Circuit structure and method for realizing capacitance value calculation

Abstract

The present invention relates to a circuit structure for realizing capacitance value calculation, which includes a detection control circuit module, a charging resistor R1, a first reference capacitor (C1), a capacitor under test (C2), a third reference capacitor (C3), a first switch (S1), a second switch (S2), a fifth switch (S5) and a sixth switch (S6). The detection control circuit module is connected across the charging resistor (R1). One end of the charging resistor (R1) is connected to the first reference capacitor (C1), a third switch (S3) and the first switch (S1). The other ends of the first reference capacitor (C1) and the third switch (S3) are grounded. The other end of the charging resistor (R1) is connected to the second switch (S2), the fifth switch (S5) and the sixth switch (S6). The present invention also relates to a method for realizing capacitance value calculation. By adopting the circuit structure and method for realizing capacitance value calculation of the present invention, the error caused by the resistor can be eliminated, the influence of environmental factors on the detection of a single capacitor can be reduced, the influence of parasitic capacitance on the detection can be eliminated, and the measurement is more accurate.

Description

Technical Field

[0001] The present invention relates to the field of MEMS products, and particularly to the field of capacitance detection of MEMS products. Specifically, it refers to a circuit structure and method for realizing capacitance value calculation. Background Art

[0002] In recent years, stimulated by the general environment of Sino-US trade confrontation, many domestic MEMS products have emerged on the market. Many of these products have capacitance characteristics, and the changes in the environment, products, etc. are characterized by detecting the changes in capacitance. However, the capacitance values of many MEMS products are small (in the order of dozens of picofarads), and they are easily interfered; the linearity and consistency are relatively poor. And regardless of high-grade or low-grade capacitive components, the long-term stability is not ideal, and most of them drift seriously after long-term use. For example, the capacitance value of a humidity-sensitive capacitor changes in the pF level, and the change of 1% RH is less than 0.5 pF. The drift change of the capacitance value often causes an error of dozens of RH%.

[0003] Case 1: The working diagram of measuring capacitance with 555 is as Figure 1 shown. C2 is the capacitor to be measured. Among them, R4, R2, and C2 form a charging circuit, and R2 and C2 form a discharging circuit. It is charged to 0.67VCC through R4 and R2, and then discharged to 0.33VCC through R2.

[0004] Then t high = C2 * (R2 + R4) * ln2......(1)

[0005] t low = C2 * R2 * ln2......(2)

[0006] F = 1 / (t high + t low ) = 1 / (C2 * (R4 + 2 * R2) * ln2)......(3)

[0007] Output duty cycle = t high * F = R2 / (R4 + 2 * R2)......(4)

[0008] It can be obtained from formula (3) that the capacitance change affects the frequency change. On the contrary, it can be obtained that the frequency can characterize the capacitance change, and further characterize the change of the environment or the product.

[0009] There are problems with the method of separately testing the capacitance in Case 1:

[0010] 1. Resistance has temperature characteristics. In the actual working circuit, with the change of resistance, it will affect the charging and discharging time, and then lead to the change of the measured frequency, and further affect the determination of the capacitance value.

[0011] 2. Errors caused by parasitic capacitance due to reasons such as wiring, circuit board manufacturing, and impurity accumulation during long-term operation cannot be eliminated. For example, the base value and sensitivity of a humidity-sensitive capacitor are both in the picofarad range. The existence of external parasitic capacitance will affect the charging and discharging time, resulting in a change in the measured frequency, and thus affecting the determination of the capacitance value.

[0012] Case 2: A capacitance measurement method of the prior art is as Figure 2 shown. The capacitor under test C2, the reference capacitor C1, and the charging resistor R1 are the core components. The control module first closes S2, and charges C1 through R1 first. When the threshold value is reached, S3 is closed to discharge to the ground. After a certain time, S3 is opened, and the charging and discharging are repeated. The time T used to record the number of oscillations X1 is recorded. During the time T, the number of oscillations X2 of the capacitor under test C2 is counted. Since the charging voltage and the flip threshold voltage are the same, the following relationship can be obtained through formula conversion:

[0013] C1×X1 = C2×X2 = ……(5)

[0014] For the reference capacitor C1, the number of oscillations X1 of the reference capacitor remains fixed. From the formula, it can be obtained that for different capacitances, the corresponding number of oscillations X2 is different. Conversely, the capacitance change can be characterized by X2.

[0015] Although the technical solution of Case 2 introducing the test circuit of the reference capacitor effectively solves the influence brought by the charging resistor, etc., there are still the following problems:

[0016] 1. Errors caused by parasitic capacitance due to reasons such as wiring, circuit board manufacturing, and impurity accumulation during long-term operation cannot be eliminated. For example, the base value and sensitivity of a humidity-sensitive capacitor are both in the picofarad range. The error brought by the parasitic capacitance has a great influence on the actual measured humidity.

[0017] 2. The circuit discharge is controlled by an N transistor to discharge to GND, and the time is fixed. This method believes that since the discharge time is very small, it is ignored. However, in actual use, there may be situations where the capacitor under test is much larger or much smaller than the reference capacitor. At this time, the number of oscillations will also show a large difference. Ignoring the discharge time at this time will introduce new errors. Summary of the Invention

[0018] The object of the present invention is to overcome the above-mentioned shortcomings of the prior art, and provide a circuit structure and method for calculating the capacitance value that meet the requirements of small error, high precision, and accurate measurement.

[0019] In order to achieve the above object, the circuit structure and method for calculating the capacitance value of the present invention are as follows:

[0020] The circuit structure for calculating the capacitance value of a capacitor has the main feature that the circuit structure includes a detection and control circuit module, a charging resistor, a first reference capacitor, a capacitor under test, a third reference capacitor, a first switch, a second switch, a fifth switch, and a sixth switch.

[0021] The detection and control circuit module is connected across the charging resistor. The first reference capacitor, the third switch, and the first switch are all connected to one end of the charging resistor. The other ends of the first reference capacitor and the third switch are both grounded. The other end of the charging resistor is connected to the second switch, the fifth switch, and the sixth switch. The other end of the fifth switch is grounded through the capacitor under test, and the other end of the sixth switch is grounded through the third reference capacitor.

[0022] Preferably, the circuit structure further includes a fourth switch, one end of which is connected to the ends where the fifth switch and the sixth switch are connected, and the other end is grounded. When reaching the threshold point, the first reference capacitor, the capacitor under test, and the third reference capacitor are discharged to ground through the fourth switch. The capacitor under test and the third reference capacitor are discharged through the fourth switch, and the first reference capacitor is discharged through the charging resistor and then through the fourth switch.

[0023] Preferably, the circuit structure further includes a third switch, one end of which is connected to the ends where the first reference capacitor and the charging resistor are connected, and the other end is grounded. When reaching the threshold point, the first reference capacitor, the capacitor under test, and the third reference capacitor are discharged to ground through the third switch. The first reference capacitor is discharged through the third switch, and the capacitor under test and the third reference capacitor are discharged through the charging resistor and then through the third switch.

[0024] The method for calculating the capacitance value of a capacitor using the above circuit structure has the main feature that the method includes the following steps:

[0025] (1) Close the second switch and charge the first reference capacitor through the charging resistor. After reaching the threshold voltage V OUT , close the third switch and quickly discharge to ground. After the discharge time t d , open the third switch to complete the oscillation. Repeat the charge and discharge within the period T and record the oscillation times X1. Then open the second switch;

[0026] (2) Through the detection and control circuit module, close the sixth switch and open the fifth switch. Close the first switch to charge and discharge the third reference capacitor, record the oscillation times X3 within the period T, and then open the first switch;

[0027] (3) Through the detection and control circuit module, close the fifth switch and open the sixth switch. Close the first switch to charge and discharge the capacitor under test, record the oscillation times X2 within the period T, and then open the first switch;

[0028] (4) Calculate the capacitance value of the capacitance to be measured based on the obtained oscillation times X1, X2, and X3.

[0029] Preferably, in step (4) of calculating the capacitance value of the capacitance to be measured, specifically:

[0030] Calculate the capacitance value of the capacitance to be measured according to the following formula:

[0031] C2 = C1 - (C1 - C3) × (X2X3 - X1X3) / (X3X2 - X1X2);

[0032] Wherein, C1 and C3 are the capacitance values of the first reference capacitance and the third reference capacitance respectively, and X1, X2, and X3 are the oscillation times.

[0033] By adopting the circuit structure and method for realizing capacitance value calculation of the present invention, compared with the prior art, the error caused by the resistor can be removed, the influence of the single capacitance detection vulnerable to environmental factors can be reduced, and compared with the prior art, the influence of the parasitic capacitance on the detection can be removed, and the discharge time is taken into account, and the measurement is more accurate. Description of the Drawings

[0034] Figure 1 It is a schematic diagram of the circuit structure for directly testing the charging and discharging of the capacitance in Embodiment 1 of the prior art.

[0035] Figure 2 It is a schematic diagram of the circuit structure of the capacitance measurement method in Embodiment 2 of the prior art.

[0036] Figure 3 It is a schematic diagram of the circuit structure for realizing capacitance value calculation of the present invention.

[0037] Figure 4 It is a schematic diagram of the circuit structure of the embodiment for realizing capacitance value calculation of the present invention.

[0038] Figure 5 It is a schematic diagram of the circuit structure of another embodiment for realizing capacitance value calculation of the present invention. Detailed Embodiments

[0039] In order to be able to more clearly describe the technical content of the present invention, the following will be further described in conjunction with specific embodiments.

[0040] The present invention optimizes the case 2 of the prior art, takes into account the advantages, effectively removes the error caused by the parasitic capacitance, improves the capacitance test accuracy, and can increase the service life, and effectively removes the new parasitic capacitance introduced due to the accumulation of dust and impurities caused by long-term use. The present invention mainly removes the error caused by the parasitic capacitance for capacitance calculation due to reasons such as wiring, board making, and accumulation of impurities during long-term operation.

[0041] The circuit structure of the present invention introduces two reference capacitors C3 and C2 and their corresponding control parts compared with the circuit structure of the prior art case 2. By introducing two reference capacitors and through formula derivation, they cancel each other out, effectively removing the error caused by the resistor and also removing the error caused by the parasitic capacitance due to various factors.

[0042] The circuit structure for implementing the capacitance value calculation of the present invention includes a detection and control circuit module, a charging resistor R1, a first reference capacitor C1, a capacitor under test C2, a third reference capacitor C3, a first switch S1, a second switch S2, a fifth switch S5, and a sixth switch S6.

[0043] The detection and control circuit module is connected across the charging resistor R1. One end of the first reference capacitor C1, the third switch S3, and the first switch S1 are all connected to one end of the charging resistor R1. The other ends of the first reference capacitor C1 and the third switch S3 are both grounded. The other end of the charging resistor R1 is connected to the second switch S2, the fifth switch S5, and the sixth switch S6. The other end of the fifth switch S5 is grounded through the capacitor under test C2, and the other end of the sixth switch S6 is grounded through the third reference capacitor C3.

[0044] As a preferred embodiment of the present invention, the circuit structure further includes a fourth switch S4, one end of which is connected to the ends of the fifth switch S5 and the sixth switch S6 that are connected together, and the other end is grounded. In the case of reaching the threshold point, the first reference capacitor C1, the capacitor under test C2, and the third reference capacitor C3 are discharged to ground through the fourth switch S4. The capacitor under test C2 and the third reference capacitor C3 are discharged through the fourth switch S4, and the first reference capacitor C1 is discharged through the charging resistor R1 and then through the fourth switch S4.

[0045] As a preferred embodiment of the present invention, the circuit structure further includes a third switch S3, one end of which is connected to the ends of the first reference capacitor C1 and the charging resistor R1 that are connected together, and the other end is grounded. In the case of reaching the threshold point, the first reference capacitor C1, the capacitor under test C2, and the third reference capacitor C3 are discharged to ground through the third switch S3. The first reference capacitor C1 is discharged through the third switch S3, and the capacitor under test C2 and the third reference capacitor C3 are discharged through the charging resistor R1 and then through the third switch S3.

[0046] The working principle of the present invention is as follows:

[0047] Close S2, and the voltage V IN charges the reference capacitor C1 through the charging resistor R1. When reaching the threshold voltage V OUT then close S3 to quickly discharge to ground. Since there is no discharge current-limiting resistor, it can be ensured that the capacitor is discharged to ground within the time t d The discharge time t dAfter arrival, disconnect S3 to complete one oscillation. Repeat the charge and discharge within time T, and record the number of oscillations X1. Then turn on S2. Next, through the control circuit, first close S6 and open S5, then close S1 to charge and discharge C3, record the number of oscillations X3 within time T, and then turn off S1. Then, through the control circuit, first close S5 and open S6, close S1 to charge and discharge C2, record the number of oscillations X2 within time T, and then turn off S1. In this way, the number of oscillations X1, X2, and X3 of the three capacitors within the same time T are obtained through the above circuit. The following algorithm also takes these three quantities as inputs and performs calculations with some fixed quantities set in the circuit.

[0048] At this time, the charge and discharge time of the reference capacitor C1 each time is:

[0049] t1 = T / X1......(6)

[0050] The charge and discharge time of the reference capacitor C3 each time is:

[0051] t3 = T / X3......(7)

[0052] The charge and discharge time of the capacitor under test C2 each time is:

[0053] t2 = T / X2......(8)

[0054] And,

[0055] t1 = t 1c +t d ......(9)

[0056] Where t 1c is the charging time of C1 each time,

[0057] t3 = t 3c +t d ......(10)

[0058] Where t 3c is the charging time of C3 each time,

[0059] t2 = t 2c +t d ......(11)

[0060] Where t 2c is the charging time of C2 each time.

[0061] In summary,

[0062] T / X1 - T / X3 = t 1c -t 3c ......(13)

[0063] According to the capacitor charging formula:

[0064] t = -RCln(1 - V OUT / V IN ) ……(14)

[0065] Let

[0066] A = -Rln(1 - V OUT / V IN ) ……(15)

[0067] Derived from formulas (13), (14), and (15):

[0068] T / X1 - T / X3 = A×(C1 - C3) ……(16)

[0069] Similarly,

[0070] T / X1 - T / X2 = A×(C1 - C2) ……(17)

[0071] From formulas (16) and (17):

[0072] C2 = C1 - (C1 - C3)(X2X3 - X1X3) / (X3X2 - X1X2) ……(18)

[0073] The verification process of the above working principle is as follows:

[0074] The reference capacitor C1 oscillates a fixed number of times X1, the fixed capacitor C3 oscillates X3 times, and the capacitance value of C2 is changed. At this time, the recorded X2 will change. Conversely, X2 can be used to characterize the change of capacitance.

[0075] At this time, if the design of the reference capacitor and the capacitor to be measured is ensured to be completely symmetrical in the design, assuming that the parasitic capacitance ΔC is introduced simultaneously.

[0076] Then formula (18) becomes

[0077] C2 = C1 - (C1 - C3)(X2X3 - X1X3) / (X3X2 - X1X2) ……(19)

[0078] Finally,

[0079] C2 = C1 - (C1 - C3)(X2X3 - X1X3) / (X3X2 - X1X2) ……(20)

[0080] According to the formula derivation, the influence of the parasitic capacitance brought by external factors can be completely offset, proving that the circuit design is completely feasible.

[0081] In the specific implementation of the present invention, according to the above working principle, the method for calculating the capacitance value of the present invention using the above circuit structure includes the following steps:

[0082] (1) Close the second switch S2, charge the first reference capacitor C1 through the charging resistor R1 until the threshold voltage V OUT is reached, then close the third switch S3 to discharge rapidly to the ground. During the discharge time t d after which, open the third switch S3 to complete the oscillation. Repeat the charge and discharge within the period T, record the number of oscillations X1, and then open the second switch S2;

[0083] (2) Through the detection and control circuit module, close the sixth switch S6 and open the fifth switch S5. Close the first switch S1 to charge and discharge the third reference capacitor C3, record the number of oscillations X3 within the period T, and then open the first switch S1;

[0084] (3) Through the detection and control circuit module, close the fifth switch S5 and open the sixth switch S6. Close the first switch S1 to charge and discharge the capacitor under test C2, record the number of oscillations X2 within the period T, and then open the first switch S1;

[0085] (4) Calculate the capacitance value of the capacitor under test C2 based on the obtained number of oscillations X1, X2, and X3.

[0086] As a preferred embodiment of the present invention, in step (4), calculating the capacitance value of the capacitor under test C2 is specifically as follows:

[0087] Calculate the capacitance value of the capacitor under test C2 according to the following formula:

[0088] C2 = C1 - (C1 - C3) × (X2X3 - X1X3) / (X3X2 - X1X2);

[0089] where C1 and C3 are the capacitance values of the first reference capacitor C1 and the third reference capacitor C3 respectively, and X1, X2, and X3 are the number of oscillations.

[0090] Adopting the circuit structure and method for calculating the capacitance value of the present invention can remove the error caused by the resistor compared with the prior art, and at the same time reduce the influence of environmental factors on the detection of a single capacitor. Moreover, compared with the prior art, it can remove the influence of parasitic capacitance on the detection and take the discharge time into account, making the measurement more accurate.

[0091] In this specification, the present invention has been described with reference to its specific embodiments. However, it is obvious that various modifications and transformations can still be made without departing from the spirit and scope of the present invention. Therefore, the specification and the drawings should be regarded as illustrative rather than restrictive.

Claims

1. A method for calculating the capacitance value by using a circuit structure for realizing capacitance value calculation, the circuit structure comprising: Detection control circuit module, charging resistor ( ), first reference capacitor ( ), capacitor under test ( ), third reference capacitor ( ), first switch ( ), second switch ( ), fifth switch ( ), and sixth switch ( ). The described detection and control circuit module is connected across the charging resistor ( ). One end of the described first reference capacitor ( ), the third switch ( ), and the first switch ( ) are all connected to one end of the charging resistor ( ). The other ends of the described first reference capacitor ( ) and the third switch ( ) are both grounded. The other end of the charging resistor ( ) is connected to the second switch ( ), the fifth switch ( ), and the sixth switch ( ). The other end of the fifth switch ( ) is grounded through the capacitor under test ( ). The other end of the sixth switch ( ) is grounded through the third reference capacitor ( ). The other ends of the first switch ( ) and the second switch ( ) are both connected to the voltage ; Characterized in that the method comprises the following steps: (1) Close the second switch ( ), charge the first reference capacitor through the charging resistor (R1) ( ), close the third switch ( ) after reaching the threshold voltage, quickly discharge to the ground, and open the third switch ( ) after the discharge time ( ) to complete the oscillation. Repeat the charge and discharge within the period ( ), record the number of oscillations ( ), and open the second switch ( );​ (2) By detecting that the control circuit module closes the sixth switch ( ) and opens the fifth switch ( ), closes the first switch ( ) to charge and discharge the third reference capacitor ( ), records the number of oscillations within the period , and opens the first switch ( ); (3) By detecting that the control circuit module closes the fifth switch ( ), and opens the sixth switch ( ), closes the first switch ( ) to charge and discharge the capacitor under test ( ), records the number of oscillations within the period , and opens the first switch ( ); (4) Calculate the capacitance value of the capacitance to be measured through the obtained oscillation times X1, X2, and X3 ( ) 2. The method for calculating the capacitance value according to claim 1, characterized in that, In the step (4), the capacitance value of the capacitance to be measured ( ) is calculated as follows: Calculate the capacitance value of the capacitance to be measured ( ) according to the following formula: ； Among them, and are the capacitance values of the first reference capacitor ( ) and the third reference capacitor ( ) respectively, and and are the number of oscillation times.

3. The method for calculating the capacitance value according to claim 1, wherein The described circuit structure further includes: a fourth switch ( ), one end of which is connected to one end of a fifth switch ( ) and one end of a sixth switch ( ), and the other end is grounded. When reaching the threshold point, the first reference capacitor ( ), the capacitor under test ( ), and the third reference capacitor ( ) are discharged to ground through the fourth switch ( ). The capacitor under test ( ) and the third reference capacitor ( ) are discharged through the fourth switch ( ). The first reference capacitor ( ) is charged through a charging resistor ( ) and then discharged through the fourth switch ( ).

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