TWI383158B - Capacitance measurement circuit and method - Google Patents

Description

Capacitance measuring circuit and method

The present invention relates to a method and a circuit for measuring a capacitance value of a capacitor or a variation amount of a capacitance value, and more particularly to a method and a circuit for measuring a capacitance value of a capacitance or a variation amount of a capacitance value by a capacitance integration circuit.

A method and circuit for detecting or measuring a change in capacitance of a capacitor to be measured using a charge transfer technique is disclosed in U.S. Patent No. 6,466,036. As shown in (1), the technique shifts the charge in the capacitor to be tested into a reference capacitor. After multiple shifts, the charge in the reference capacitor gradually increases, and the voltage of the reference capacitor also follows. Ascending, by measuring the voltage change on the reference capacitor, the capacitance value of the capacitor to be measured is calculated.

The shortcoming of this technique is that the amount of charge transferred from the capacitor to be measured to the reference capacitor is different each time. The latter charge is less than the previous one. After the accumulated multiple times, the charge transferred in the last few times is relatively For the first time, it is much less. The voltage of the reference capacitor rises, and the slower the rise becomes later, which causes the problem of measuring the time of the capacitor to be tested and the accuracy.

Therefore, the inventor of the present invention has developed a method and circuit for measuring the capacitance value of a capacitor or the amount of change in a capacitance value, in particular, a method according to a known capacitance value and a capacitance of three switches to a capacitance integration circuit. A method and circuit for measuring the capacitance value of an unknown capacitor or the amount of change thereof, which can improve the time of measuring the capacitance to be tested in the prior art and the accuracy is poor. shape.

The present invention relates to a method and a circuit for measuring a capacitance value of a capacitor or a variation amount of a capacitance value, which utilizes a capacitance integration circuit to achieve a high-precision capacitance value or a sense of change in capacitance value.

The method includes at least: (a) grounding one end of the unknown capacitor and opening the other end, and simultaneously short-circuiting the known capacitor and connecting to a negative input terminal and an output terminal of an operational amplifier; wherein the operational amplifier The positive input is coupled to a first reference voltage; (b) the known capacitor is open at both ends, the negative input of the operational amplifier is coupled to one end of the known capacitor, and the output of the operational amplifier is coupled to the known The other end of the capacitor; (c) connecting the other end of the unknown capacitor to a second reference voltage to charge the unknown capacitor to a predetermined voltage; (d) opening the other end of the unknown capacitor; and (e) the unknown capacitor The other end is connected to the negative input of the operational amplifier in a manner that the switch is turned on.

Preferably, the circuit includes at least: a known capacitor; a first reference voltage; a second reference voltage different from the first reference voltage; and a first switch having one end coupled to the unknown ground capacitance One of the ungrounded terminals controls charging of the unknown ground capacitance to the second reference voltage; and a second switch is coupled to one end of the unknown grounded capacitor that is not grounded, and controls charging the known capacitor a third switch for controlling discharge of the known capacitor; a capacitor integrating circuit comprising the known capacitor, an operational amplifier, a first reference voltage and the third switch, using the first reference voltage amount to make a second There is a fixed voltage difference across the switch to provide a fixed current pair Knowing the capacitance charging; a voltage measuring circuit for measuring the voltage of the output terminal of the operational amplifier, the voltage is the voltage of one end of the reference capacitor and a signal processing and control circuit is used to control the first, second, and The three switches process the measurement result of the voltage measurement circuit to measure the capacitance value of the unknown capacitance to be tested or the change amount of the capacitance value.

In order to enable the reviewing committee to have a better understanding and approval of the structural purpose and efficacy of the present invention, the following examples are described in detail with reference to the illustrated examples.

Figure 2 is a circuit diagram of a preferred embodiment of the present invention.

A switch 201 of the circuit of the present invention is used to initialize the capacitor 211 (such as the capacitance of the metal conductor plate to the ground). When the switch 201 is turned on, the potential of the capacitor 211 to be tested is initialized, and the capacitor 211 to be tested is charged to a voltage. 212. When the initialization is completed, the switch 201 is turned off, and the voltage across the capacitor 211 to be tested is maintained at the voltage 212.

A switch 203 of the circuit of the present invention is used to initialize the reference capacitor 207. When the switch 203 is turned on, the reference capacitor 207 is initialized, and the voltage across the reference capacitor 207 is discharged to 0 volts. When the initialization is completed, the switch 203 is turned off.

The main feature of the present invention is to measure the capacitance value of the capacitor 211 to be tested or the amount of change in the capacitance value by using a reference capacitance integrating circuit 220. The capacitance to be tested can be a metal ball or a metal plate. The capacitance integrating circuit 220 can be implemented by various circuits. The present invention uses an operational amplifier 206, a reference voltage 204 and a reference capacitor 207. The reference capacitor 207 is connected across the inverting input and output of the operational amplifier 206.

When the reference capacitor 207 is fully charged, a voltage measuring circuit 208 is used to measure the voltage 205 at the output of the operational amplifier, and then the measured signal is processed by a signal processing and control circuit 209 to calculate the capacitance of the capacitor 211 to be tested. Or the amount of change in capacitance. The function of the signal processing and control circuit includes the control of three switching elements, the control of the starting voltage measuring circuit 208, the reading of the result of the voltage measuring circuit, and the processing of the measurement of the capacitance 211 to be measured.

Preferably, the voltage measuring circuit 208 can be an analog digital converter or a comparator. The signal processing and control circuit 209 can be selected from a microcontroller, a microprocessor, a digital signal processor, an FPGA, or an integrated circuit.

Preferably, the circuit of the present invention can be fabricated in an integrated circuit by means of an integrated circuit, or can be composed of a plurality of integrated circuits. The circuit of the invention can also be used for measuring the capacitance value of the capacitor or the variation of the capacitance value, and can also measure the capacitance value of the characteristic of the other component or the variation of the capacitance value, and is particularly suitable for measuring the ground of any object. The capacitance value, which includes various parts of the human body, can be used as a touch switch due to the amount of change in capacitance caused by contact or proximity to a sensing plane. The touch switch can be a multi-channel switch.

Preferably, the capacitor 211 to be tested of the present invention may be a metal plate or a capacitor generated by one or more pins of an integrated circuit, or may be formed by a wire on a printed circuit board (PCB). Parasitic capacitance, or a capacitor formed by making a wire into a circular shape or other shape, or a plane 213 formed by a human body or other object contacting or being close to a wire or wire on the printed circuit board (PCB). Capacitance.

Preferably, the reference voltage 204 can be implemented in various manners, can be generated by the power supply of the integrated circuit in the integrated circuit, or can be realized by using other components such as resistors, capacitors, and transistors. The voltage value of voltage 204 can be any voltage, but is different from reference voltage 212. The reference voltage 204 is connected to the positive terminal input of the operational amplifier 206. Since the operational amplifier 206 operates under negative feedback, the positive terminal of the operational amplifier 206 and the negative terminal of the operational amplifier are virtual short circuits, so the negative terminal voltage of the operational amplifier 206 The voltage is equal to the voltage of the reference voltage 204.

The implementation steps of Figure 2 are as follows, wherein steps a~e are shown in Figure 3: a. First, the circuit of this embodiment is set in an initialized state, switches 201 and 202 are turned off, switch 203 is turned on, and reference capacitor 207 on the integrating circuit is used. Initialization, the voltage at both ends of the reference capacitor 207 is discharged to OV; as shown in FIG. 3, the capacitor 211 is open, the reference capacitor 207 is short-circuited; b. The switch 203 is turned off, and the reference capacitor 207 is opened by FIG. 3; c. The capacitor 211 to be tested is initialized, and the voltage of the capacitor 211 to be tested is charged to the reference voltage 212; d. The switch 201 is turned off, and the capacitor 211 is opened again by FIG. 3; e. The switch 202 is turned on for a fixed time (Δt) After that, the switch 202 is turned off; f. repeating steps c, d, e N times, to the next step g; g. voltage measuring circuit 208 measures the voltage value of 205; and h. using voltage measurement The measurement result of the circuit 208 calculates the capacitance value of the capacitor 211 to be tested or the amount of change of the capacitance value by a signal processing and control circuit.

The description of this embodiment is as follows: a metal round conductor plane 213 and a metal round conductor connected thereto The parasitic capacitance 211 generated on the surface of the surface 213 to the integrated circuit increases the capacitance of the capacitor 211 to be tested when the human finger approaches or approaches the metal circular conductor plane 213. Therefore, by measuring the amount of increase in the capacitance of the capacitor 211 to be tested, it can be determined whether the human finger is close to or close to the metal circular conductor plane 213.

After the step a, b is performed, the voltage across the reference capacitor 207 is OV; after the step c, the voltage of the capacitor 211 to be tested is fixed to the voltage 212; after the step e is performed, the voltage across the reference capacitor 207 is charged. To ΔV; this ΔV=I*Δt/Cs; I is the reference voltage 212, 204 subtracted, divided by the resistance value when the switch 202 is turned on.

Δt is the time when the switch 202 is turned on; Cs is the capacitance value of the reference capacitor 207.

Moreover, the amount of charge change stored in step e of the capacitor 211 to be tested is ΔQcx=Cx*ΔVcx=I*Δt; ΔQcx is the amount of charge change of the capacitor 211 to be tested; Cx is the capacitance of the capacitor 211 to be tested. Value; ΔVcx is the Δt time, the voltage on the capacitor 211 to be tested changes; therefore ΔV=I*Δt/Cs=Cx*ΔVcx/Cs.

After step f is performed, the voltage across the reference capacitor 207 is charged to an NΔV; ΔVcs=N*ΔV=NI*Δt/Cs=NCx*ΔVcx/Cs. ………(public Formula 1) ΔVcs is the voltage change across the reference capacitor 207.

From (Formula 1), the voltage across the reference capacitor 207 will be related to Cx. Therefore, when N, ΔVcx, and Cs are unchanged, Cx changes, and the voltage difference across the reference capacitor 207 is changed.

After the execution of steps g, 205, the voltage value is measured, because one end of the reference capacitor 207 is connected to the negative terminal of the operational amplifier 206. This voltage is the same as the positive terminal of the operational amplifier 206 due to the virtual short circuit of the operational amplifier 206. For reference voltage 204, the other end of reference capacitor 207 is coupled to 205, which is the voltage measured by voltage measurement circuit 208. Therefore, the voltage difference across the reference capacitor 207 is again equal to the value subtracted by the reference voltage 204 from the voltage measured by the voltage measuring circuit 208, so Vopout = Vb - ΔVcs = Vb - N * Cx * ΔVcx / Cs, Vopout For the voltage value of 205, Vb is the reference voltage 204. After step h is performed, the measurement result of the voltage measuring circuit 208 can be used to detect the change of Cx through the voltage value of the monitoring 205, and then the measured value is obtained. The capacitance value of the capacitor 211 or the amount of change in the capacitance value.

This feature has the following advantages: 1. Since the voltage of the reference capacitor 207 of the capacitance integrating circuit 220 is linearly changed, the voltage of the reference capacitor 207 of the capacitance integrating circuit 220 can be easily controlled within a range as long as the magnitude of N is controlled, and the voltage measuring circuit 208 can Measure to a more accurate voltage value.

2. Changing the capacitance value of the reference capacitor 207 of the capacitance integrating circuit 220, the ΔV size can be changed because: △V=I△t/Cs=Cx△Vcx/Cs, ΔV is inversely proportional to Cs. The smaller the Cs is, the larger the ΔV is. The larger the ΔV is, the smaller N can be, then the capacitance to be measured is measured. The time of the capacitance value can be shortened.

3. Since the voltage of the reference capacitor 207 of the capacitance integrating circuit 220 is linearly changed, it is easy to control and process, and the resolution is also relatively high.

Similarly, the operating principle of the present invention can also be applied to multi-channel unknown capacitance measurement as shown in FIG. 4, and FIG. 4 adds a set of unknown capacitors 211a/b and switches 201a/b and 202a/b, when the switches 201a/b and 202a/b is driven by four interleaved clocks, and the skilled person can easily detect the capacitance value or the variation of the unknown capacitor 211a/b according to the foregoing disclosure, and details are not described herein again.

The above description is only exemplary of the invention, and the scope of the invention is not limited thereto. That is to say, the equivalent changes and modifications made by the applicant in accordance with the scope of the patent application of the present invention should still fall within the scope of the patent of the present invention. I would like to ask your review committee to give a clear explanation and pray for it.

201, 201a, 201b, 202, 202a, 202b, 203‧ ‧ switch

204‧‧‧ voltage

206‧‧‧Operational Amplifier

205‧‧‧Operation Amplifier Output

207,211,211a,211b‧‧‧ capacitor

208‧‧‧Voltage measurement circuit

209‧‧‧Signal Processing and Control Circuit

212‧‧‧ voltage

213,213a, 213b‧‧‧ conductor plane

220‧‧‧Reference capacitance integration circuit

FIG. 1A is a schematic diagram of a prior art; FIG. 2 is a schematic diagram of an example used in the present invention; FIG. 3 is an equivalent circuit diagram of FIG. 2; and FIG. 4 is a schematic diagram of a multi-channel example of the present invention.

201,202,203‧‧" switch

204‧‧‧ voltage

206‧‧‧Operational Amplifier

205‧‧‧Operation Amplifier Output

207,211‧‧‧ capacitor

208‧‧‧Voltage measurement circuit

209‧‧‧Signal Processing and Control Circuit

212‧‧‧ voltage

213‧‧‧Conductor plane

220 reference capacitance integration circuit

Claims (13)

A circuit for measuring an unknown capacitance to ground with a known capacitance, comprising at least: a known capacitance; a first reference voltage; a second reference voltage different from the first reference voltage; a first switch, One end is coupled to the ungrounded end of the unknown ground capacitance to control charging the unknown ground capacitance to the second reference voltage; and a second switch having one end coupled to the unknown ground capacitance not grounded One end controls the charging of the known capacitor; a third switch controls the discharge of the known capacitor; and a capacitive integrating circuit includes the known capacitance of the system and the third switch, using the first reference voltage Measure the capacitance value of the unknown capacitance to be tested or the amount of change of the capacitance value; a voltage measurement circuit that measures a digital voltage value at the output end of the operational amplifier; and a signal processing and control circuit that controls the first The second and third switches process the output of the capacitance integrating circuit. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the unknown capacitor can be a metal conductor plate. A circuit for measuring an unknown capacitance to ground, as in the first aspect of the patent application, wherein: the first, second, and third switches, the capacitance integrating circuit, the signal The processing and control circuitry is fabricated on a single wafer. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the voltage measuring circuit is an analog-to-digital converter. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the voltage measuring circuit is a comparator. For example, the circuit for measuring an unknown capacitance to ground according to the first item of the patent scope, wherein: the signal processing and control circuit is selected by a microcontroller, a microprocessor, a digital signal processor, an FPGA, or an integrated circuit. . A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the reference voltage is generated by a power supply of the integrated circuit. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the second reference voltage is a power supply of the integrated circuit. A circuit for measuring an unknown capacitance to ground, as in the first aspect of the patent application, wherein: the second reference voltage is a power ground of the integrated circuit. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the circuit is capable of measuring a capacitance change of an unknown capacitance to ground. A circuit for measuring an unknown capacitance to ground, as in the first application of the patent scope, wherein: The voltage of this known capacitor is linearly changed. A circuit for measuring an unknown capacitance to ground according to the first aspect of the patent application, wherein: the known capacitor is connected to the negative input terminal and the output terminal of the operational amplifier via the third switch, and the first reference voltage is connected to the circuit The positive input of the operational amplifier. A circuit for measuring an unknown capacitance to ground, as in claim 1, wherein the first reference voltage is connected to ground.