CN101739186B - Image display system, capacitive touch panel and capacitance measuring device and method thereof - Google Patents

Abstract

The invention provides an image display system, a capacitive touch panel, a capacitance measuring device and a capacitance measuring method thereof. The capacitance measuring device is provided with a reference voltage source and a comparator, wherein the reference voltage source can generate a correction reference voltage according to noise on a parasitic capacitor of the capacitive touch panel. In a discharging process, the comparator compares the corrected reference voltage with the input voltage from the selected electrode to judge the contact capacitance on the selected electrode. Because the noise can be eliminated under the differential comparison of the comparator, the capacitance measuring device can accurately measure the contact capacitance value of the capacitive touch panel.

Description

Image display system, capacitive touch panel and capacitance measuring device and method thereof

technical field

The present invention relates to a touch panel, in particular to an image display system, a capacitive touch panel and a capacitance measuring device and method thereof.

Background technique

Touch panels are widely used, such as automatic teller machines, point-of-sale terminals, industrial control systems, and the like. With the popularity of portable electronic products such as smart phones and personal digital assistants (PDAs), touch panels also provide a more convenient input method for users who are not familiar with key input, so the market is growing rapidly.

According to the physical principle of detecting touch points, the touch panel is mainly divided into two types: resistive touch panel and capacitive touch panel. A resistive touch panel generates a voltage when lightly pressed with a finger or a stylus; while a capacitive touch panel operates by drawing a tiny current when a finger touches the panel. Moreover, the capacitive touch panel can be further divided into surface capacitive touch panel and projected capacitive touch panel. The projected capacitive touch panel can achieve multi-touch -Touch) function, so it is more attractive to the industry to compete for research and development. If the projected capacitive touch panel is integrated into the LCD screen to form an in-cell multi-touch panel (In-CellMulti-Touch Panel), the thinness of the original panel can be maintained. In thin film transistor liquid crystal displays (TFTLDs), transparent electrodes (ITO) are commonly used to lock and store charges. The same principle is reversed. This layer of ITO can also be used as a sensor and can provide high-density detection. However, in TFT LCD, since the noise caused by TFT is very large, the capacitive sensing control circuit is very important.

Referring to FIG. 1 , it is a schematic diagram of a prior art projected capacitive touch panel 200A. As shown in the figure, the projected capacitive touch panel 200A includes a capacitive touch panel unit 300A and a controller 30A. The capacitive touch panel unit 300A has an insulating substrate (not labeled), vertical strip electrodes 32A, and horizontal strip electrodes 34A, wherein the strip electrodes 32A and the strip electrodes 34A form an angle of 90 degrees. 1 is only a simplified schematic diagram. It should be noted that the vertical strip electrodes 32A and the horizontal strip electrodes 34A are on different planes (such as the front and back sides) of the insulating substrate, and the numbers are M and N respectively. Therefore, the input lines of the controller 30A are at least (M+N) lines. The controller 30A can sense a contact position on the capacitive touch panel unit 300A by capturing the capacitance change between the vertical strip electrodes 32A and the horizontal strip electrodes 34A.

Referring to FIG. 2 , it is a sensing circuit 100A of a related art capacitive touch panel. For example, the sensing circuit 100A can be built into the controller 30A shown in the first figure, so as to measure the capacitance change And know the contact position. Assuming that Cs is the contact capacitance generated by the finger touching the kth vertical strip electrode 32A, and Cp is the stray capacitance, then the sensing circuit 100A will first charge the charge Q to the contact capacitance Cs by a charging current source Ic and parasitic capacitance Cp. Then the sensing circuit 100A is first discharged through a larger first current source 20A, and then discharged through a smaller second current source 22A. From the formula Q=C×V, it can be seen that if the capacitance C changes due to finger contact with the electrode, the voltage at the connection point P will also change, so a comparator 50A is used to compare the voltage at the connection point P (compared with a reference voltage), that is It can be known whether the finger touches the kth electrode strip.

In more detail, if the controller 30A periodically switches the charge switch SW_C, the first discharge switch SW_A, and the second discharge switch SW_B, the output of the comparator 50A is a pulse width modulation whose pulse width changes with the contact or not. (PWM) signal. By reading the output PWM signal of the sensing circuit 100A, the controller 30A can know whether the kth vertical strip electrode 32A is touched by a finger and the position of the touch. The sensing circuit 100A can sequentially select and measure the vertical strip electrode 32A and the horizontal strip electrode 34A in FIG. 1 , and the controller 30A can know the capacitance change on these electrodes and the contact position on the projected capacitive touch panel. .

However, the sensing circuit 100A of the related art capacitive touch panel has two disadvantages:

1. Since the parasitic capacitance Cp is generally large, about 1000 times of the contact capacitance Cs of the touch panel, if there is noise on the parasitic capacitance Cp, it will affect the detection accuracy. Furthermore, when the projected capacitive touch panel is used in a liquid crystal display, since the ITO electrodes (forming electrode strips) on the upper panel of the liquid crystal display and the DC common voltage (DCVCOM) electrodes are very close, the parasitic capacitance Cp will increase. , making it difficult to measure the contact capacitance Cs. In addition, the sensing circuit 100A on the lower board often has a large noise coupled to the parasitic capacitance Cp of the upper board, causing measurement errors.

2. Since the M vertical strip electrodes 32A and the N horizontal strip electrodes 34A need (M+N) signal lines to lead to the sensing circuit 100A, the wiring is relatively complicated.

Contents of the invention

Therefore, an object of the present invention is to provide a capacitance measurement device for a capacitive touch panel that can reduce noise.

Therefore, another object of the present invention is to provide a capacitive touch panel capable of reducing noise.

In order to achieve the above object, the present invention provides a capacitance measuring device for a capacitive touch panel, the capacitance measuring device includes: a first discharge current source electrically connected to the connection point to provide a first discharge current; a second A discharge current source, electrically connected to the connection point to provide a second discharge current, wherein the first discharge current is greater than the second discharge current; a differential comparator has a first input terminal and a second input terminal, the first discharge current An input terminal is electrically connected to the connection point; a reference voltage source is electrically connected to the second input terminal, and the reference voltage source receives a reference voltage and a DC common voltage (DCVCOM) of a liquid crystal display to generate the DC common voltage A related modified reference voltage, wherein the DC common voltage of the liquid crystal display is also electrically connected to the touch panel. The differential comparator compares an input voltage on the connection point with the modified reference voltage to generate a pulse width modulation signal representing the contact capacitance, thereby reducing noise interference on the DC common voltage.

In order to achieve the above object, the present invention provides a capacitive touch panel, comprising: first electrodes having a first number M in the vertical direction; second electrodes having a second number N in the horizontal direction; and a capacitance measuring device. The capacitance measurement device includes: a first discharge current source, electrically connected to the connection point to provide a first discharge current; a second discharge current source, electrically connected to the connection point to provide a second discharge current, wherein the The first discharge current is greater than the second discharge current; a differential comparator has a first input terminal and a second input terminal, the first input terminal is electrically connected to the connection point; a reference voltage source is electrically connected to the first input terminal Two input terminals, the reference voltage source receives a reference voltage and a DC common voltage (DCVCOM) of a liquid crystal display to generate a modified reference voltage related to the DC common voltage, wherein the DC common voltage is also electrically connected to the touch panel . The differential comparator compares an input voltage on the connection point with the modified reference voltage to generate a pulse width modulation signal representing the contact capacitance, thereby reducing noise interference on the DC common voltage.

In order to achieve the above object, the present invention provides a capacitance measurement method of a capacitive touch panel, the capacitance measurement method measures a contact capacitance of an electrode of the capacitive touch panel, and the capacitance measurement method includes: a. providing a first discharge current source electrically connected to a connection point to provide a first discharge current, the connection point being electrically connected to the electrode; b providing a second discharge current source electrically connected to the connection point to provide a A second discharge current, wherein the first discharge current is greater than the second discharge current; c. providing a differential comparator with a first input and a second input, the first input is electrically connected to the The connection point; d provides a reference voltage source, electrically connected to the second input terminal, the reference voltage source superimposes a reference voltage and a DC common voltage of a liquid crystal display to generate a related to the DC common voltage modifying the reference voltage, wherein the DC common voltage is also electrically connected to the touch panel; e. after precharging the electrode via the connection point, discharging the electrode with the first discharge current; f. The electrode is discharged with the second discharge current; g. During the discharge period of the second discharge current, compare the modified reference voltage with an input voltage on the connection point to determine the contact capacitance.

Furthermore, according to a preferred embodiment of the present invention, the capacitive touch panel further has a multiplexer, which has a plurality of switches corresponding to the first electrodes and the second electrodes, and generates a third number of switches. A control signal, wherein the third number is the larger of the first number and the second number plus one. The multiplexer controls the switches through the control signals to select one of the first electrodes and the second electrodes to be electrically connected to the capacitance measuring device to measure the contact capacitance of the selected electrode , thereby reducing the number of signal lines.

Description of drawings

FIG. 1 is a schematic diagram of a prior art projected capacitive touch panel.

FIG. 2 is a circuit diagram of a sensing circuit of a related art capacitive touch panel.

FIG. 3 is a circuit diagram of a capacitance measuring device for a capacitive touch panel according to a preferred embodiment of the present invention.

FIG. 4A is a block diagram of the multiplexer of the present invention.

FIG. 4B is a partial circuit diagram of the multiplexer in FIG. 4A.

FIG. 5 is a circuit diagram of a reference voltage source according to a preferred embodiment of the present invention.

FIG. 6 is a circuit diagram of a first discharge current source according to a preferred embodiment of the present invention.

FIG. 7 is a circuit diagram of a differential comparator according to a preferred embodiment of the present invention.

FIG. 8 is a circuit diagram of a first low-pass filter according to a preferred embodiment of the present invention.

Fig. 9 is a waveform diagram illustrating the operation of the capacitance measuring device of a preferred embodiment of the present invention.

FIG. 10 is a flowchart illustrating the capacitance measurement method of the present invention.

FIG. 11 is a schematic block diagram of an image display system according to another embodiment of the present invention.

Figure number:

Touch panel 200A Touch panel unit 300A

Controller 30A

Strip electrodes 32A, 34A

Sensing circuit 100A Charging current source Ic

The first current source 20A The second current source 22A

Multiplexer 10 Capacitance measuring device 100

Charging current source Ic The first discharging current source 20

The second discharge current source 22 reference voltage source 30

The first low-pass filter 40 The second low-pass filter 42

Differential Comparator 50 Output Buffer 60

Charge switch SW_C First discharge switch SW_A

The second discharge switch SW_B

X direction switch SX1-SXM Y direction switch SY1-SYN

XY control signal SWXY switch control signal SW1-SWM

Electrode X1-XM

First switching capacitors C11, C12, C13

The second switching capacitor C21, C22 storage capacitor Cx

Thin film transistors to A-F transistors Q1-Q7

Capacitive touch panel 200 Display module 300

Input module 400 Image display system 500

Steps S100-S106

Detailed ways

Please refer to FIG. 3 , which is a circuit diagram of a capacitance measuring device 100 of a capacitive touch panel according to a preferred embodiment of the present invention. The capacitance measuring device 100 can be used in conjunction with a multiplexer 10 to selectively measure The shown contact capacitance Cs of a certain electrode on the projected capacitive touch panel unit 300A (details will be described later). In order to simplify the description, part of the similar components use the similar figure numbers as in Fig. 1 and Fig. 2, although they are not shown in the drawings, it should be noted that the multiplexer 10 and the capacitance measuring device 100 are controlled by a controller (not shown), To select electrodes, provide operating clocks and judge contact positions.

As shown in FIG. 3 , the capacitance measuring device 100 includes a differential comparator 50, respectively connected to the first input terminal (for example, non-inverting input terminal V+) and the second input terminal (for example, V+) of the differential comparator 50. The first low-pass filter 40 and the second low-pass filter 42 of the inverting input terminal V-), the reference voltage source 30 connected to the second low-pass filter 42, and the first low-pass filter 40 and the multiple The charging current source Ic, the first discharging current source 20 (providing the first discharging current IL), and the second discharging current source 22 (providing the second discharging current IS) are electrically connected to the contact P of the converter 10 . Furthermore, the charging current source Ic, the first discharging current source 20 and the second discharging current source 22 are respectively switched and controlled by the charging switch SW_C, the first discharging switch SW_A and the second discharging switch SW_B. The output of the comparator 50 is connected to an output buffer 60 for further analysis by the controller.

FIG. 4A is a block diagram of the multiplexer 10. The multiplexer 10 can be used for the projected capacitive touch panel unit 300A shown in FIG. 1, so FIG. 4A also includes the projected capacitive touch panel unit 300A. Eli explains. The multiplexer 10 includes M sets of switches SX1-SXM corresponding to M vertical (longitudinal) strip electrodes 32A and N sets of switches SY1-SYN corresponding to N horizontal (horizontal) strip electrodes 34A. Assuming M>N, the multiplexer 10 only needs M+1 sets of control signals to control M sets of X-direction switches SX1-SXM and N sets of Y-direction switches SY1-SYN. In more detail, if the multiplexer 10 wants to select the kth group of X-direction switches to output the kth electrode signal to the connection point P, then the multiplexer 10 can select the X-direction switch by the XY control signal SWXY, and then pass the switch Control signals SW1-SWM to select the kth group of X-direction electrodes to output to the connection point P, and ground the remaining unselected electrodes. Furthermore, if the multiplexer 10 wants to select the kth group of Y-direction switches to output the kth electrode signal to the connection point P, then the multiplexer 10 can select the Y-direction switch by the XY control signal SWXY, and then pass the switch control signal SW1-SWN selects the kth group of electrodes in the Y direction to output to the connection point P, and grounds the remaining unselected electrodes. Therefore, the multiplexer 10 only needs 1+Max(M,N) signal lines (in this example, 1+M, because M>N) and (M+N) sets of switches to control M sets of X-direction switches SX1 -SXM and N groups of Y direction switches SY1-SYN greatly reduce the number of signal lines used.

FIG. 4B is a partial circuit diagram of the multiplexer 10 , which mainly includes circuits corresponding to M groups of X-direction switches SX1-SXM, and circuits corresponding to N groups of Y-direction switches SY1-SYN. As shown in the figure, each switch includes four transistors, and these switches are selectively connected to the connection point P or ground by control signals SWk (k=1..M) and SWXY. More specifically, each switch (SX1-SXM, SY1-SYN) has four transistors, and two of the transistors are connected in series between the connection point P and an electrode (X1-XM), and the other two transistors are connected in parallel. Between a ground and electrodes (X1-XM).

Referring to FIG. 5 , it is a circuit diagram of the reference voltage source 30 used in the capacitance measuring device 100 according to a preferred embodiment of the present invention. The reference voltage source 30 includes a reference voltage Vref, a DC common voltage (DCVCOM), five switching capacitors C11, C12, C13, C21, C22 respectively controlled by the first clock and the second clock, and a storage Capacitor Cx. Referring to FIG. 9, the first clock CLK1 and the second clock CLK2 are clocks that do not overlap with each other in time. When the first clock CLK1 is turned on (the second clock CLK2 is turned off), the first switching capacitor C11, C12, C13 are closed and the second switching capacitors C21, C22 are open, so the reference voltage Vref can be stored in the storage capacitor Cx. When the second clock CLK2 is turned on (the first clock CLK1 is turned off), the second switch capacitors C21 and C22 are closed and the first switch capacitors C11, C12 and C13 are turned on, so the reference voltage Vref and the DC common voltage can be (DCVCOM) is superimposed to generate a modified reference voltage Vref′, and the modified reference voltage Vref′ is sent to the second input terminal of the differential comparator 50 for processing. If the DC common voltage (DCVCOM) has noise, referring to FIG. 3 , the noise will also be coupled to the first input terminal of the differential comparator 50 through the parasitic capacitance Cp. With the reference voltage source 30 of the present invention, since the corrected reference voltage Vref' also has a superimposed component from the DC common voltage (DCVCOM), noise can be eliminated in a differential manner and measurement precision can be increased. When the capacitive touch panel is used as a touch panel of a liquid crystal display, the reference voltage source 30 can eliminate the noise from the DC common voltage. More specifically, in the liquid crystal display, the flicker phenomenon must be reduced by adjusting the DC common voltage (DCVCOM), and the common electrode (common electrode) of the common voltage of the upper and lower plates is connected through the conductive gold glue outside the array. Since the capacitive touch panel is located on the upper plate when used in the touch panel of the liquid crystal display, the capacitance measurement device of the capacitive touch panel must overcome the noise from the lower plate and be coupled to the upper plate through the common electrode; The correction reference voltage Vref' of the DC common voltage (DCVCOM) noise and the differential comparison can reduce the noise interference from the lower board.

Referring to FIG. 6 , it is a circuit diagram of the first discharge current source 20 used in the capacitance measuring device 100 according to a preferred embodiment of the present invention. The first discharge current source 20 includes an input TFT transistor and six thin film transistor (TFT) pairs (A-F) to provide a large discharge current source when the voltage at the connection point P changes from -5V to 7V (or greater). Current source for the discharge current. When the voltage at the connection point P is -5V, the six TFT transistor pairs (A-F) operate in the saturation region. If the voltage at the connection point P changes from -5V to 7V, the TFT transistor pairs (F-B) will enter the linear region one by one; however, VDDD The voltage and the voltage of the connection point P drop on the TFT transistor pair A, so the TFT transistor pair A still operates in a saturation mode and provides a certain current. A current source that can provide a large discharge current when the voltage across the connection point P changes.

Referring to FIG. 7 , it is a circuit diagram of the differential comparator 50 used in the capacitance measuring device 100 according to a preferred embodiment of the present invention. The differential comparator 50 includes seven transistors Q1-Q7, wherein the positive voltage VDDA is connected to the transistors Q1-Q3, the gate of Q5 provides the first input terminal, and the gate of Q4 provides the second input terminal. Transistors Q6-Q7 are connected to reference bias voltage Vb and negative voltage VSSA to provide a bias reference. The junction of transistors Q3 and Q7 provides the output of differential comparator 50, thereby providing a differential comparator for comparing the input voltage Vin with the corrected reference voltage Vref'.

Referring to FIG. 8 , it is a circuit diagram of the first low-pass filter 40 used in the capacitance measuring device 100 according to a preferred embodiment of the present invention. Existing low-pass filter, for example, can be resistance-capacitance circuit (RC circuit), and the load terminal is connected on the electric capacity, and the first low-pass filter 40 of the present invention comprises three RC circuits connected in series, and each series The resistance value of the stage (stage) is 1/3 of the original single RC low-pass filter, and the capacitance value is also 1/3 of the original low-pass filter, so that the low-pass cut-off frequency can be reduced and noise can be effectively suppressed. According to the inventor's analysis, if the RC circuit connected in series exceeds four stages, the filter waveform will be deteriorated. Furthermore, the second low-pass filter 42 can also adopt the same multi-stage implementation method as the first low-pass filter 40 to effectively reduce noise.

Referring to FIG. 9 , it is a waveform diagram illustrating the operation of the capacitance measuring device 100 in a preferred embodiment of the present invention. The waveform shown in the figure is a complete detection process for an electrode. Referring to FIG. 3 and FIG. 9 at the same time, firstly, the controller controls the pre-charging switch SW_C to charge the contact capacitance Cs and the parasitic capacitance Cp with the charging current from the charging current source Ic. Then the controller controls the first charging switch SW_A and the second charging switch SW_B respectively. The charges of the contact capacitance Cs and the parasitic capacitance Cp are respectively discharged by the larger first current source 20 (providing the current IL) and the smaller second current source 22 (providing the current IS). During the discharge phase of the second current source 22 , the controller can control the first clock CLK1 and the second clock CLK2 to generate the modified reference voltage Vref′ of the reference voltage source 30 . The differential comparator 50 compares the corrected reference voltage Vref′ with the input signal Vin from the contact capacitor Cs and the parasitic capacitor Cp to generate a pulse width modulated (PWM) output signal (Vout). The controller can judge the pulse width of the PWM output signal to judge the capacitance change. Therefore, as shown in FIG. 4A and FIG. 4B, when the multiplexer 10 selects the X or Y direction switch through the control signal SWXY, and then selects a specific electrode to output to the connection point P through the control signal SW1-SWM, and the remaining unused The selected electrode is grounded; the controller can judge the capacitance change by the pulse width of the PWM output signal, and then judge the contact position. Furthermore, the controller can compare the modified reference voltage and the input voltage during the discharge period of the second discharge current to generate a plurality of pulse width modulation signals, and determine the The contact capacitance.

Referring to FIG. 10 , it is a flowchart illustrating a capacitance measurement method using a capacitance measurement device of a capacitive touch panel, wherein the capacitance measurement method measures a contact capacitance Cs of an electrode of the capacitive touch panel. In the capacitance measuring method, firstly, after precharging the electrode via a connection point P, the electrode is discharged with a first discharge current, wherein the first discharge current can be provided by the first discharge current source 20 shown in FIG. 6 (S100 ). Then discharge the electrode with a second discharge current, wherein the first discharge current is greater than the second discharge current ( S102 ). The capacitance measurement method then provides a modified reference voltage Vref', which is the superposition result of a reference voltage Vref and a DC common voltage (DCVCOM), and the DC common voltage (DCVCOM) is also electrically connected to the contactor. control panel (S104). Finally, during the discharge period of the second discharge current, compare the revised reference voltage Vref′ with an input voltage Vin on the connection point P to determine the value of the contact capacitance ( S106 ).

Furthermore, in the above step S104, the modified reference voltage Vref′ may be provided by the reference voltage source 30 shown in FIG. 5 . The reference voltage source 30 includes a plurality of first switching capacitors C11, C12, C13, a plurality of second switching capacitors C21, C22 and a storage capacitor Cx, wherein the first switching capacitors C11, C12, C13 and The second switching capacitors C21, C22 are alternately switched by non-overlapping clocks (CLK1, CLK2), so that the reference voltage Vref and the DC common voltage (DCVCOM) are superimposed to generate the modified reference voltage Vref′. Similarly, in conjunction with FIG. 3 and FIG. 8 , the modified reference voltage Vref′ and the input voltage Vin can be subjected to low-pass filtering before comparison, wherein the low-pass filtering is performed by a plurality of resistor-capacitor circuits connected in series. Furthermore, in the above-mentioned step S104, during the discharge period of the second discharge current, the modified reference voltage Vref' and the input voltage can be continuously compared to generate a plurality of pulse width modulation signals, and the pulse width modulation signals are generated by the pulses The average value of the wide modulation signal is used to determine the contact capacitance.

Referring to FIG. 11 , it is a schematic block diagram of an image display system 500 according to another embodiment of the present invention. The image display system 500 includes a display module 300, wherein the display module 300 has a capacitive touch panel 200 (such as the capacitive touch panel 200A shown in FIG. The capacitance measuring device 100 and the multiplexer 10 shown can be disposed on the capacitive touch panel 200 or outside the capacitive touch panel 200 . An input module 400 is coupled to the display module 300 for processing an input signal including an image signal to the display module 300 to display the image. The display module 300 can be a liquid crystal display module or an organic electroluminescence (OLED) display module. The image display system 500 can be, for example, a personal digital assistant (PDA), a display (display), a notebook computer (notebook computer), a digital camera (digital camera), a car display (car display) ), a panel computer, a television, a global positioning system (GPS), an avionics display, a digital photo frame, and a portable DVD player machine (portable DVDplayer) or a mobile phone (mobile phone) or other portable electronic devices.

In the capacitive touch panel and its capacitance measuring device and method of the present invention, the modified reference voltage generated by the reference current source is used to superimpose noise, and then the noise is removed by differential comparison, which can increase the comparison accuracy. Furthermore, the capacitance measuring device of the present invention provides a first discharge current source with a stable voltage, which can improve the comparison resolution.

To sum up, the present invention has industrial applicability, novelty and progress, and the structure of the present invention has never been seen in similar products and public use. It fully meets the requirements of the invention patent application, so the application is filed in accordance with the Patent Law.

Claims (10)

1. A capacitance measuring device for a capacitive touch panel, characterized in that the capacitance measuring device is electrically connected to an electrode of the capacitive touch panel via a connection point to measure the contact capacitance of the electrode, The capacitance measuring device includes: a first discharge current source electrically connected to the connection point to provide a first discharge current; a second discharge current source electrically connected to the connection point to provide a second discharge current, wherein the first discharge current is greater than the second discharge current; a differential comparator having a first input terminal and a second input terminal, the first input terminal being electrically connected to the connection point; A reference voltage source electrically connected to the second input terminal, the reference voltage source receives a reference voltage and a DC common voltage of a liquid crystal display to generate a modified reference voltage related to the DC common voltage, wherein the A DC common voltage is also electrically connected to the touch panel; The differential comparator compares an input voltage on the connection point with the modified reference voltage to generate a pulse width modulated signal representing the contact capacitance. 2. The capacitance measuring device according to claim 1, wherein the reference voltage source comprises a plurality of first switch capacitors electrically connected to the reference voltage and the DC common voltage, a plurality of second switch capacitors A switching capacitor and a storage capacitor, wherein the first switching switching capacitors and the second switching switching capacitors are alternately switched by clock pulses that do not overlap in time, so that the reference voltage and the DC common voltage are superimposed to generate the modified reference voltage. 3. A capacitance measuring method of a capacitive touch panel, characterized in that, the capacitance measuring method measures a contact capacitance of an electrode of the capacitive touch panel, and the capacitance measuring method comprises: a. providing a first discharge current source electrically connected to a connection point to provide a first discharge current, the connection point being electrically connected to the electrode; b providing a second discharge current source electrically connected to said connection point to provide a second discharge current, wherein said first discharge current is greater than said second discharge current; c. providing a differential comparator having a first input terminal and a second input terminal, the first input terminal being electrically connected to the connection point; d. providing a reference voltage source electrically connected to the second input terminal, the reference voltage source superimposing a reference voltage and a DC common voltage of a liquid crystal display to generate a modified reference voltage related to the DC common voltage , wherein the DC common voltage is also electrically connected to the touch panel; e. after precharging the electrode via the connection point, discharging the electrode with the first discharge current; f. Discharging the electrodes with the second discharge current; and g. During the discharge period of the second discharge current, comparing the modified reference voltage and an input voltage on the connection point to determine the contact capacitance. 4. capacitance measuring method as claimed in claim 3, is characterized in that, in step c, further comprises: providing a plurality of first switching capacitors, a plurality of second switching capacitors and a storage capacitor, wherein the first switching capacitors and the second switching capacitors are alternately switched by clock pulses that do not overlap in time, so as to The reference voltage is superimposed with the DC common voltage to generate the modified reference voltage. 5. capacitance measurement method as claimed in claim 3, is characterized in that, further comprises in step d: During the discharge period of the second discharge current, compare the modified reference voltage and the input voltage to generate a plurality of pulse width modulation signals, and judge the contact according to the average value of the pulse width modulation signals capacitance. 6. A capacitive touch panel, characterized in that, the capacitive touch panel comprises: having a first number M of first electrodes in the longitudinal direction; having a second number N of second electrodes in a lateral direction; A capacitance measuring device is electrically connected to an electrode to be measured among the first electrodes and the second electrodes via a connection point, so as to measure the contact capacitance of the electrode to be measured, and the capacitance measuring device includes: a first discharge current source electrically connected to the connection point to provide a first discharge current; a second discharge current source electrically connected to the connection point to provide a second discharge current, wherein the first discharge current is greater than the second discharge current; a differential comparator having a first input terminal and a second input terminal, the first input terminal being electrically connected to the connection point; A reference voltage source electrically connected to the second input terminal, the reference voltage source receives a reference voltage and a DC common voltage of a liquid crystal display to generate a modified reference voltage related to the DC common voltage, wherein the A DC common voltage is also electrically connected to the touch panel; Wherein the differential comparator compares an input voltage on the connection point with the modified reference voltage to generate a pulse width modulated signal representing the contact capacitance. 7. The capacitive touch panel according to claim 6, wherein the capacitive touch panel further comprises a multiplexer having multiplexers corresponding to the first electrodes and the second electrodes switches, and generating a third number of control signals, wherein the third number is the greater of M and N plus one; Wherein the multiplexer controls the switches through the control signals to select one of the first electrodes and the second electrodes to be electrically connected to the capacitance measuring device to measure the selected electrodes the contact capacitance. 8. The capacitive touch panel according to claim 7, wherein the control signals include an XY control signal and switch control signals whose number is the larger of M and N, wherein the XY control signal The signal is electrically connected to each switch to select the first electrodes in the vertical direction or the second electrodes in the horizontal direction; the switch control signals are electrically connected to the corresponding switches to select the first electrodes. electrode and one of the second electrodes. 9. An image display system, characterized in that the image display system comprises: A display module having a capacitive touch panel as claimed in claim 6; and An input module is coupled to the display module to send an input signal to the display module so that the display module displays images. 10. The image display system according to claim 9, wherein the image display system is a personal digital assistant, a display, a notebook computer, a digital camera, a vehicle display, a tablet computer, A television, a global positioning system, an aviation monitor, a digital photo frame, a portable DVD player or a mobile phone.

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