DE4135990C1 - Capacitance-frequency converter with offset compensation - has third changeover switch connected to compensating capacitor for switching to different potentials - Google Patents

Abstract

The converter, providing a rectangular output signal with a pulse repetition frequency corresp. to the capacitance of a measured capacitor, uses a pair of change-over switches (S1, S2) on opposite sides of the measured capacitor (Cx), a further capacitor (C1), a current source (R*), a comparator (K1) and a pulse generation circuit (P) controlled by the output of the comparator (C1). A compensation (Ckomp) is coupled to one side of the measured capacitor at one pole and is coupled via a third change-over switch (S3) at its opposite pole, allowing it to be coupled to two different potentials (-kUB, UB), ADVANTAGE - Eliminates stray capacitance effect.

Description

The present additional invention relates to further training a capacitance-frequency converter for generating an im essential rectangular converter output signals with one of the capacitance of a capacitor to be measured dependent pulse repetition frequency, according to Patent No. 40 39 006.

From the standard textbook Tietze-Schenk, semiconductor circuit technology, 9th edition, 1989, page 925 is already a Capacitance-voltage converter known. To this capacity Voltage converter to a capacitance-frequency converter too expand, an additional voltage-frequency Converter. The output voltage of the known capacitance-to-voltage converter depends on the input reference voltage, so that fluctuations in the input reference voltage ent speaking measurement errors. Stray capacities at the entrance of the known capacitance-voltage converter in parallel of the capacitance to be measured are in the Output voltage directly on. The result is that the known Capacitance-to-voltage converters only for such applications can be used with acceptably low measurement errors, where due to short leads to the measured Capacity too low stray capacities occur or where the capacity to be measured has a high value.

From the data book for semiconductor circuits: Linear Databook 1990 by Linear Technology Corporation, page 11 to 23, right figure above is a voltage-frequency converter known of a switched capacitance between the output and the inverting input of an operational amplifier has, the non-inverting input on the one hand via a resistor with ground and on the other hand via a  further capacity is connected to the output and where furthermore the inverting input via the emitter of a Transistor connected to a negative reference voltage whose base is at the common node of a series connection another resistance and another Capacitor is present, this series connection between the output of the operational amplifier and ground. It is conceivable with such a voltage-frequency converter the switched capacity by a capacity too replace, whose capacity value is to be determined and thus with a fixed input voltage this circuit as a capacitance-frequency converter to use. However, one would also such a modification of the known circuit a stray capacitance against ground in parallel with the one to be measured Capacity. Two reference points would also be required their possible variations directly in the output frequency and would therefore go into the achievable measurement accuracy.

In sensors, there is often the case that a capacitance which corresponds to a physical value changes a small area of their total capacity while the physical value to be recorded has a large value Area has changed. In such a case, the capacitance-to-frequency converter can only one after the main patent deliver increased measurement accuracy of the physical value, if increased accuracy in the measurement of the output frequency is achieved. The capacity-frequency converter can achieve this goal according to the main patent only in limited Dimensions are enough.

The present additional invention therefore has the task to create a capacitance-to-frequency converter on the one hand stray capacity especially in the area of Supply lines of the capacity to be measured only a small one And the measuring accuracy due to fluctuations the operating voltage of the capacitance-frequency converter  is only slightly affected and on the other hand a more accurate measurement of small changes in capacity is achieved.

This task is accomplished by a capacitance-to-frequency converter solved according to claims 1 and 4.

Preferred developments of the capacitance-frequency converter according to the invention are specified in the subclaims.

Below are with reference to the accompanying Drawings preferred embodiments of the invention Capacity-frequency converter explained in more detail. It shows:

Figure 1 shows a first embodiment of the capacitance-to-frequency converter according to the main patent.

Fig. 2 shows a second embodiment of the capacitance-to-frequency converter according to the main patent;

3 shows a third embodiment of the capacitance-to-frequency converter according to the main patent.

Fig. 4 shows an embodiment of the capacitance-to-frequency converter according to the invention additive;

Figure 5 is an alternative embodiment of the capacitance-to-frequency converter according to the invention additive.

Fig. 6 is an embodiment of the shown in Figure 4 capacitance-to-frequency converter according to the invention for a plurality of additive with respect to their capacity to be measured capacitors. and

FIG. 7 shows an embodiment of the embodiment of the capacitance-frequency converter shown in FIG. 5 according to the additional invention, likewise for a plurality of capacitors to be measured with regard to their capacitance.

As shown in FIG. 1, the capacitance-to-frequency converter, which is designated in its entirety by reference number 1 , comprises a first changeover switch S1, which is connected to a first pole of a capacitor C _X to be measured with regard to its capacitance, and this connects to ground in a first switching state or connects to an operating voltage U _B in a second switching state, which is shown in the figure. The second pole of the capacitor C _X to be measured with respect to its capacitance is connected to a second changeover switch S2, which connects it in a first switching state to a node KN of the capacitance-frequency converter 1 or in a second state (shown here) to ground. Both poles or connections of the capacitor C _X to be measured with regard to its capacitance have stray capacitances C _S1 , C _S2 to ground. The stray capacitances can be, for example, the capacitances of connecting lines, which can have a high value in particular if the capacitor with the capacitance C _x to be measured is spaced a long distance from the capacitance-frequency converter and to it Coaxial lines is connected. Another capacitor C _I lies between the node KN and ground. The node KN is connected to a voltage-controlled current source R1, R2, K2, R3, T, which applies a constant current to the node, as will be explained below.

The capacitance-frequency converter 1 further comprises a comparator K1, the non-inverting input of which is connected to the node KN and the inverting input of which is connected to ground. The output of this comparator K1 forms the output of the capacitance-frequency converter, which has a substantially rectangular converter output signal, the frequency f _{out of which is} proportional to the capacitance of the capacitor to be measured.

The converter output A is connected to the input of a pulse generation circuit P connected to generate one pulse each of predetermined duration T when a falling occurs or rising edge of the substantially rectangular Converter output signal is used.

In the pulse generation circuit P, which is in the state of the Known technology, it can either be an analog built or digital circuit. At many applications of the capacity-frequency converter it will be possible to use a clock signal whose clock frequency for controlling a digitally constructed Pulse generation circuit can be used.

The output of the pulse generation circuit is used for the first Switch S1 and the second switch S2 in the bring first or second switching state.

The voltage-controlled current source, which is connected to the node KN, comprises an n-channel field effect transistor T, the source electrode of which is connected to the node KN and the drain electrode of which is connected to the operating voltage U _B via a third resistor R3. The drain electrode of the field effect transistor T is connected to the inverting input of an operational amplifier K2, the non-inverting input of which is connected to the common node of a voltage divider formed by two resistors R1, R2. The voltage divider is connected on the one hand to ground and on the other hand to the operating voltage U _B.

The current I _S output by the voltage-controlled current source R1, R2, R3, T, K2 can be expressed as a function of the operating voltage U _B and the resistance values of the first to third resistors R1, R2, R3:

The mode of operation of the circuit described above is explained in more detail below. It is assumed that the potential at the further capacitor C _I rises and just passes through the reference potential of the comparator K1 in the direction of the operating voltage U _B rising. The comparator K1 switches, whereupon the pulse generation circuit P sets the two changeover switches S1, S2 to the second switching state, which is shown in FIG. 1, for a predetermined time period T. In this switching state, the capacitor C _X to be measured with regard to its capacitance is charged to the operating voltage U _B. After this time has elapsed, the two changeover switches S1, S2 assume the first switching state, which is opposite to the switching state shown in FIG. 1. In this switching state, the capacitor C _X to be measured transfers its charge Q to the further capacitor C _I , so that its potential jumps in the voltage direction opposite the operating voltage, as a result of which the comparator K1 falls back into its starting position, whereupon the further capacitor C _I and the capacitor C _X to be measured with regard to its capacitance is discharged via the current source device T, R1 to R3, K2 in the direction of the operating voltage U _B. As soon as the potential at the node KN passes through the reference potential of the comparator K1 in the direction of the operating voltage U _B , the process described above is repeated.

The output frequency of the circuit results from the oscillation condition for the circuit, which in turn requires that the charge supplied to the current source device T, R1, R2, R3, K2 is equal to that charge which the node KN has via the condensation C _X to be measured with regard to its capacitance is fed. This results in the following equation:

As can be seen, the dependency on the operating voltage U _B drops out in this connection, so that the following equation applies to the output frequency:

The following consideration will show that the influences of stray capacities at least largely fall out.

As far as the first stray capacitance C _{S1 is} concerned, it is either charged to the operating voltage U _B or discharged to the reference potential via the first changeover switch S1. Under the usually fulfilled condition that the volume resistance of the first switch S1 is correspondingly low, the capacitor to be measured with respect to its capacitance C _X becomes within the pulse width T, which is determined by the pulse generation circuit P, despite the first stray capacitance C _S1 to the operating voltage U _B charged. The first stray capacitance thus has no influence on the measurement of the capacitor C _X to be measured with regard to its capacitance.

As far as the second stray capacitance C _{S2 is} concerned, it is charged with the further capacitor C _I on the one hand by the capacitor to be measured with regard to its capacitance C _X and on the other hand discharged via the effective resistance R * of the current source device. As long as it is ensured that the comparator K1 switches as soon as the voltage at the node KN passes through the reference potential, the second stray capacitance C _{S2 is} completely discharged at the switching time, so that it likewise no longer has any influence on the determination of the capacitance value of the capacitor C _X to be measured Has.

For this reason, in the case of the capacitance-to-frequency converter 1, the capacitor C _X to be measured with regard to its capacitance value can be arranged far from the measurement location. In particular, it is possible to connect this capacitor via coaxial cables whose shielding is at reference potential and whose inner conductor connects the capacitor C _X to be measured with regard to its capacitance and the second changeover switch S2.

FIG. 2 shows a simplified embodiment of the circuit shown in FIG. 1, which can be used if one only wants to determine the capacitance values of small capacitors C _X. In this case, the current source device according to FIG. 1 is replaced by a resistor R *. The prerequisite for this is that the capacitance of the further capacitor C _{I is} very much larger than that of the capacitor C _X to be measured.

If the latter condition is met, the maximum voltage U _{CI max} after connecting the capacitor C _X to be measured and the further capacitor C _{I in} parallel is very much lower than the operating voltage U _B. The maximum voltage occurring at the further capacitor C _I can therefore be expressed as follows:

The average discharge voltage U _{CI medium across} the further capacitor C _I can be expressed as follows:

The current I _R averaged over a period through the resistor R * can be expressed as follows:

It can be seen from the seventh equation that the following equation applies to capacitance values of the further capacitor C _I , which are much larger than the capacitance value of the capacitor to be measured:

Although the above equation for particularly high values of capacitance of the further capacitor C _I met particularly well, it is not possible yet to choose arbitrarily high capacitance values of the further capacitor C _I. The dimensioning of the further capacitor C _I must take into account the properties of the comparator K1, which, as a real circuit for decreasing changes in input voltage, shows increasing switching times for switching the output signal.

Taking these properties of the comparator K1 into account, however, efforts will be made to choose the capacitance value of the further capacitor C _I as large as possible, in order to also reduce any parasitic currents which flow off to the reference potential. In a first approximation, these can be expressed as follows:

It follows from this that the influences of parasitic resistances decrease as the capacitance value of the further capacitor C _{I increases} .

With a suitably high choice of the capacitance value of the further capacitor C _I , the capacitance-to-frequency converter 1 according to the invention thus enables the capacitance C _X to be measured with poor quality, as is the case, for. B. is the case with capacitive humidity sensors.

The equivalent circuit diagram of a poor quality capacitor corresponds to an ideal capacitor with a resistor connected in parallel. This resistance R is _parasitic towards the reference potential as long as the capacitor C _X to be measured with regard to its capacitance is connected in parallel with the further capacitor C _I during the discharge phase of the capacitance-to-frequency converter circuit according to the invention.

The circuit according to FIG. 2 is more advantageous compared to the circuit according to FIG. 1 in that it is simpler, cheaper and more temperature-stable, is more stable with regard to aging and works in a broadband manner.

The circuit shown in FIG. 2, on the other hand, is disadvantageous compared to the circuit shown in FIG. 1 in that the output frequency has little dependence on the operating voltage U _B , since the capacitance value of the further capacitor C _I cannot be chosen to be infinitely large. When using a simple voltage regulator for the operating voltage U _B , however, the output frequency can be regarded as being dependent solely on the capacitance value C _X to be measured.

Furthermore, the circuit according to FIG. 2 has a certain non-linearity of the output frequency depending on the capacitance value of the capacitor C _X to be measured and the capacitance value of the further capacitor C _I , as has already been described. If the degree of linearity achieved by the circuit is insufficient, the circuit can be freed from linearity errors by calibration according to the above calculation.

The circuit shown in FIG. 3 corresponds to the circuit according to FIG. 2 with the exception of the deviation explained below. However, this is used to measure the capacitance of a plurality N of capacitors C _X1 , C _X2,. . . , C _S2 . A corresponding plurality N of first switches S1₁,. . . , S1 _N is one of the capacitors C _X1,. . . , C assigned to _XN . Between the second switch and the other pole of each of the capacitors C _X1,. . . , C _XN is a switch from a corresponding plurality N of third switches S3₁,. . . , S3 _N.

A switch control SS1, SS2, which is used by a switching input SE to select the capacitor C _X1,. . . , C _XN is activated, activates one of the first changeover switches S1 1,. . . , S1 _N as a function of the output signal of the pulse generation circuit P and switches the remaining first changeover switches to their first switching state. Furthermore, the switch control closes the corresponding third switch S3₁,. . . , S3 _N and opens all other of the third switch S3₁,. . . , S3 _N.

So far only capacities C _N1 . . . , C _XN can be measured with high quality, the third switch S3₁,. . . , S3 _N are omitted since the capacities of no interest with regard to the capacitance to be measured only act as stray capacitances which are not included in the output voltage.

As far as not only capacitors with high quality, but also those with poor quality are to be measured, it is advantageous to use the third switches S3 1,. . . To make use of S3 _N since they prevent the flow of parasitic currents via the capacitances C _X1,. . . , C prevent _XN .

FIG. 4 shows a modification of the embodiment of the capacitance-to-frequency converter according to FIG. 2 with a capacitance offset compensation. Those parts of the circuit which are designated by the same reference numerals correspond to the circuit according to FIG. 2, so that it is not necessary to explain them again.

The capacitance-to-frequency converter circuit shown in FIG. 4, which is designated in its entirety by reference number 1 , comprises, in addition to the circuit according to FIG. 2, a compensation capacitor C _comp , a third changeover switch S3 and an inverter INV. The compensation capacitor C _comp is connected with one pole to the other pole of the capacitor C _X to be measured with regard to its capacitance. The other pole of the compensation capacitor C _comp can be switched via the third changeover switch S3 either to ground in a first switching state or to a potential of the inverter INV on the output side in a second switching state. This potential on the output side - k · U _B is a potential which is proportional to the operating voltage U _B and inversely to it.

The operation of this circuit is explained below insofar as it differs from the operation of the circuit according to FIG. 2. In the second switching state shown, the charging phase of the capacitor C _X to be measured and of the compensation capacitor C _comp takes place.

If the first, second and third changeover switches are brought into the first switching state (not shown) by a switch control, which can be brought about by the pulse generation circuit P, then the further capacitor C _{I is} only removed from the capacitor C _X to be measured with the difference in charges and applied to the compensation capacitor C _comp , with which the output frequency f _out only depends on the difference in the charges.

Equations (1) and (2) also apply to the circuit shown in FIG. 4.

However, it follows from the oscillation conditions for this circuit the slightly changed relationships listed below:

From this it follows:

With

Therefore, for the circuits according to FIG. 4, with regard to the differential measuring capacitance ΔC _X, those relationships apply which apply to the measuring capacitance C _X itself in the circuit according to FIG. 2.

The same applies to the stray capacitance C _S1 , C _S2 as explained with reference to FIG. 2.

Corresponding stray capacitances can be present at the compensation capacitance C _comp . Since the actuation of the compensation capacitance C _comp and its wiring correspond to the actuation and the wiring of the measuring capacitance C _X , the stray capacitances also have no effect on the output frequency of the circuit. Therefore, the compensation capacitance C _comp as well as the measuring capacitance can be connected, for example, via a coaxial cable and be located away from the circuit without this affecting the functioning of the circuit.

FIG. 5 shows a modification of the embodiment according to FIG. 4. In this embodiment, the compensation capacitor C _{comp is} connected to ground with its one pole, while the other pole is connected to the other pole of the capacitor C by means of the third switch S3 either in a first switching state _X or in a second switching state can be connected to the operating voltage U _B. The inverter circuit INV shown in FIG. 4 is omitted.

The constant k given in equation (3d) is for this Circuit 1.

With regard to the mode of operation, the circuit shown in FIG. 5 completely agrees with the circuit shown in FIG. 4, with the exception of its behavior with regard to the stray capacitance of the compensation capacitor C _comp .

In Fig. 5, a further stray capacitance C of the _S3 was Kompen sationskondensators C shown _comp. With regard to this stray capacitance, the circuit according to FIG. 5 is disadvantageous compared to the circuit of FIG. 4, since, as explained, the circuit according to FIG. 4 is also insensitive to stray capacitances of the compensation capacitor.

The circuit according to FIG. 5 can therefore be considered if the absolute amount of the compensation capacitance C _comp is not important and if it is ensured that the stray capacitance C _S3 does not change over time.

The circuits according to FIGS. 6 and 7 show the application of the capacitance-frequency converter with capacitance offset compensation according to FIGS. 4 and 5 to the capacitance-frequency converter circuit shown in FIG. 3 with a variety of capacitance values capacitors to be measured C _X1 to C _XN . With regard to the basic structure, the circuits according to FIGS. 6 and 7 therefore correspond to those according to FIG. 3. There are only designated in Fig. 3 as the third switch S3₁ to S3 _N switch in Figs. 6 and 7 as the fourth switch S4₁ to S4 _N. The circuit of FIG. 6 thus has a plurality of N with respect to their capacity to be measured capacitors C _X1. . . , C _XN , a corresponding plurality of first switches S1₁,. . . , S1 _N , which are connected to these capacitors, a corresponding plurality of fourth switches S4₁,. . . , S4 _N between the capacitors C _X1,. . . , C _XN and the second switch S2, and a corresponding plurality N of compensation capacitors C _komp1 to C _kompN . In this respect, the circuit according to FIG. 6 is identical to that according to FIG. 7.

In the circuit of Fig. 6 is a corresponding plurality of N third switches S3₁. . . , S3 _{N are} provided, with each of which the other pole of an assigned compensation capacitor C _komp1,. . . , C _kompN can be _switched either in a first switching state against the reference potential or ground potential or in a second switching state against the output voltage of the inverter INV, which is inverse to and proportional to the operating voltage.

Between the one pole of each compensation capacitor C _komp1 to C _kompN and the second changeover switch S2 there is a corresponding plurality N of fifth switches S5 1,. . . , S5 _N. A switch control SS1, SS2 activates one of the first and third changeover switches depending on the output signal of the pulse generation circuit P and switches the remaining of the first and third changeover switches to the first switching state. It also closes the corresponding fourth and fifth switches and leaves the remaining fourth and fifth switches open.

In the alternative circuit according to FIG. 7, each compensation capacitor C _komp1 to C _{kompN is} connected to _ground with one pole. The other pole of each compensation capacitor C _komp1 to C _kompN is by means of an assigned third switch S3₁ to S3 _N in a first switching state with the other pole of the assigned capacitor C _X1 to C _XN to be measured or in a second switching state with the operating voltage U _B connectable.

Claims (16)

1. Capacitance-frequency converter for generating an essentially rectangular converter output signal with a pulse repetition frequency that is dependent on the capacitance of a capacitor to be measured, with:

• a first changeover switch (S1) with which a pole of the capacitor (C _X ) can be switched either in a first switching state against a first potential or in a second switching state against a second potential (U _B ),
• a second changeover switch (S2) with which the other pole of a capacitor (C _X ) can either be connected to a node (KN) in a first switching state or can be switched to a third potential in a second switching state,
• a further capacitor (C _I ) which is connected on the one hand to the node (KN) and on the other hand to a fourth potential,
• - a current source device (R *; R1, R2, R3, K2, T) connected to the node (KN),
• a comparator (K1), the first input of which is connected to the node (KN) and the second input of which is connected to a fifth potential and the output of which provides the converter output potential, and
• - A pulse generating circuit (P) connected to the output of the comparator (K1) for generating one pulse each of a predetermined duration (T) when an edge of the substantially rectangular converter output signal occurs, the output of which pulse with the inputs of the first and second changeover switches controlling the switching state ( S1, S2) is connected, according to Patent No. 40 39 006,

characterized by

• - That a compensation capacitor (C _comp ) is provided, which is connected with its one pole to the other pole of the capacitor (C _X ), and
• - That a third changeover switch (S3) is provided, with which the other pole of the compensation capacitor (C _comp ) either in a first switching state against a sixth potential or in a second switching state against a seventh potential (-kU _B ), one against the second potential (U _B ) has inverse polarity, is switchable.

2. capacitance-frequency converter according to claim 1, characterized in that the first, third, fourth, fifth and sixth potential are equal to a reference potential. 3. capacitance-frequency converter according to claim 1 or 2, characterized in that
that the second potential is equal to the operating voltage (U _B ) of the capacitance-frequency converter, and
that the seventh potential is proportional to the inverse operating voltage. 4. capacitance-frequency converter for generating an essentially rectangular converter output signal with a pulse repetition frequency that is dependent on the capacitance of a capacitor to be measured, with:

• a first changeover switch (S1) with which a pole of the capacitor (C _X ) can be switched either in a first switching state against a first potential or in a second switching state against a second potential (U _B ),
• a second changeover switch (S2) with which the other pole of the capacitor (C _X ) can either be connected to a node (KN) in a first switching state or can be switched to a third potential in a second switching state,
• a further capacitor (C _I ) which is connected on the one hand to the node (KN) and on the other hand to a fourth potential,
• - a current source device (R *; R1, R2, R3, K2, T) connected to the node (KN),
• - a comparator (K1), the first input of which is connected to the node (KN) and the second input of which is connected to a fifth potential and the output of which provides the converter output signal, and
• - A pulse generation circuit (P) connected to the output of the comparator (K1) for generating one pulse each of a predetermined duration (T) when an edge of the essentially rectangular converter output signal occurs, the output of which pulse with the inputs of the first and second changeover switches controlling the switching state ( S1, S2) is connected, according to Patent No. 40 39 006,

characterized,

• - That a compensation capacitor (C _comp ) is provided, which is connected with its one pole to an eighth potential, and
• - That a third changeover switch (S3) is provided with which the other pole of the compensation capacitor (C _comp ) either in a first switching state with the other pole of the capacitor (C _X ) or in a second switching state with a ninth potential (U _B ) is connectable.

5. capacitance-frequency converter according to claim 4, characterized in that the first, third, fourth, fifth and eighth potential are equal to the reference potential. 6. capacitance-frequency converter according to claim 4 or 5, characterized in that the second and ninth potential are equal to the operating voltage (U _B ). 7. capacitance-frequency converter according to one of claims 1 to 6, characterized, that the current source device is a voltage controlled Current source (R1, R2, R3, K2, T), 8. capacitance-frequency converter according to claim 7, characterized in
that the current source device has a voltage divider (R1, R2), an operational amplifier (K2), the non-inverting input of which is connected to the voltage divider node of the voltage divider (R1, R2), a FET transistor (T) and a resistor (R3),
that the resistor (R3) is connected between the second potential (U _B ) and the drain electrode of the FET transistor (T),
that the gate of the FET transistor (T) is connected to the output of the operational amplifier (K2) and its source electrode (S) to the node (KN) of the capacitance-to-frequency converter ( 1 ), and
that the inverting input of the operational amplifier (K2) is connected to the drain electrode (D) of the FET transistor (T). 9. capacitance-frequency converter according to one of claims 1 to 8, characterized in
that the capacitance of the capacitor (C _X ) is much smaller than the capacitance of the further capacitor (C _I ), and
that the current source device is formed by a resistor (R *) connected between the node (KN) and a third voltage potential (U _B ). 10. capacitance-frequency converter according to claim 9, characterized in that the third voltage potential is equal to the first voltage potential (U _B ). 11. Capacity-frequency converter according to one of claims 1 to 3 or according to one of claims 7 to 10 in relation to claim 1, characterized in that
that a plurality (N) of capacitors to be measured (C _X1 , _... , C _XN ) are provided,
that a corresponding plurality (N) of first switches (S1₁,..., S1 _N ) are connected to the capacitors (C _X1 ,..., C _XN ),
that a corresponding plurality (N) of fourth switches (S4₁,..., S4 _N ) are connected between the capacitors (C _X1 ,..., C _XN ) and the second changeover switch (S2),
that a corresponding plurality (N) of compensation capacitors C _komp1,. . . , C _kompN ) are provided,
that a corresponding plurality (N) of third switches (S3₁,.., S3 _N ) are provided with which the other pole of an assigned compensation capacitor (C _komp1 , _... , C _kompN ) either in a first switching state against the sixth Potential or in a second switching state is switchable against the seventh potential, and
that between the one pole of each compensation capacitor (C _komp1 , _... , C _kompN ) and the second changeover switch (S2) one of a corresponding plurality (N) of fifth switches (S5 1,..., S5 _N ) is arranged . 12. capacitance-frequency converter according to one of claims 4 to 10 in relation to claim 4, characterized in that
that a plurality (N) of capacitors to be measured (C _X1 , _... , C _XN ) are provided,
that a corresponding plurality (N) of first switches (S1₁,..., S1 _N ) are connected to the capacitors (C _X1 ,..., C _XN ),
that a corresponding plurality (N) of fourth switches (S4₁,..., S4 _N ) are connected between the capacitors (C _X1 ,..., C _XN ) and the second changeover switch (S2),
that a corresponding plurality (N) of compensation capacitors (C _komp1 , _... , C _kompN ) are provided, and
that a corresponding plurality (N) of third switches (S3₁,.., S3 _N ) are provided, with each of which the other pole of one of the associated compensation capacitors (C _komp1 , _... , C _kompN ) either in a first switching state can be connected to the other pole of one of the assigned capacitors (C _X ) or in a second switching state to the ninth potential (U _B) . 13. capacitance-frequency converter according to claim 11, characterized in that a switch control (SS1, SS2) each one of the first and third changeover switches (S1₁,..., S1 _N , S3₁,..., S3 _N ) depending on the output signal of the pulse generating circuit (P) is activated and the rest of the first and third changeover switches are switched to the first switching state and the corresponding fourth and fifth switches (S4₁,..., S4 _N , S5₁,..., S5 _N ) are closed and the remaining fourth and fifth switches opens. 14. Capacity-frequency converter according to claim 12, characterized in that a switch control (SS1, SS2) each one of the first and third changeover switches (S1₁,..., S1 _N ; S3₁,..., S3 _N ) depending on the output signal of the pulse generating circuit (P) is activated and the remaining first and third changeover switches are switched to the first switching state and the corresponding fourth switch (S4 1,..., S4 _N ) closes and the other fourth switches are opened.