DE4216176A1 - Integratable conductivity measurement arrangement for liquids - has current source and current-coupling capacitors for feeding rectangular wave to liq., and voltage measuring circuit consisting of coupling capacitors, operational amplifier and capacitor-switch circuit. - Google Patents

Abstract

A current source (SQ) is connectable to two current conductors, via which the rectangular wave current is fed into the liquid. A measurement circuit (CM,CR,S5-S11) measures the voltage drop in the liquid between two voltage measurement elements, which depends on the liquid's conductivity. The measurement circuit is a switching capacitor circuit with a measurement capacitor connected to and separated from the voltage measurement elements via switches according to the current variations. The current delivery elements are in the form of current coupling capacitors (CK1,CK4). ADVANTAGE - No polarisation effects are experienced, no galvanic connection is made to measurement liquid and enables simple conductivity measurement.

Description

The present invention relates to an integrable guide ability measuring device for measuring the electrical conductivity ability of liquids with a power source device device that is connected to two power supply elements, through which a current can be fed into the liquid, and with a measurement connected to two voltage measuring elements circuit for determining the voltage drop between the two voltage measuring elements, which of the electrical Conductivity of the fluid being tested depends on the measuring circuit is a switch-capacitor circuit, which has a measuring capacitor which with the voltage measurement elements via a switch device depending on the time of the course of the substantially rectangular Electricity can be coupled and separated, according to the German Patent application P4113033.2-52 (main application).

It is well known for the purpose of determining the electrical conductivity of a liquid in this one Impress current and the voltage drop within the Liquid which is inversely proportional to conductivity the liquid is to be measured.

In the simplest case, only two electrodes are used for this used. The current is fed into the Liquid imprinted and at the same time the voltage drop measured over the same electrodes. Here occur so-called te polarization effects on the actual measurement signal  distort. These effects always occur when a Current over an interface between an electrode and an electrolyte flows. There is a current in an electrolyte flow associated with the migration of ions form at the interface between the electrolyte and the electr trode accumulations of ions of a charge type, which the ur weaken the original field and reduce the measurement signal.

In order to avoid this disadvantage, conductivity measurements devices with so-called four-electrode arrangements used where a power source with two power electro which is provided for impressing a measuring current. Two other electrodes called voltage electrodes can be used to measure the over the liquid falling voltage. The voltage drop caused by the Voltage electrodes is tapped by a High-impedance amplifier connected to voltage electrodes reinforced. Due to the high input impedance of the amplifier The current flowing over the voltage electrodes can be less be kept low, so that the Po larization effects are reduced, resulting in a ver better measurement accuracy compared to conductivity measurement with only two electrodes. However, also leads to the four-electrode arrangement of the over the voltage electro the flowing measuring current to polarization and thus to a falsification of the measurement signal.

It is also known to be used in conductivity measuring devices against the type just described, the impressed current as to generate sinusoidal alternating current by this measure to avoid decomposition processes in the liquid, in the case of a measurement with impressed direct current would kick.

To address these problems of known integrable conductive keitsmeßvorrichtungen using two voltage electrodes and two current electrodes are designed to clear out beats the main application P4113033.2-52 proposes that the power sources  device generates a rectangular current that the is supplied to both current electrodes, and that the measuring scarf device is designed as a switch-capacitor circuit, the has a measuring capacitor with the voltage electro that via a switch device as a function of time on the course of the substantially rectangular current can be coupled and separated.

With the subject of the main application, those in the stand measurement errors due to Polarisa completely avoided.

Imprinting the current in the liquid (the electro lyte) and the voltage measurement are carried out via a direct galvanic contact between the conductivity measurement device and the electrolyte. These are electrodes used, which consist of precious metals, steel or coal and are referred to as so-called "Kohlrausch" cells. The However, galvanic contact leads to undesirable electro chemical effects at the interface between the elec tread and the electrolyte. These effects include:

Additional voltage drops, especially when the current flows must be compensated for by measurement.

Electrolysis processes, d. H. Discharges and Depositions from Ions on the electrodes.

Absorption of ions and contamination of the electrodes, which can lead to drift errors.

In the event of an error, there is a direct current path through the Electrolytes, which can lead to its electrolysis. At certain applications, such as in invasive medical diagnostics, such a case must be supplemented by circuitry measures are intercepted.  

Due to the galvanic coupling, the electrolyte is on fixed potential, what in certain applications is not desired.

The presence of a metal or other conductive capable substance can lead to undesired chemical reactions lead to, for example, the catalytic effect of Platinum counts.

To avoid the effects just mentioned, have already been non-contact measuring method for determining the conductivity Developed by liquids that have two principles are different: while in one method with one inductive coupling between a measuring circuit and the When electrolytes are used, the other method is used a capacitive coupling.

With the inductive method, for example, in a closed tube introduced electrolyte coupling of otherwise separate windings of a transformer. At an alternating voltage is applied to a primary winding current flow in the electrolyte. This stream flux causes a voltage in a secondary winding Height depends on the conductivity of the electrolyte. The However, this method requires a great deal of equipment.

A capacitive coupling according to the other measurement method is achieved in that the walls of the elec vessel containing trolytes partly as capacitors be formed. Part of such a Kon formed by a metal layer which of is covered with a thin layer of glass. The counter electrode the electrolyte forms. The electrical equivalent circuit Such an arrangement consists of two such Capacitors with ohmic electrolyte resistance between. In the known capacitive measuring methods these elements are placed in an RF resonant circuit in which the electrolyte resistance, for example, the damping ver  hold determines which is then evaluated. As well as with the inductive method, the apparatus is also here Big effort. In addition, here is due to the only small achievable capacities require a high measuring frequency. Furthermore, there is no simple linear relationship between the Given conductivity and the measured variable.

Based on the prior art described above the present invention has for its object an inte specify gratable conductivity measuring device in which no polarization effects occur in which a galva African connection to the regarding their conductivity measuring liquid is avoided and its a simple one Allows detection of the conductivity of the liquid.

This task is accomplished through an integrable conductivity Measuring device according to claim 1 solved.

The subject of the main application P4113033.2-52 stands out the integrable conductivity measuring device according to the in short, this additional application in that at least those at the main filing as current electrodes executed power supply elements in the sense of the present Invention are designed as current coupling capacitors. According to an essential aspect of the invention Voltage measuring elements as voltage decoupling capacitors trained so that a complete electrical isolation between the conductivity measuring device and the regarding their conductivity to be measured liquid is reached.

The conductivity measuring device according to the invention is based on a purely capacitive coupling between the conductive keitsmeßvorrichtung and the liquid or the electroly ten, the coupling capacities preferably integrated Technically manufactured elements can be in which a thin insulating layer over a conductive layer located. The counter electrode is replaced by the liquid speed or the electrolyte formed. The resulting  Capacitors can be manufactured in a sufficient size which allows a constant for a certain period of time Power over two capacities serving as power supply elements to let flow. The resulting voltage drop in the Liquid is also constant for this period of time can be measured with the help of two voltage measuring elements Voltage decoupling capacitors are executed, and the preferably between the two current coupling condensers gates are arranged, detect and process.

Due to the galvanic isolation, it is in the Invention moderate conductivity measuring device possible, the formation a direct current path through the electrolyte or To prevent fluid. It also prevents the Liquid is placed at a fixed potential. This he enables the application of the conductivity according to the invention device in areas with high security requirements.

The measuring circuit is monolithic with all components integrable into a semiconductor substrate. The small size of the integrable conductivity measuring device according to the invention tion leaves use with small sample volumes or on hard to reach places.

Below are exemplary embodiments of the integrable Conductivity measuring device after the main application as well according to the present invention with reference to the accompanying drawings explained in more detail. Show it:

Fig. 1 is a circuit diagram of the conductivity measuring device according to the parent application;

FIG. 2 shows a time diagram of currents or voltages as they occur in the conductivity measuring device according to FIG. 1;

FIG. 3a shows a cross-sectional view of a coupling capacitance of the conductivity measuring device according to the invention;

FIG. 3b is a plan view of the coupling capacitance shown in FIG. 3a;

Fig. 4 is a circuit diagram of an embodiment of the inventive conductivity measuring device;

FIG. 5a is a top view of the arrangement of the Koppelkapa capacities of the conductivity measuring device according to Fig. 4;

Fig. 5b shows the electrical equivalent circuit of the arrangement of the coupling capacitors according to Fig. 5a;

FIG. 6a, the effective electrical equivalent circuit diagram of the arrangement of the coupling capacitors in the impressing the measuring current;

Fig. 6b voltage drops on the elements of the equivalent circuit picture;

Fig. 6c voltages at points within the equivalent circuit picture;

Figure 7a shows the effective electrical equivalent circuit diagram of the arrangement of the coupling capacitors as shown in FIG 5a in measuring the signal voltage..; and

Fig. 7b shows the waveforms of the voltages at the measuring capacitor of the conductivity measuring device in accordance with Fig. 4.

The preferred embodiment shown in Fig. 1 of the integrable conductivity measuring device after the main application, which is designated in its entirety by the reference numeral 1 , comprises a current source SQ for generating an impressed direct current, which via a first to fourth switch S1, S2, S3, S4 can be connected to two current electrodes E1, E4 in a first polarity or in a polarity opposite to the first polarity, depending on the switching state thereof.

The current electrodes produce one in an electrolyte EL rectangular current without DC component.

A control device (not shown) controls the first one to fourth switches S1 to S4 so that the current source le SQ alternately during a first time period T1 in the first polarity and during a second period of time T2 in the second polarity connected to the current electrodes E1, E4 becomes. The first and second time periods T1, T2 are the same long.

There are two voltage electrodes between the current electrodes E1, E4 electrodes E2, E3 arranged in the electrolyte EL, which the through the impressed rectangular current between the Current electrodes E1, E4 occurring over the electrolyte Measure voltage drop V1.

The time course of the voltage drop V1 in relation to the first and second time periods T1, T2 is shown in FIGS. 2a to 2c.

The voltage electrodes E2, E3 are via a fifth to eighth switches S5, S6, S7, S8 in a first or second Connect the polarity to the electrodes of a measuring capacitor C1 bar.

The fifth, sixth, seventh and eighth switches S5 to S8 are likewise controlled by the control device (not shown), which can be designed as a microprocessor. The control takes place in such a way that the voltage electrodes E2, E3 are connected to the measuring capacitor C1 during a third period T3 in the first polarity and during a fourth period T4 in a second polarity. As can be seen from FIGS. 2d, 2e with regard to FIGS. 2a, 2b, the third time period T3 lies within the first time period T1 and the fourth time period T4 within the second time period T2.

A ninth and tenth switches S9, S10 lie between the two electrodes of the measuring capacitor C1 and the invertie renden or non-inverting input of an operation amplifier OPV, the output of which via a feedback con capacitor C2 in connection with its inverting input stands.

The control device (not shown) connects the measuring capacitor C1 to the inputs of the operational amplifier OPV for a fifth time period that lies outside the third and fourth time periods T3, T4. Accordingly the capacitance ratio of the crystallizer Rückkopplungskonden C2 and the measuring capacitor C1 is thereby boosts the voltage at the measuring capacitor _C1 to a V at the output of Operationsver stärkers generated voltage V _OUT.

In the embodiment shown, the (not shown) control device after the expiry of fifth time period parallel to the feedback condenser sator C2 switched eleventh switch S11, so that the ge showed switch capacitor circuit S5 to S11, C1, C2, OPV works as an amplifier circuit. However, it is also possible to switch the eleventh switch S11 after several pe riodes T1, T2 close so that the switch capacitor Circuit in this case works as an integration circuit.

As can be seen from the observation of the course of the voltage drop if the voltage electrodes E2, E3 according to FIG. 2c, the first and second time periods are each selected to be sufficiently long that the switching effects subside and the voltage V1 assumes a substantially constant value. Only after the switching processes have subsided is the measuring capacitor connected to the voltage electrodes during the period T3. As a result, charge carriers flow via the voltage electrodes E2, E3 onto the electrodes of the measuring capacitor C1. At the beginning of the period T3, this current flow leads to a disturbance in the original field between the current electrodes E1, E4 and to a temporary polarization. With increasing charging of the measuring capacitor C1, the measuring current tends to zero at the voltage electrodes E2, E3 exponentially, so that the voltage electrodes E2, E3 become de-energized with a sufficient length of the third time period T3. In the case of a sufficient third time period T3, which depends on the individual case but can be determined experimentally but easily, polarization effects no longer have a negative influence on the measurement accuracy that can be achieved.

The conductivity measuring circuit is suitable for an integra tion of the electrodes E1 to E4, the current source circuit SQ and the amplifier electronics including the switch Capacitor circuit on a single semiconductor substrate. Through the monolithic integration on a semiconductor The conductivity measuring device can strongly minia substrate be turized so that measurements in small sample volumes or in places that are difficult to access, such as se in the field of invasive medical diagnostics, possible are.

The circuit components can be made in CMOS technology leads. In this case, the elec trodes compatible to the CMOS process because only the additional process step of applying a Edelme tallschicht for the electrodes is required.

Although the conductivity measuring circuit is preferably for a complete integration is suitable, also measuring scarf lines with separately arranged electrodes after the be concept can be realized.

The integrable conductivity measuring device according to the invention stands out from the conductivity measuring device described with reference to FIG. 1, essentially because the electrodes E1 to E4 there are replaced by coupling capacitors CK1, CK2, CK3, CK4. Gene in übri the basic circuit corresponds to the Fig. 4 is identical to that of FIG. 1 so that a renewed description of the circuitry can be omitted. Merely for the sake of completeness it should be mentioned that here the measuring capacitor is designated by the reference symbol CM, while the feedback capacitor is designated by the reference symbol CR.

The coupling capacitors can be integrated with the conductivity measuring device and preferably have the structure which is described below with reference to FIGS . 3a, 3b.

On a semiconductor substrate 30 is above an oxide layer 31, which in the case of a silicon substrate, a silicon-oxide layer 31 is a conductive layer of polysilicon preferably arranged 32nd This poly silicon layer 32 forms one side of the coupling capacitor. On this polysilicon layer 32 there is a thin insulating layer 33 made of silicon oxide and silicon nitride, which galvanically separates the circuit or the poly terminal from the electrolyte 34 . The counter electrode of this capacitor is formed by the electrolyte 34 itself, which is in direct contact with the insulating layer 33 . The surface of the arrangement is covered by a protective oxide layer 35 which has an opening 37 in the area of the capacitor upper surface 36 .

As can also be seen in FIG. 3b, the conductive polysilicon layer 32 extends to an attachment area 38 for connection to the rest of the circuit.

The effective distance of the capacitor thus formed is the thickness of the insulating layer 33 . The component thus created acts as a capacitor, although only one of its sides is in a conventional, solid form and consists of a material in which electrons are responsible for the current transport. The other side, however, is liquid, since it is formed by the electrolyte 34 , with the charge transport being carried out by dissociated ions. Since for a migration of both electrons within the circuit and of ions in the electrolyte 34 , the field strength at the charge carrier is finally decisive and this continues through the insulating layer 33 , it is possible to let a displacement current flow through the component. As with conventional capacitors, charge carriers of different signs accumulate on the capacitor plates. The only difference is that, for example, a negative charge of electrons is formed on the one hand and a positive charge of ions on the other hand.

As is shown in Fig. 4a, the conductivity measuring arrangement according to the invention comprises a total of four such coupling capacitors C _K1 , C _K2 , C _K3 , C _K4 , which are arranged planar on a common semiconductor substrate 30 .

In contrast to the electrodes E1 to E4 used in the embodiment of FIG. 1, in which a conductive layer has direct contact with the electrolyte, no charge transfer between the circuit and the liquid can take place here. In contrast to measuring cells with galvanic contact, in which there is always a direct current path through the electrolyte, which can lead to its electrolysis in the event of a fault, this disadvantageous effect is avoided here by using galvanically separated cells.

Fig. 5b shows the equivalent circuit diagram of the arrangement of the coupling capacities according to Fig. 5a. As can be seen there, _ohmic resistors R _EL1 , R _EL2 , R _{EL3 lie} between the coupling capacitances C _K1 to C _K4 . The CMOS switches S ₅ and S ₆ are replaced (in the equivalent circuit diagram according to FIG. 7a) by their on-resistance R _ON . The on-resistance of these switches S ₅ , S ₆ is only of importance during the clock phase T ₃ , in which the measuring capacitor C _{M is} charged.

In general, in the following consideration, a fairly angular current is assumed, which flows through the arrangements from A to D or vice versa for the same amount of time. As shown in Fig. 3, the current is generated by a direct current source SQ, the polarity of which is periodically reversed with the aid of controlled switches S1 to S4.

The method described is used for explanation in two Sub-processes broken down, namely on the one hand into the explanation the processes in connection with the imprint of the measurement current and on the other hand the measurement of a specific Resistance of the electrolyte proportional voltage.

FIG. 6a shows the effective electrical equivalent circuit diagram for the impressing of the test current. To simplify the illustration, only the current flow in the direction from A to D (clock phase T ₁ ) is considered, since only the signs change when the current is reversed (clock phase T ₂ ). Point D is assumed to be on the ground for the duration under consideration. The constant current then leads to a linear rise in voltage across the capacitor C _K4 in accordance with the following relationship:

The element of the electrical equivalent circuit diagram following the fourth coupling capacitor C _K4 , namely the third ohmic electrolytic resistor R _El3 , causes a constant voltage drop due to the measuring current in accordance with the following context:

V _REl3 = I₀R _El3 (2)

Likewise, the other two _ohmic resistors R _El2 , R _El1 lead to the following voltage _drops :

V _REl2 = I₀R _El2 (3)

V _REl1 = I₀R _El1 (4)

The voltage at point C results as follows as the sum of the voltages across the fourth coupling capacitor C _K4 and the third electrolytic resistor R _El3 :

Correspondingly, the voltage at point B results the following relationship:

The upper capacitor C _K1 is of the same size and configuration as the capacitor C _K4 and causes an equally large voltage drop as this, which satisfies the following relationship:

It is crucial for the further evaluation that the Voltage at points B and C is absolutely ramp-shaped run, but their difference is constant and only from depend on the measuring current and the electrolyte resistance. Therefore applies:

V _B - V _C = I₀R _El2 (8)

In order to detect this differential voltage, two further coupling capacitors C _K2 and C _{K4 are} used, which are arranged between the capacitors C _K1 and C _K4 described above. These are connected to the measuring capacitor C _M during a time T ₃ after the polarity change of the measuring current, that is, after steady-state conditions have occurred with respect to the ohmic voltage drop across the electrolyte, via the switches S ₅ and S ₆ . The procedure is the same for reverse current flow. Then the switches S ₇ and S _{8 are} used within the time period of T ₂ during the period T ₄ accordingly.

To analyze the process, the ohmic voltage drop across the partial electrolyte resistor R _{El2 is shown} in Figure 4 as a voltage source. This is permissible, although the original current distribution is disturbed by the charging of the branch with the measuring capacitor C _M , since this branch becomes currentless again after a short time, so that the same conditions then exist as without the measuring capacitor C _M.

The following boundary conditions apply to the loading process of C _M :

Charging takes place exponentially, since only resistors and Capacities are in the relevant circle.

The time constant is defined by the double on-state on resistance of the CMOS switch and by the series connection of the coupling capacitances C _K2 , C _K3 and the measuring capacitance C _M. Here, C _{K2 is} equal to C _K3 . The following relationship applies to the charging time constant t _M :

The time constant t _M achieved in the implemented circuit is in the range of a few 10 ns, so that a charging of the measuring capacitance C _{M is} ensured within a very short time compared to the duration of the voltage ramp of a few 10 μs. The final value of the voltage at the measuring capacity C _M is obtained from the capacitive division ratio of the existing capacitors according to the following relationship:

With this voltage V _CMEnd , a _{measurement signal} proportional to the electrolyte resistance R _El2 is now available. Due to the predetermined geometric relationships of the measuring arrangement, the second electrolyte part resistance R _{El2 is linked} via a constant to the specific resistance or by means of reciprocal formation with the specific conductance of the electrolyte. This constant can be understood as a "cell constant" of the arrangement.

The time profiles of the voltage across the measuring capacitor C _M are shown in FIG. 7b.

After the measurement signal has been transmitted to the measurement capacitance C _M , the switches S ₅ and S _{6 are} opened again, the charge on the measurement capacitance C _M being retained (cf. FIG. 7a). For further processing, it is introduced and amplified by means of further switches S ₉ and S ₁₀ in the switch capacitor circuit according to FIG. 3. The charge stored on the measuring capacitor C _M is fed to the feedback capacitor C _R , the capacitance ratio of C _M to C _R defining the gain factor. The output voltage V _OUT is available as an output value after each charging process. When the operational amplifier OPV is connected as an integrator, in which the voltage across the feedback capacitor C _{R is} only reset after a number of cycles by the eleventh switch S ₁₁ , the output signal can be further amplified.

In the conductivity measuring device according to the invention Polarization effects excluded, can not be electrical lysis processes occurring in the electrolyte are drift errors due to absorption of ions or pollution likewise excluded or in the area of the measuring elements greatly reduced. The electrolyte is not on a solid  Potential, which makes the scope of the inventions The inventive conductivity measuring device is further enlarged becomes. Because the electrolyte is not in contact with metals must, undesirable chemical reactions, such as for example, catalytic reactions avoided.

Claims (13)

1. Integrable conductivity measuring device for measuring the electrical conductivity of liquids,
with a current source device (SQ, S1, S2, S3, S4) which can be connected to two current supply elements, via which an essentially rectangular current can be fed into the liquid, and
with a measuring circuit (OPV, S5 - S11, CM, CR) connected to two voltage measuring elements for determining the voltage drop in the liquid between the voltage measuring elements, which depends on the electrical conductivity of the liquid under investigation,
wherein the measuring circuit is a switch-capacitor circuit having a measuring capacitor CM, which can be coupled and separated with the voltage measuring elements via a switch device S5, S6, S7, S8 depending on the course of the substantially rectangular current, after which German patent application P4113033.2-52,
characterized,
that the power supply elements as Stromeinkoppelkonden capacitors (C _K1 , C _K4 ) are formed. 2. Integrable conductivity measuring device according to claim 1, characterized in that the voltage measuring elements are designed as voltage decoupling capacitors (C _K2 , C _K3 ). 3. Integrable conductivity measuring device according to claim 1 or 2, characterized in that each coupling capacitor (C _K1 , C _K2 , C _K3 , C _K4 ) as a conductive layer ( 32 ) on an insulating intermediate layer ( 31 ), which in turn on a Semiconductor substrate ( 30 ) lies, is formed and is covered by an insulating surface layer ( 33 ). 4. Integrable conductivity measuring device according to claim 3, characterized in that
that the insulating surface layer ( 33 ) consists of silicon oxide and silicon nitride,
that the conductive layer ( 32 ) consists of polysilicon, and
that the insulating intermediate layer ( 33 ) consists of silicon oxide. 5. Integrable conductivity measuring device according to one of claims 1 to 4, characterized in that the current source device has a direct current source (SQ) via a first, second, third and fourth switch (S1, S2, S3, S4) with the Stromein coupling capacitors ( C _K1 , C _K4 ) can be connected in a first and a second polarity. 6. Integrable conductivity measuring device according to one of claims 1 to 5, characterized in that a control device is provided which controls the most, second, third and fourth switches (S1, S2, S3, S4) in such a way that they control the current source (SQ ) connect alternately with the current coupling capacitors (C _K1 , C _K2 ) for a first time period (T1) in the first polarity and for a second time period (T2) in the second polarity. 7. Integrable conductivity measuring device according to claim 6, characterized in that
that the switch device (S1, S2, S3, S4) has a fifth, sixth, seventh and eighth switch (S5, S6, S7, S8),
that the control device controls the fifth to eighth switches (S5, S6, S7, S8) in such a way that they disconnect the voltage coupling capacitors (C _K2 , C _K3 ) during a third period (T3) in a first polarity and for a fourth period ( Connect T4) in a second polarity to the measuring capacitor (C1), and
that the third period (T3) within the first period (T1) and the fourth period (T4) within the second period (T2). 8. Integrable conductivity measuring device according to claim 6, characterized, that the first and second periods (T1, T2) are of equal length are so that the rectangular current in time Medium has no DC component. 9. Integrable conductivity measuring device according to one of claims 1 to 8, characterized in that the measuring circuit is an amplifier circuit (OPV, CR) has that with the measuring capacitor (CM) via a ninth and tenth switches (S9, S10) can be connected. 10. Integrable conductivity measuring device according to claim 3, characterized,  that the control device the ninth and tenth scarf ter (S9, S10) drives such that they the amplifier circuit (OPV, CR) for a fifth period (T5) outside of the third and fourth periods (T3, T4) is connected to the measuring capacitor (CM). 11. Integrable conductivity measuring device according to claim 9 or 10, characterized in that the measuring circuit has a feedback capacitor (CR) in the feedback branch of the amplifier circuit (OPV, CR) having. 12. Integrable conductivity measuring device according to claim 11, characterized in that
that an eleventh switch (S11) lies in parallel with the feedback capacitor (CR), and
that the control device controls the eleventh switch (S11) such that the feedback capacitor (CR) is discharged after every fifth period (T5). 13. Integrable conductivity measuring device according to claim 11, characterized in that
that an eleventh switch (S11) lies in parallel with the feedback capacitor (CR), and
that the control device controls the eleventh switch (S11) in such a way that the feedback capacitor (CR) is discharged after each of a plurality of periods (T1, T2).