) is arranged in series with the evaluation coil (220) in the feedback branch (260) of the transconductance amplifier (210), characterized by , that in the feedback branch (260) of the transconductance amplifier (210) a switch is provided for disconnecting the feedback branch (260), the input of the transconductance amplifier (210) can be connected to an AC voltage source (420) and the output of the voltage amplifier (240) can be connected to a voltage measuring device (410). [7] Evaluation circuit (200) according to claim 6, wherein the voltage measuring device (410) and the transconductance amplifier (210) can be connected to a control device which is configured to adjust a gain factor (G) of the transconductance amplifier (210) such that an output voltage (415) detected by the voltage measuring device (410) has an amplitude below a predetermined threshold, in particular a minimum amplitude. [8] Evaluation circuit (200) according to one of claims 6 or 7, wherein the compensation circuit (230) has a negated frequency response of a coil with ohmic winding resistance. [9] Evaluation circuit (200) according to one of claims 6 to 8, wherein the compensation circuit (230) an operational amplifier (300) with an inverting input (310), a non-inverting input (320) and an output connected to an output (370) of the compensation circuit (230), a compensation resistor (330) arranged between an input (360) of the compensation circuit (230) and the inverting input (310) of the operational amplifier (300), a compensation capacitor (340) connected in parallel to the compensation resistor (330) and has a negative feedback resistor (350) arranged between the output (370) of the compensation circuit (230) and the inverting input (310) of the operational amplifier (300). [10] Evaluation circuit (200) according to one of claims 6 to 8, wherein the compensation circuit (230) an operational amplifier with an inverting input, a non-inverting input and an output connected to an output of the compensation circuit (230) and a compensation resistor arranged between an input of the compensation circuit (230) and the inverting input of the operational amplifier, and A negative feedback resistor and a negative feedback coil are provided in series between the output of the compensation circuit (230) and the inverting input of the operational amplifier. [11] Evaluation circuit (200) according to one of claims 6 to 10, wherein circuitry means are provided to determine an amplitude of an oscillation in the evaluation circuit (200). [12] Evaluation circuit (200) according to one of claims 6 to 11, wherein the evaluation circuit (200) has a digital counter for determining a frequency of an oscillation in the evaluation circuit (200).

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