DE4135991C1 - Capacitance-frequency converter producing rectangular pulse train signal - has two changeover switches connected to respective plates of capacitor for connection to different potentials - Google Patents

Abstract

The converter provides a rectangular output signal with a pulse repetition frequency dependant on the capacitance value of a measured capacitor. It uses a pair of change-over switches (S1, S2) on opposite sides of the capacitor (Cx), a comparator (K1) and a pulse generator circuit (P) controlled by the comparator output. The output of the pulse generator circuit provides the circuit output and is fed to a further switch coupled to the switch (S1) on the first side of the capacitor and allowing the current direction of a current source (UB) connected to the second side of the capacitor via the second switch to be reversed. ADVANTAGE - Eliminates DC error in measured values caused by capacitance of leads.

Description

The present additional invention relates to further training a capacitance-frequency converter for generating an im we substantial rectangular converter output signals with one depends on the capacitance of a capacitor to be measured dependent pulse repetition frequency, according to the patent 40 39 006.

From the standard textbook Tietze-Schenk, semiconductor scarf tungstechnik, 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 Voltage converter depends on the input reference voltage gig, so that fluctuations in the input reference voltage ent speaking measurement errors. Stray capacities at the on gear of the known capacitance-voltage converter in parallel of the capacitance to be measured are in the Output voltage directly on. This leads to the fact that the te capacitance-voltage converter only for such applications can be used with acceptably low measurement errors, in which to measure due to short leads to the only 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 is the base of which is the common node of a row circuit of another resistor 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 Converters the switched capacity by a capacity replace, whose capacity value is to be determined and thus with a fixed input voltage this circuit as capacitance Frequency converter. However, one would also such a modification of the known circuit a Streuka capacitance to ground in parallel with the one to be measured Capacity. Furthermore, two reference voltages would be required their possible variations directly in the output frequency and would therefore go into the achievable measurement accuracy.

In the capacitance-frequency converter circuit after the main The con applied to the capacitor with a measuring voltage that a ge know has equal share. In some use cases this is undesirable.

In the case of capacitive humidity sensors, the measuring voltage is allowed have no DC component over a long period of time, as this would destroy the sensor.

Therefore, the additional invention is based on the task, one Capacity-frequency converter of the type mentioned create, on the one hand, stray capacities in particular in the area of the supply lines of the capacity to be measured only have little influence and on the other hand a at least largely freed from DC voltage components Measurement is reached.

This task is accomplished by a capacitance-to-frequency converter solved according to claim 1.  

Preferred further developments of the capacity Frequency converters are specified in the subclaims.

Below, with reference to the Drawings preferred embodiments of the Invention according to capacity-frequency converter explained. It shows

Fig. 1 shows a first embodiment of the capacity-Fre frequency converter according to the main patent;

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

Fig. 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 additional invention;

Fig. 5 is a timing chart of signal waveforms in nerhalb of the capacitance-to-frequency converter of the invention; and

Fig. 6 shows an alternative embodiment of the capacity-Fre frequency converter according to the additional invention.

As shown in FIG. 1, the capacitance-to-frequency converter, which is designated in its entirety by reference numeral 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 is connected via coaxial lines. 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 acts on the node with a constant current, as will be explained below.

The capacitance-to-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-to-frequency converter, which has an essentially rectangular converter output signal whose frequency f _{out is} proportional to the capacitance of the capacitor to be measured.

The converter output A is connected to the input of a pulse generator supply circuit P connected to generate one Im each pulses of predetermined duration T when one fal occurs loin or rising edge of the essentially rectangular shaped 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 to control a digital structure th pulse generation circuit can be used.

The output of the pulse generation circuit is used by the  Most changeover switch S1 and the second changeover 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 switched on the one hand against Mas se and on the other hand against the operating voltage U _B.

The current I _s emitted 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 rises on the further capacitor C _I 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 respect to its capacitance is charged to the operating voltage U _B. After this time, 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 outputs its charge Q to the further capacitor C _I , so that its potential jumps into the voltage direction opposite the operating voltage, whereby the comparator K1 falls back into its initial position, whereupon the further one Capacitor C _I and the capacitor C _X to be measured with regard to its capacitance are 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 vibration 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 the charge that the node KN has over the capacitor to be measured with regard to its capacitance C _{X is} supplied. This results in the following equation:

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

The following consideration will show that the one flows from stray capacities at least largely fall out len.

As far as the first stray capacitance is C _S1 , 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 C _X to be measured with respect to its capacitance is within the pulse width T, which is determined by the pulse generating 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 from the capacitor to be measured with respect to its capacitance C _X on the one hand 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 to be measured _X has.

For this reason, in the capacitance-frequency converter 1, the capacitor C _X to be measured with respect to its capacitance value can be arranged far from the measurement location. In particular, it is possible to make the connection of this capacitor via coaxial cable, the shielding of which lies on 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 *. 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} at 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:

From the seventh equation it can be seen 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 is met particularly well for particularly high values of the capacitance of the further capacitor C _I , it is nevertheless not possible to select capacitance values of the further capacitor C _{I of} any desired magnitude. The dimen sioning of the further capacitor C _I must take into account the properties of the comparator K1, which shows as a real circuit for decreasing input voltage changes 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, such as this. B. sensors with capacitive moisture sensors is the case.

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

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 a slight 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 seen 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 as a function of 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 deviations 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 _XN assigned. 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 from a switching input SE to select the capacitor to be measured 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 pulser generating circuit P and switches the remaining first switch ter in 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 _X1 . . ., 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,. . ., S3 _N Ge to make use, since they prevent the flow of parasitic currents over the capacitances C _X1,. . ., Prevent C _XN .

Fig. 4 shows a modification of the capacitance-frequency converter shown in Fig. 2, the embodiment of Fig. 4 is suitable for DC-free measurement. Diejeni gene circuit parts, which are designated by the same reference numerals, agree with the circuit shown in FIG. 2, so that it is not necessary to explain them again.

As shown in FIG. 4, the first switch S1 in its second switching state shown in FIG. 4 is not directly connected to the operating voltage, as is the case in the embodiment according to FIG. 2, but is coupled to a switching unit S _B . Likewise, the connection of the resistor R * which forms the current source device, which is turned away from the further capacitor C _I , is likewise not connected to the operating voltage U _B , but is also connected to the switching unit S _B. As shown, the switching unit S _B thus connects a contact of the first switch S₁ and the further capacitor C _I abge facing connection of the resistor R * in its first switching state with a second potential, which can be the operating voltage, and in their second switching state with a potential inverse to the second potential, which can be the inverse operating voltage. The inverse operating voltage can be obtained from the operating voltage by using an inverter. In contrast to the embodiment according to FIG. 2, the output of the capacitance-frequency converter is not formed by the comparator output, but by a further control output of the pulse generating circuit P, the function of which will be explained in more detail later with reference to FIG. 5. The mode of operation of the circuit shown in FIG. 4 and the mode of operation of the pulse generation circuit P according to FIG. 4 will be explained in more detail below with reference to FIGS. 5a to 5e. At the time t 1, the output signal of the comparator K1 changes its potential, as shown in FIG. 5b. This triggers the following actions:

• - The influence of the comparator output signal on the pulse generating circuit P is blocked for the period t₂₁-t. The integration period Delta t 1 begins. The switching unit S _B switches the operating voltage supplied to the first switch and the resistor R * to + U _B.
• The first and second changeover switches S1, S2 are switched to their position shown in FIG. 4, which corresponds to the second switching state of these changeover switches. As a result, the capacitor C _{X is} charged at the operating voltage U _B.

Within the period of time delta t 1, the capacitor C _{X is} charged to + U _B in the switch position shown in FIG. 4. The further capacitor C _I is also charged via the resistor R *, as shown in FIG. 5a.

When the time period delta t 1 defined within the pulse generation circuit P has elapsed, the changeover switches S1, S2 are switched to their first switching state, as a result of which the capacitance C _{X is} connected in parallel to the further capacitor C _I. As a result, the voltage across the further capacitor C _I jumps into the negative, as is shown in FIG. 5a.

Within the period of time delta t₂, the capacitances C _I and C _{X are} loaded up to the reference potential ground via the resistor R *.

Within the period t₂₁-t₁ defined in the pulse generation circuit P, the comparator output signal has no influence on the processes within the circuit. The period of time is such that after the time t₂ the comparator output (see Fig. 5b) has enough time to be safely at the low potential.

After the time t₂₁ the comparator output voltage to control the pulse generation circuit P again give.

At time t₃ the comparator output switches. Hereby the following processes are triggered:

• - The influence of the comparator output signal is blocked during the period (t₄₁-t₃). (See hatching in Fig. 5c).
• - The switching unit S _B switches in its second switching state, ie it switches to the negative operating voltage U _B.
• - The changeover switches S1, S2 are switched to their other switching state in order to charge the capacitor C _X to the value -U _B. The voltage curve for driving the order switches S1, S2 is shown in Fig. 5d.

Here begins the period delta t₃.

During the period delta t₃, the capacitor C _{X is} charged to -U _B. During this period, the further capacitor C _I is discharged via the resistor R * in the direction of the negative operating voltage -U _B.

The period of time delta t₃ is equal to the period of time delta t₁, so that the measuring voltage applied to the capacitor C _X becomes zero on average over time if the condition is fulfilled that the positive or inverted operating voltage U _B applied to the switching unit have the same absolute value.

At the time t₄, the capacitance C _{X is} switched in parallel with the further capacitance C _I via the two switches S1, S2. As a result, the voltage across the further capacitor C _I jumps into the positive.

During the period delta t₄, the two capacitances C _I and C _{X are} discharged via the resistor R * to the negative operating voltage -U _B.

At time t₄₁, the comparator output voltage Control of the pulse generation circuit P after the Time (t₄₁-t₃) released again.  

The time t₅ corresponds to the time t₁. Here, the potential at the further capacitor C _I runs through the reference potential, so that the comparator K1 switches again and triggers the processes described at the time t 1.

The output frequency (see FIG. 5e) of the circuit described is derived below.

It is assumed that the capacitance C _X to be measured is much smaller than the capacitance of the further capacitor C _I , with which it can be assumed that the further capacitor C _I is discharged and charged with a constant current via R *.

For the period t₁ + delta t₂ = T₁, fol gender context:

The following applies:

From this it follows:

For the period delta t₃ + delta t₄ = T₂:

With

follows:

T₂ = C _X · R * (14)

this calculates the output frequency

Fig. 6 shows one of Fig. 3 corresponding capacitance-Fre frequency converter circuit with a plurality of capacitors to be measured. As can be seen, a DC-free measurement can also be brought about in this circuit according to the principle shown in FIG. 4, by in each case one pole of the first changeover switch S1 1,. . ., S1 _{N is} connected to the switching unit S _B , to which the resistor R * forming the current source device is connected, the circuit unit here also being acted upon with both the positive and the negative operating voltage U _B , -U _B and is alternately switched by the pulse generation circuit.

Claims (5)

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 against a first potential in a first switching state,
• 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 connected to the node (KN) (R *, R1, R2, R3, K2, T),
• - 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
• - A connected to the output of the comparator (K1) pulse generation circuit (P) for generating a pulse of a predetermined duration (T) when an edge of the comparator output signal occurs, the output of which controls the switching state of the inputs of the first and second switches (S1 , S2) is connected, according to Patent No. 40 39 006,

characterized by

• that the output of the pulse generation circuit (P) delivers the converter output signal,
• - That a switching unit (S _B ) is provided which is connected to the first switch (S1) and in the second switching state of the first switch (S1) the pole of the capacitor (C _X ) facing it in its first switching state with a second potential and applied in its second switching state with a potential inverse to the second potential, and
• - That the switching unit (S _B ) from the current source device (R *; R1, R2, R3, K2, T) current generated the node (KN) in its first switching state in positive direction and in its second switching state in the negative direction feeds.

2. Capacity-frequency converter according to claim 1, characterized ge features that the first, third, fourth and fifth potential equal to the reference potential of the capacitance-frequency Converter. 3. Capacity-frequency converter according to claim 1 or 2, characterized in that the current source device is formed by a resistor (R *) connected between the switching unit (S _B ) and the node (KN). 4. capacitance-frequency converter according to one of claims 1 to 3, characterized in
that a plurality (N) of capacitors to be measured with regard to their capacitance (C _X1 , _... , C _XN ) are provided,
that a corresponding plurality (N) of first Umschal tern (S1₁,.., S1 _N ) are connected to the capacitors (C _X1 ,..., C _XN ),
that a corresponding plurality (N) of third scarf stars (S3₁,..., S3 _N ) are connected between the capacitors (C _X1 ,..., C _XN ) and the second changeover switch (S2),
that a switch control (SS1, SS2) activates one of the first changeover switches (S1₁,..., S1 _N ) as a function of the output signal of the pulse generating circuit (P) and switches the rest of the first changeover switch to the first switching state and the corresponding third switch (S3₁,..., S3 _N ) closes and the other third switches open, and
that the switching unit is connected on the output side to one of the contacts of each first change-over switch (S1 1,..., S1 _N ).