# Patent

## Title
Sensing method and circuit of fingerprint sensor

## Abstract

A sensing method and circuit of fingerprint sensor is disclosed. In a first phase, the sensing method supplies a first to third voltages to an electrode plate to be measured, a read-out circuit of the electrode plate to be measured and a conductor adjacent to the electrode plate to be measured, respectively. In a second phase, the sensing method stops supplying the first to third voltages and supplies voltage to the conductor and connects the electrode plate to be measured to the read-out circuit so the read-out circuit reads out a measurement result of the electrode plate to be measured. According to the sensing method and circuit, the measurement result of the electrode plate to be measured is not affect by capacitors between the electrode plate to be measured and the conductor.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of United States provisional application filed on Dec. 15, 2014 and having application Ser. No. 62/092,225 and the benefit of United States provisional application filed on Feb. 16, 2015 and having application Ser. No. 62/116,885, the entire contents of which are hereby incorporated herein by reference.
This application is based upon and claims priority under 35 U.S.C. 119 from Taiwan Patent Application No. 104136842 filed on Nov. 9, 2015, which is hereby specifically incorporated herein by this reference thereto.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fingerprint sensor, especially to a sensing method and circuit of a fingerprint sensor.
2. Description of the Prior Arts
With reference to FIGS. 12 and 13, a schematic diagram of a projected capacitive fingerprint sensor 50 is shown. The projected capacitive fingerprint sensor 50 has multiple electrode plates 51 arranged in a matrix, a protection electrode 52 and a sensing circuit 53 connected to the electrode plates 51. The sensing circuit 53 is used to detect a capacitance value of a capacitor formed between the finger 60 and each electrode plate 51, such as a capacitor CSA between the finger 60 and the electrode plate (Plate A). According to the capacitance value of the capacitor formed between the finger 60 and each electrode plate 51, a fingerprint image of the finger can be obtained. However, a measurement result of the capacitance value of the capacitor formed between the finger 60 and the electrode plate 51 is affected by other factors. For example, when detecting the electrode plate (Plate A), multiple capacitors respectively formed between the electrode plate (Plate A) and other electrode plates (Plate B ̃Plate D) around the electrode plate (Plate A), and a capacitor CFAS formed between the electrode plate (Plate A) and the protection electrode 52 (such as an electrostatic discharge protection electrode) also affect the measurement result.
SUMMARY OF THE INVENTION
Based on the aforementioned drawbacks of the prior art, an objective of the present invention provides a sensing method and circuit of a fingerprint sensor.
To achieve the aforementioned objective, the present invention provides the sensing method of the fingerprint sensor having steps of:
(a) in a first phase, supplying a first voltage to a first node connected to an electrode plate to be measured, supplying a second voltage to a second node connected to a read-out circuit, and supplying a third voltage to a conductor adjacent to an electrode plate to be measured, wherein the first voltage minus the second voltage leaves a non-zero value and the first voltage is larger or smaller than the second voltage; and
(b) in a second phase, stopping supplying the first, second and third voltages to the first, second and third nodes, supplying a fourth voltage to the conductor, connecting the first node to the second node, wherein a voltage of the second node corresponds to a measurement result of the electrode plate to be measured, wherein when the first voltage is larger the second voltage and the third voltage is larger than the fourth voltage, or when the first voltage is smaller the second voltage and the third voltage is smaller than the fourth voltage.
To achieve the aforementioned another objective, the present invention provides the sensing circuit of the fingerprint sensor having:
a read-out circuit to read out a measurement result of an electrode plate to be measured;
a first switching unit coupling to a first voltage and a second voltage and having a first node connected to the electrode plate to be measured and a second node connected to the read-out circuit, wherein the first voltage minus the second voltage leaves a non-zero value and the first voltage is larger or smaller than the second voltage;
a second switching unit to connect a conductor to a third voltage or a fourth voltage, wherein the third voltage minus the fourth voltage leaves a non-zero value and when the first voltage is larger the second voltage and the third voltage is larger than the fourth voltage, or when the first voltage is smaller the second voltage and the third voltage is smaller than the fourth voltage; and
a control unit controlling the first and second switching units, wherein
in a first phase, the control unit controls the first and second units to connect the first node to the first voltage, to connect the second node to the second voltage, to disconnect the first node to the second node and to connect the conductor to the third voltage;
in a second phase, the control unit controls the first and second switching units to respectively disconnected the first, second and third nodes to the first, second and third voltages, to connect the first node to the second node and to control the second switching unit to couple the conductor to the fourth voltage.
Based on the foregoing description, the sensing method and circuit of the fingerprint sensor respectively connects the adjacent conductor to the third and fourth voltages with different electric potentials in the first and second phases to improve the drawback of the prior art. The drawback is that an fringe capacitor formed between the electrode plate to be measured and the adjacent conductor affects a measurement result of the electrode plate to be measured.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic view of a partial structure of a fingerprint sensor and a sensing circuit thereof in accordance with the present invention;
FIG. 1B is another schematic view of a partial structure of a fingerprint sensor and a sensing circuit thereof in accordance with the present invention;
FIG. 2 is another schematic view of a partial structure of a fingerprint sensor and a sensing circuit thereof in accordance with the present invention;
FIG. 3 is a circuit diagram of a first embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIGS. 4A and 4B are two different circuit diagrams of the sensing circuit of FIG. 3 respective in first and second phases;
FIG. 4C is a diagram combined a time sequence diagram of each switch and a voltage-variation waveform diagram of each node in FIGS. 4A and 4B;
FIG. 5 is a circuit diagram of a second embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIG. 6 is a circuit diagram of a third embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIG. 7A is a circuit diagram of a fourth embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIG. 7B is a circuit diagram of a fifth embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIG. 8 is a circuit diagram of a sixth embodiment of a sensing circuit of the fingerprint sensor in accordance with the present invention;
FIGS. 9A to 9C are three different circuit diagrams of the sensing circuit of FIG. 8 respective in first to third phases;
FIG. 9D is a diagram combined a time sequence diagram of each switch and a voltage-variation waveform diagram of each node in FIGS. 9A, 9B and 9C;
FIG. 10A is a diagram of dynamic voltage range and switching number in accordance with the present invention;
FIG. 10B is another diagram of dynamic voltage range and switching number in accordance with the present invention;
FIGS. 11A to 11C are three different circuit diagrams of the sensing circuit of FIG. 2 respective in first to third phases;
FIG. 11D is a diagram combined a time sequence diagram of each switch and a voltage-variation waveform diagram of each node in FIGS. 11A, 11B and 11C;
FIG. 12 is a schematic view of a projected capacitive fingerprint sensor; and
FIG. 13 is a schematic view of a finger and a projected capacitive fingerprint sensor.



DETAILED DESCRIPTION OF THE EMBODIMENTS
The present invention provides a sensing method and circuit of a fingerprint sensor. The sensing circuit of the present invention may be applied to a fingerprint sensor as shown in FIG. 12. Multiple electrode plates 51 on the fingerprint sensor are arranged in a matrix. A grid space is defined among the electrode plates 51. In FIG. 1A, a protection electrode 52 is formed inside the grid space and is coplanar with the electrode plates 51. In another embodiment, as shown in FIG. 1B, the protection electrode 52 is located above the electrode plates 51 and a dielectric layer 502 is formed between the protection electrode 52 and the electrode plates 51. In another embodiment, the protection electrodes 52 are respectively formed on a plane coplanar with the electrode plates 51, and a plane above the electrode plates 51. Two layers of the protection electrodes 52 are electrically connected to each other.
In following descriptions and drawings, four electrode plates (Plate A ̃Plate D) located on a left-upper area of a projected capacitive fingerprint sensor of FIG. 12 are used to describe the sensing circuit of the present invention and operations thereof, and the measurement of electrode plate (Plate A) is used as an example to describe the process. In FIG. 3, four symbols CSA ̃CSD respectively represent four capacitors, which are respectively formed between a finger and the electrode plates (Plate A ̃Plate D). Three symbols CFAB, CFAC, CFAD respectively represent three capacitors, which are respectively formed between the electrode plate (Plate A) and each of adjacent electrode plates (Plate B ̃Plate D). A symbol CFAS represents a capacitor, which is formed between the electrode plate (Plate A) and the protection electrode 52 (such as an electrostatic discharge protection electrode; ESD protection electrode). A symbol Cp1a represents other parasitic capacitors related to a node N1a and a symbol Cp2a represents other parasitic capacitors related to a node N2a. Other similar symbols have similar representations and the details are not described below for the sake of brevity.
FIG. 1A and FIG. 3 illustrate a first embodiment of the sensing circuit 10 of the present invention. The sensing circuit 10 has multiple detecting units 11 and a control unit 12. The multiple detecting units 11 are respectively connected to the electrode plates (Plate A ̃Plate D).
The following description uses the electrode plate (Plate A) as an electrode plate to be measured as an example. The detecting unit 11 connected to the electrode plate to be measured (Plate A) has a first switching unit 111 and a read-out circuit 13. Each of other detecting units 11 connected to the electrode plates (Plate B ̃Plate D) has a second switching unit 112a and a read-out circuit 13. The protection electrode 52 provides an electrostatic protection for the electrode plate and connects to a second switching unit 112b. 
With reference to FIG. 3, the first switching unit 111 has a first node N1a connected to the electrode plate to be measured (Plate A) and a second node N2a connected to the read-out circuit 13. The first switching unit 111 couples a first voltage VR1 to the first node N1a and couples the second voltage VR2 to the second node N2a. The first voltage VR1 minus the second voltage VR2 leaves a non-zero value. In the first switching unit 111, a first switch SW1A is connected between the first voltage VR1 and the first node N1a, a second switch SW3A is connected between the second voltage VR2 and the second node N2a and a third switch SW2A is connected between the first node N1a and the second node N2a.
In FIG. 3, three second switching units 112a are respectively connected to the electrode plates (Plate B ̃Plate D) through three third nodes N1b ̃N1d and respectively connected to three read-out circuit 13 through three fourth node N2b ̃N2d. Each of the second switching units 112a is coupled to the first voltage VR1 and the second voltage VR2. The read-out circuit 13 is used to read out a measurement of the electrode plate to be measured (Plate A). The read-out circuit 13 may be a differential circuit or a buffer. In the present embodiment, the read-out circuit 13 is a differential circuit having a non-inverting input (+), an inverting input (−) and a voltage output VOA. A gain of the differential circuit is GA.
The first switching unit 111 has the first switch SW1A, the second switch SW3A, the third switch SW2A and the fourth switch SW4A. The first switch SW1A is used to couple the first voltage VR1 to the first node N1a. The second switch SW3A is used to couple the second voltage VR2 to the second node N2a and the second node N2a is connected to the non-inverting input (+) of the differential circuit. The third switch SW2A is located between the first node N1a and the second node N2a and used to determine whether the first node N1a connects to the second node N2a or not. The fourth switch SW4A is coupled between the second voltage VR2 and the inverting input (−) of the differential circuit.
The second switching unit 112a connected to the electrode plate (Plate B) has a first switch SW1B, the second switch SW3B, the third switch SW2B and the fourth switch SW4B. The first switch SW1B is used to couple the first voltage VR1 to the third node N1b. The second switch SW3B is used to couple the second voltage VR2 to the fourth node N2b and the fourth node N2b is connected to the non-inverting input (+) of the differential circuit. The third switch SW2B is located between the third node N1b and the fourth node N2b and used to determine whether the third node N1b connects to the fourth node N2b or not. The fourth switch SW4B is coupled between the second voltage VR2 and the inverting input (−) of the differential circuit.
The three second switching units 112a has the same switches and the same connections arrangement, wherein multiple similar symbols in FIG. 3 have similar representations and thus the details of the other two second switching units 112a connected to the electrode plates (Plate C and Plate D) are not described here for the sake of brevity. A grounded capacitor Cg is connected to the non-inverting input (+) of the differential circuit.
An electrostatic discharge (hereafter ESD) protection circuit 14 is coupled between the protection electrode 52 and the second switching units 112b to provide a discharging path for static electricity. The ESD protection circuit 14 has a first diode D1, a second diode D2 and a resistor element R. An anode of the first diode D1 is connected to the protection electrode 52 and a cathode thereof is connected to a high voltage VDD. A cathode of the second diode D2 is connected to the anode of the first diode D1 and the protection electrode 52. An anode of the second diode D2 is connected to ground. One terminal of the resistor element R is connected to a connection node between the first and second diodes D1, D2 and the other terminal of the resistor element R is connected the second switching unit 112b. The second switching unit 112b has a first switch SWSE and the second switch SWSP. The first switch SWSE is coupled between the first voltage VR1 and the resistor element R and the second switch SWSP is coupled between the voltage VR2 and the resistor element R. Using the resistor element R with a larger resistance can avoid static electricity flowing to the second switching unit 112b. When static electricity with a voltage larger than an amount of the high voltage VDD and a turn-on voltage of the first diode D1 occurred on the protection electrode 52, the first diode D1 is turned on and the static electricity is discharged quickly. Similarly, When the static electricity with a negative voltage less than a difference calculated by subtracting a turn-on voltage of the second diode D2 from the ground voltage is occurred on the protection electrode 52, the second diode D2 is turned on and the static electricity with the negative voltage is discharged quickly.
In FIG. 3, the first to fourth switches SW1A ̃SW1D, SW3A ̃SW3D, SW2A ̃SW2D, SW4A ̃SW4D and the first and second switches SWSE, SWSP of the second switching unit 112b are controlled to turn on or turn off by the control unit 12.
A following description describes the operations of detecting electrode plate (Plate A).
In a first phase (excitation phase or pre-charge phase), a status of each switch is shown in FIG. 4A. The first, second and fourth switches SW1A, SW3A, SW4A are turned on, such that the first node N1a connects to the first voltage VR1 and the second node N2a connects to the second voltage VR2. The third switch SW2A is turned off to disconnect the first node N1a to the second node N2a. In each second switching unit 112a, the first, second and fourth switches SW1B ̃SW1D, SW3B ̃SW3D, SW4B ̃SW4D are turned on, such that the third node N1b ̃N1d connects to the first voltage VR1 and the fourth node N2b ̃N2d connects to the second voltage VR2. Thus, the first voltage VR1 is supplied to the electrode plate (Plate B ̃Plate D). The third switch SW2B ̃SW2D of each second switching unit 112a are turned off to disconnect the third node N1b ̃N1d to the fourth node N2b ̃N2d. In the second switching unit 112b, the first switch SWSE is turned on and the second switch SWSP is turned off to connect the protection electrode 52 to the first voltage VR1 through the resistor element R. Briefly, in the first phase, the electrode plate to be measured (Plate A), the other electrode plate (Plate B ̃Plate D) and the protection electrode 52 are connected to the first voltage VR1.
In the second phase (reading phase or evaluation phase), the status of each switch of FIG. 3 is shown in FIG. 4B. In the first switching unit 111, the first, second and fourth switches SW1A, SW3A, SW4A are turned off, such that the first node N1a disconnects to the first voltage VR1 and t the second node N2a disconnects to the second voltage VR2. The third switch SW2A is turned on to connect the first node N1a to the second node N2a. In each second switching unit 112a, the first switches SW1B ̃SW1D are turned off to disconnect the third nodes N1b ̃N1d to the first voltage VR1. The second and fourth switches SW3B ̃SW3D, SW4B ̃SW4D are still turned on so the fourth nodes N2b ̃N2d are connected to the second voltage VR2. The third switches SW2B ̃SW2D are turned on to connect the third nodes N1b ̃N1d to the fourth nodes N2b ̃N2d. Thus, an electric potential of each electrode plate (Plate B ̃Plate D) is switched to the second voltage VR2. In the second switching unit 112b, the first switch SWSE is turned off and the second switch SWSP is turned on to connect the protection electrode 52 to the second voltage VR2. In another embodiment, the fourth switches SW4B ̃SW4D may be turned off.
An output voltage of the differential circuit connected to the first switching unit 111 is VOA=GA×(VIPA−VINA)+VOCM and is used to represent the measurement result of the electrode plate to be measured (Plate A). After the first and second phases, the output voltage VOA of the read-out circuit 13 connected to the electrode plate to be measured (Plate A) can be represented as following equation: VOA=GA×{[CSA×(VR1−VR2)]/CT+[CFPPA×(VR1−VR2−VR1+VR2)]/CT+[CFAS×(VR1−VR2−VR1+VR2)]/CT+[Cp1a×(VR1−VR2)]/CT}+VOCM; wherein, GA is the gain of the differential circuit, CFPPA is a sum of CFAB, CFAC and CFAD. The foregoing equation is further calculated to VOA=GA×{[CSA×(VR1−VR2)]/CT+[Cp1a×(VR1−VR2)]/CT}+VOCM, wherein CT=CSA+CFPPA+CFAS+Cp1a+Cp2a, and VOCM is a common mode voltage of the differential circuit. Based on the final equation, a numerator does not include fringe capacitors CFPPA and CFAS. Accordingly, the present embodiment indeed improves the influence to the output of the read-out circuit 13, wherein the influence is caused by the capacitors formed between the electrode plate to be measured (Plate A) and the adjacent electrode plates (Plate B ̃Plate D) and the capacitor formed between the electrode plate to be measured (Plate A) and the protection electrode 52.
In the first phase shown in FIG. 4A and the second phase shown in FIG. 4B, a time sequence of each switch and a voltage variation of each node in one embodiment are shown in FIG. 4C. In a time sequence diagram of each switch, a high voltage level represents that the switch is turned on and a low voltage level represents that the switch is turned off. In this embodiment, the second voltage VR2 is larger than the first Voltage VR1. From the first phase to the second phase, the first switches SW1A ̃SW1D, SWSE, the second switch SW3A and the fourth switch SW4A are turned off before the third switches SW2A ̃SW2D and the second switch SWSP turn on. A voltage level VEVA_A represents an electric potential of a node IPA when detecting the electrode plate to be measured (Plate A) in the second phase. A second embodiment of FIG. 5 is similar to a first embodiment, and a difference therebetween is the inverting input (−) is directly connected to the second voltage VR2.
FIG. 6 shows a third embodiment and a difference between the first and third embodiments is that a read-out circuit 13a employs a buffer. In FIG. 6, the second node N2a of the first switching unit 111 is connected to a single input IA of the buffer. The fourth node N2b ̃N2d of the second switching units 112a are respectively connected to three single inputs IB ̃ID of other three buffers. An output voltage of the buffer connected to the first switching unit 111 is VOA=GA×VIA+Vshift. After the foregoing operations of the first and second phases, the output voltage VOA of the buffer connected to the electrode plate to be measured (Plate A) can be represented as an equation: VOA=GA×{[CSA×(VR1−VR2)]/CT+[CFPPA×(VR1−VR2−VR1+VR2)]/CT+[CFAS×(VR1−VR2−VR1+VR2)]/CT+[Cp1a×(VR1−VR2)]/CT+VR2}+Vshift. The equation is further calculated to VOA=GA×{[CSA×(VR1−VR2)]/CT+[Cp1a×(VR1−VR2)]/CT+VR2}+Vshift, wherein CT=CSA+CFPPA+CFAS+Cp1a+Cp2a and Vshift is a voltage offset of the buffer.
Based on the foregoing final equation, the numerator does not include the fringe capacitors CFPPA and CFAS. Accordingly, the present invention indeed improves the influence, which is caused by the fringe capacitors CFPPA and CFAS, to the output of the read-out circuit by the controls of the first and second phases.
In a fourth embodiment of FIG. 7A, the electrode plates (Plates A ̃Plate D) are connected to a differential circuit through a multiplexer 131. When detecting the electrode plate (Plate A), the control unit 12 controls the multiplex 131 to connect the non-inverting input (+) of the differential circuit to the second node N2a. In FIG. 7A, other elements and operations thereof are the same with those in FIG. 3 so that the details are not described here.
In a fifth embodiment of FIG. 7B, the electrode plates (Plates A ̃Plate D) are connected to a buffer through a multiplexer 131. When detecting the electrode plate (Plate A), the control unit 12 controls the multiplex 131 to connect the input of the buffer to the second node N2a. In FIG. 7B, other elements and operations thereof are the same with those in FIG. 6 so that the details are not described here.
In the fourth and fifth embodiments of FIGS. 7A and 7B, the electrode plates (Plate A ̃Plate D) commonly use one read-out circuit so that the number of the read-out circuits can be reduced.
FIG. 8 illustrates a sixth embodiment. In FIG. 8, the first switching unit 111 connected to the electrode plate (Plate A) and each of the second switching units 112a connected to the electrode plates (Plate B ̃Plate D) further has a fifth switch SW5A, SW5B, SW5C, or SW5D connected to a third voltage VR3 and a sixth switch SW6A, SW6B, SW6C, or SW6D connected to a fourth voltage VR4. The fifth and sixth switches SW5A ̃SW5D, SW6A ̃SW6D are used to respectively couple the third voltage VR3 and the fourth voltage VR4 to the electrode plates (Plate A ̃Plate D) to switch the electric potentials of the electrode plates (Plate A ̃Plate D). In the second switching unit 112b, the first and second switches SWSE, SWSP are respectively coupled to the third and fourth voltages VR3, VR4. A difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to a difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR3−VR4=VR1−VR2). The control unit 12 controls all of the switches to turn on or turn off. Other elements and connections thereof are the same with those in FIG. 3 so that the details are not described here. An arrangement of FIG. 8 can also improves the influence, which is caused by fringe capacitors CFPPA, CFAS, to the output voltage of the read-out circuit 13. The following description also uses detecting the electrode plate (Plate A) as an example to describe the operations of the circuit shown in FIG. 8.
In the first phase (excitation phase or pre-charge phase), a status of each switch of FIG. 8 is shown in FIG. 9A. The first, second and fourth switches SW1A, SW3A, SW4A of the first switching unit 111 are turned on, such that the first node N1a connects to the first voltage VR1, the second node N2a connects to the first voltage VR2, and the inverting input INA of the differential circuit connects to the second voltage VR2. The third switch SW2A is turned off to disconnect the first node N1a to the second node N2a. The fifth switch SW5A and the sixth switch SW6A are turned off. In each second switching unit 112a, the fifth switches SW5B ̃SW5D are turned on and the first and sixth switches SW1B ̃SW1D, SW6B ̃SW6D are turned off, such that the electrode plates (Plate B ̃Plate D) connect to the third voltage VR3. The third switches SW2B ̃SW2D are turned off to disconnect the third nodes N1b ̃N1d to the fourth nodes N2b ̃N2d. The second switches SW3B ̃SW3D and the fourth switches SW4B ̃SW4D are turned on. The first switch SWSE of the second switching unit 112b is turned on and the second switch SWSP is turned off such that the protection electrode 52 connects to the third voltage VR3 through the resistor element R. Briefly, in the first phase, the electrode plate to be measured (Plate A) is connected to the first voltage VR1 and the other electrode plates (Plate B ̃Plate D) and the protection electrode 52 are connected to the third voltage VR3. In another embodiment, the second switches SW3B ̃SW3D and the fourth switches SW4B ̃SW4D may be turned off.
In the second phase (reading phase or evaluation phase), a status of each switch of FIG. 8 is shown in FIG. 9B. The first, second and fourth switches SW1A, SW3A, SW4A of the first switching unit 111 are turned off to disconnect the first node N1a to the first voltage VR1 and to disconnect the second node N2a to the first voltage VR2. The third switch SW2A is turned on to connect the first node N1a to the second node N2a. The fifth switch SW5A and the sixth switch SW6A are still turned off. In each second switching unit 112a, the fifth switches SW5B ̃SW5D are turned off to disconnect the electrode plates (Plate B ̃Plate D) to the third voltage VR3. The sixth switches SW6B ̃SW6D are turned on to connect the electrode plates (Plate B ̃Plate D) to the fourth voltage VR4. The first and third switches SW1B ̃SW1D, SW2B ̃SW2D are turned off. The second switches SW3B ̃SW3D and the fourth switches SW4B ̃SW4D may be turned on or turned off. The first switch SWSE of the second switching unit 112b is turned off and the second switch SWSP is turned on to connect the protection electrode 52 to the fourth voltage VR4 through the resistor element R.
The output voltage of the differential circuit connected to the first switching unit 111 is VOA=GA×(VIPA−VINA)+VOCM, which represents a measurement result of the electrode plate to be measured (Plate A). After the operations of the first and second phases, the output voltage VOA of the read-out circuit 13 connected to the electrode plate to be measured (Plate A) can be represented as an equation: VOA=GA×{[CSA×(VR1−VR2)]/CT+[CFPPA×(VR1−VR2−VR3+V4)]/CT+[CFAS×(VR1−VR2−VR3+VR4)]/CT+[Cp1a×(VR1−VR2)]/CT}+VOCM. When the difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR3−VR4=VR1−VR2), the foregoing equation is calculated to VOA=GA×{[CSA×(VR1−VR2)]/CT+[Cp1a×(VR1−VR2)]/CT}+VOCM, wherein CT=CSA+CFPPA+CFAS+Cp1a+Cp2a, and VOCM is a common mode voltage of the differential circuit. Based on the final equation, an effect of the fringe capacitors CFPPA and CFAS is reduced. In another embodiment, the output voltage VOA is not read out in the second phase shown in FIG. 9B and then proceeding to a third phase as shown in FIG. 9C. In FIG. 9C, the first switch SW1A of the first switching unit 111 is turned on to connect the first node N1a to the first voltage VR1, again. The second switch SW3A and fourth switch SW4A are turned off. In each second switching unit 112a, the fifth switch SW5B ̃SW5D are turned on and the sixth switches SW6B ̃SW6D are turned off, such that the electrode plates (Plate B ̃Plate D) connect to the third voltage VR3, again. In the second switching unit 112b, the first switch SWSE is turned on and the second switch SWSP is turned off, such that the protect electrode 52 connects to the third voltage VR3 through the resistor element R. The statuses of the other switches are the same with those in the first phase of FIG. 9A. Briefly, in the third phase, the electrode plate to be measured (Plate A) is connected to the first voltage and the electrode plates (Plate B ̃Plate D) and the protection electrode 52 are connected to the third voltage VR3.
Next, another operation, which is the same as that of the second phase, is proceeded. At this time, the electrode plate (Plate B ̃Plate D) is connected to the fourth voltage VR4. After such the third phase of FIG. 9C and the second phase of FIG. 9B are repeated for several times, the output voltage VOA is read out in a final second phase. At this time, a voltage VP2 of the second node N2a is




  
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When the difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR3−VR4=VR1−VR2), the foregoing equation is calculated to





  
    
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wherein CT=CSA+CFPPA+CFAS+Cp1a+Cp2a, x=Cp2a/CT, x=Cp2a/CT and n is the number of switching times between the second and the third phase.

After such the third phase of FIG. 9C and the second phase of FIG. 9B are switched repeatedly for several times, multiple measurement results of the electrode plate (Plate A) are accumulated at the second node N2a. When the second voltage VR2 is larger than the first voltage VR1, the relationship between the switching times and the voltage of the second node N2a is shown in FIG. 10. A symbol Vvalley represents a voltage caused by a valley part of a fingerprint and a symbol Vridge represents a voltage caused by a ridge part of the fingerprint. As the number of switching times between the second and the third phase is increasing, a difference between Vvalley and Vridge are increased. It is helpful to determine that the present measurement result represents the valley part or the ridge part of the fingerprint, thus an accuracy of detecting fingerprint is increased. In the other embodiments (for example, using different read-out circuits 13 or different arrangement of the switching units), multiple measurement results may be also accumulated by switching between the second and third phases for several times. In the other embodiments, in the second switching unit 112b of FIGS. 8, 9A to 9C, the first switch SWSE may be connected to the first voltage VR1, the second switch SWSP may be connected to the second voltage VR2 and a difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to a difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR3−VR4=VR1−VR2). That means that using another voltage combination with the same difference to replace the third and fourth voltages VR3, VR4 connected to the second switching unit 112b is possible.
In the first phase shown in FIG. 9A, the second phase shown in FIG. 9B and the third phase shown in FIG. 9C, a time sequence of each switch and a voltage variation of each node in an embodiment are shown in FIG. 9D. In a time sequence diagram of each switch, a high voltage level represents that the switch is turned on and a low voltage level represents that the switch is turned off. In this embodiment, the second voltage VR2 is larger than the first Voltage VR1 and the fourth voltage VR4 is larger than the third Voltage VR3. From the first phase to the second phase, the first switches SW1A, SWSE, the fifth switch SW5B ̃SW5D, the second SW3A and the fourth switch SW4A are turned off before the third switch SW2A, the sixth switch SW6B ̃SW6D and the second switch SWSP are turned on. From the second phase to the third phase, the third switch SW2A, the sixth switch SW6B ̃SW6D and the second switch SWSP are turned off before the first switch SW1A, SWSE and the fifth switch SW5B ̃SW5D are turned on. From the third phase to the second phase, the first switch SW1A, SWSE and the fifth switches SW5B ̃SW5D are turned off before the third switch SW2A, the sixth switches SW6B ̃SW6D and the second switch SWSP are turned on. Three voltage levels VEVA_A_1, VEVA_A_2 and VEVA_A_3 respectively represent three electric potentials of a node IPA in the first, second and third phases. As shown in FIG. 9D, after switching between the second and the third phase for several times, the electric potential of the node IPA is decreased.
In the embodiment of FIGS. 8 and 9A to 9C, the fifth switches SW5A ̃SW5D are added to connect to the third voltage VR3 and the sixth switches SW6A ̃SW6D are added to connect to the fourth voltage VR4. In another embodiment, only the sixth switches SW6A ̃SW6D is connected to the second voltage VR2, and the first and sixth switches SW1A ̃SW1D, SW6A ̃SW6D are controlled to proceed the operations as the operations of the fifth and sixth switches SW5A ̃SW5D, SW6A ̃SW6D in FIGS. 9A to 9C when detecting the electrode plate (Plate A).
In another embodiment of FIG. 2, an isolation electrode plate 521 is formed under each electrode plate 51 to isolate most of the parasitic capacitors between each electrode plates 51 and multiple electric elements below the electrode plate 51. A dielectric layer 503 is formed between the electrode plates 51 and the isolation electrode plates 521 thereof. Using the electrode plate (Plate A) as an example, the capacitance of the parasitic capacitor between the node A and the other conductor thereof is decreased to a small capacitor Cp1a′, and another capacitor Cqa represents a capacitor between the isolation electrode plate 521 and the electrode plate (Plate A).
An embodiment of FIG. 11A is applied to the structure of FIG. 2 and further has a third switching unit 113 connected to the isolation electrode plate 521. The third switching unit 113 is used to connect isolation electrode plate 521 to a fifth voltage VR5 or a sixth voltage VR6. Each third switching unit 113 has a seventh and eighth switches. The seventh switches SW7A ̃SW7D are coupled between the fifth voltage VR5 and the isolation electrode plates 521, and the eighth switches SW8A ̃SW8D are coupled between the sixth voltage VR6 and the isolation electrode plates 521. The fifth and sixth voltages VR5, VR6 are different and the fifth voltage VR5 may be the same as or may be different from the first voltage VR1, and the sixth voltage VR6 may be the same as or may be different from the second voltage VR2. In an embodiment, a difference calculated by subtracting the sixth voltage VR6 from the fifth voltage VR5 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR5−VR6=VR1−VR2).
In the first phase, the status of each switch of the first and second switching units 111 and 112a are the same with those in FIG. 9A. The seventh switches SW7A ̃SW7D are turned on and the eighth switches SW8A ̃SW8D are turned off, so as to connect each isolation electrode plate 521 to the fifth voltage VR5. An operation of the second phase is shown in FIG. 11B. A control of first and second switching units 111 and 112a is the same as that of FIG. 9B. The seventh switches SW7A ̃SW7D are turned off and the eighth switches SW8A ̃SW8D are turned on to connect each isolation electrode plate 521 to the sixth voltage VR6. In another embodiment, at least the seventh switches SW7A and the eighth switch SW8A connected to the isolation electrode plate 521 under the electrode plate (Plate A) are controlled to perform the foregoing operations. In the second phase, the measurement result of the electrode plate (Plate A) is read out, or an operation of FIG. 11C may be further proceeded to.
In FIG. 11C, a control of the first and second switching units 111 and 112a is the same with those in FIG. 9C. The seventh switch SW7A ̃SW7D are turned on and the eighth switches SW8A ̃SW8D are turned off, so as to connect each isolation electrode plate 521 to the fifth voltage VR5. After such the third phase of FIG. 11C and the second phase of FIG. 11B are repeated for several times, an output voltage VOA is read out in a final second phase of FIG. 11B. At this time, a voltage VP2 of the second node N2a is: Vp2={[CSA×(VR1−VR2)]/CT+[CFPPA×(VR1−VR2−VR3+V4)]/CT+[CFAS×(VR1−VR2−VR3+VR4)]/CT+[Cp1a′×(VR1−VR2)]/CT+




  
    
      
        
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When the difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR3−VR4=VR1−VR2), and the difference calculated by subtracting the sixth voltage VR6 from the fifth voltage VR5 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 (VR5−VR6=VR1−VR2), the foregoing equation is calculated to





  
    
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wherein CT=CSA+CFPPA+CFAS+Cp1a′+Cp2a+Cqa, x=Cp2a/CT and n represents the number of switching times between the second phase and the third phase.

Based on the foregoing description, the parasitic capacitors Cp1a ̃Cp1d is decreased to the small parasitic capacitors Cp1a′ ̃Cp1d′ and a common mode voltage level caused by the small parasitic capacitors Cp1a′ ̃CP1D′ is relatively decreased. The effects caused by the capacitors formed between the isolation electrode plates 521 and the electrode plates 51 are decreased to a minimum condition by switching the isolation electrode plates 521 to fifth or sixth voltage VR5, VR6.
In the first phase shown in FIG. 11A, the second phase shown in FIG. 11 and the third phase shown in FIG. 11C, a time sequence of each switch and a voltage variation of each node in an embodiment are shown in FIG. 11D. In a time sequence diagram of each switch, a high voltage level represents that the switch is turned on and a low voltage level represents that the switch is turned off. In this embodiment, the second voltage VR2 is larger than the first voltage VR1, and the fourth voltage VR4 is larger than the third Voltage VR3 and the sixth voltage VR6 is larger than the fifth Voltage VR5. From the first phase to the second phase, the first switches SW1A, SWSE, the fifth switches SW5B ̃SW5D, the seventh switches SW7A ̃SW7D and the second switch SW3A are turned on before the third switch SW2A, the sixth switch SW6B ̃SW6D and the second switch SWSP and the eighth switches SW8A ̃SW8D are turned off. From the second phase to the third phase, the third switch SW2A, the sixth switches SW6B ̃SW6D and the second switch SWSP and the eighth switches SW8A ̃SW8D are turned off before the first switch SW1A, SWSE, the fifth switch SW5B ̃SW5D and the seventh switches SW7A ̃SW7D are turned on. From the third phase to the second phase, the first switch SW1A, SWSE, the fifth switches SW5B ̃SW5D and the seventh switches SW7A ̃SW7D are turned off before the third switch SW2A, the sixth switches SW6B ̃SW6D, the second switch SWSP and eighth switches SW8A ̃SW8D are turned on. The three electric potentials VEVA_A_1, VEVA_A_2 and VEVA_A_3 respectively represent three electric potentials of the node IPA in the first, second and third phases. As shown in FIG. 11D, after switching between the second and the third phases for several times, the electric potential of the node IPA is decreased.
The relationship of the switching times between FIGS. 11C and 11B and the voltage VP2 of the second node N2a is shown in FIG. 10B. With comparison with FIG. 10A, since a voltage level of the common mode voltage VOCM is decreased, the numbers of switching times can be increased as in FIG. 10B. Thus a larger voltage dynamic range Vdynamic can be obtained between a maximal operation voltage V+ and a minimal operation voltage V− of the circuit.
In another embodiment, the difference between the first and second voltage VR1, VR2 is increased, for example by using a larger voltage as the first voltage VR1. This arrangement can increase the voltage dynamic range Vdynamic. In this embodiment, the first voltage VR1 is larger than the second voltage VR2, the fifth voltage VR5 is larger than the sixth voltage VR6, and the difference calculated by subtracting the fourth voltage VR6 from the third voltage VR5 is larger to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1 [(VR5−VR6)>(VR1−VR2)]. The first voltage VR1 is a high voltage (ex. 15V), the second voltage VR2 is a low voltage (ex. 1V), the fifth voltage VR5 is another high voltage (ex. 18V) and the sixth voltage VR6 is another low voltage (ex. 0V). In any phase, an input voltage of the read-out circuit 13 falls in an operative voltage range of low voltage elements. That means, the read-out circuit 13 can be manufactured without a high voltage element manufacturing process, thus a chip size and power consumption can be saved accordingly. If the second voltage VR2 is a high voltage and the first voltage VR1 is a low voltage, the read-out circuit 13 has to be manufactured by the high voltage element manufacturing process. Since a size of the high voltage element is larger than that of the low voltage element, the size of the read-out circuit will be increased. In another aspect, if the read-out circuit is manufactured by the high voltage element manufacturing process, a high voltage source has to be provided to the read-out circuit 13 so that a size and a power consumption of a voltage doubler is added.
In the embodiment of using the larger voltage as the first voltage VR1, the difference between the third and fourth voltages VR3, VR4 may be increased to increase the voltage dynamic range Vdynamic. In such arrangement, the third voltage VR3 is larger than the fourth voltage VR4 and the difference between the third and fourth voltages VR3, VR4 is larger than that between the first and second voltages VR1, VR2 [(VR3−VR4)>(VR1−VR2)].
In other embodiments, multiple electrode plates are detected at the same time. That means that sensing signals of the multiple electrode plates to be measured are read at the same time. For example, the sensing signals of the electrode plates located on the first rows are read at the same time and other electrode plates on other rows are not read.
Each embodiment mentioned above only uses the four electrode plates as an example. In the above embodiments, in addition to the switching unit connected to the electrode plate to be measured, the switching units corresponding to other three adjacent electrode plates are controlled to perform the same operations. In other embodiments, at least one switching unit corresponding to an electrode plate adjacent to the electrode plate to be measured performs the operations. Alternatively, in addition to the switching unit connected to the electrode plate to be measure, the switching units coupling to all of the other electrode plates may perform the same operations. In another embodiment, the multiple electrode plates to be measured are detected at the same time and the switching units coupling to other electrode plates adjacent the multiple electrode plates to be measured perform the same operations.
In the different embodiments, the difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 may be larger or smaller than the difference calculated by subtracting the second voltage VR2 from the first voltage VR1. In the foregoing embodiments, the difference calculated by subtracting the sixth voltage VR6 from the fifth voltage VR5 may be larger or smaller than the difference calculated by subtracting the second voltage VR2 from the first voltage VR1. Furthermore, in the foregoing description, when the first voltage VR1 is larger than the second voltage VR2, the third voltage VR3 is larger the fourth voltage VR4 and the fifth voltage VR5 is larger than the sixth voltage VR6. When the first voltage VR1 is smaller than the second voltage VR2, the third voltage VR3 is smaller the fourth voltage VR4 and the fifth voltage VR5 is smaller than the sixth voltage VR6.
Based on the foregoing description, in a first phase of a sensing method in accordance with the present invention, the first voltage is supplied to the first node connected to the electrode plate to be measured, the second voltage is supplied to the second node connected to the read-out circuit and the third voltage is supplied to the third node connected to a conductor adjacent to the electrode plate to be measured, wherein the first voltage minus the second voltage leaves a non-zero value. In a second phase, the first, second and third voltages are stopped supplying to the first, second and third nodes and a fourth voltage is supplied to the third node, the first node is connected to the second node and the read-out circuit outputs the measurement result of the electrode plate to be measured. In the embodiment, the difference calculated by subtracting the fourth voltage VR4 from the third voltage VR3 is equal to the difference calculated by subtracting the second voltage VR2 from the first voltage VR1. The third voltage may be the first voltage and the fourth voltage may be the second voltage, or the difference calculated by subtracting the fourth voltage from the third voltage may be larger or smaller than the difference calculated by subtracting the second voltage from the first voltage.
Based on the foregoing description, in the sensing method and circuit in accordance with the present invention, the conductor adjacent to the electrode plate to be measured is connected to the third and fourth voltages with different electric potentials in the first and second phases, rather than connected to a fixed electric potential of the voltage. Thus, when reading the measurement result in the second phase, the defect that the fringe capacitor formed between the electrode plate and the adjacent conductor counteracts a part of sensing signal of the electrode plate to be measured will be improved. Therefore, larger signal value of the measurement result is obtained, and the valley part or ridge part of the fingerprint can be identified accurately.
Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

## Claims

1. A sensing method of a fingerprint sensor to sense an electrode plate to be measured of the fingerprint sensor, comprising steps of:
(a) in a first phase, supplying a first voltage to a first node connected to the electrode plate to be measured, supplying a second voltage to a second node, and supplying a third voltage to a conductor adjacent to the electrode plate to be measured, wherein the first voltage minus the second voltage leaves a non-zero value and the first voltage is larger or smaller than the second voltage; and
(b) in a second phase, stopping supplying the first, second and third voltages to the first, second and third nodes, supplying a fourth voltage to the conductor, connecting the first node to the second node, wherein a voltage of the second node corresponds to a measurement result of the electrode plate to be measured, wherein when the first voltage is larger than the second voltage and the third voltage is larger than the fourth voltage, or when the first voltage is smaller than the second voltage and the third voltage is smaller than the fourth voltage.

2. The sensing method as claimed in claim 1, wherein a difference calculated by subtracting the fourth voltage from the third voltage is equal, larger or smaller than a difference calculated by subtracting the second voltage from the first voltage.

3. The sensing method as claimed in claim 2, wherein the first voltage is equal to the third voltage and the second voltage is equal to the forth voltage.

4. The sensing method as claimed in claim 1, further comprising steps of:
(c) in a third phase, supplying the first voltage to the first node and supplying the third voltage to the conductor; and
(d) returning to the second phase.

5. The sensing method as claimed in claim 1, wherein the conductor is a first electrode plate adjacent to the electrode plate to be measured, and the first electrode plate is used to sense a fingerprint.

6. The sensing method as claimed in claim 1, wherein
in the first phase, the third voltage is further supplied to multiple second electrode plates; and
in the second phase, the fourth voltage is further supplied to the second electrode plates, wherein the second electrode plate is used to sense a fingerprint.

7. The sensing method as claimed in claim 1, wherein the conductor is a protection electrode adjacent to the electrode plate to be measured and the protect electrode is used to provide an electrostatic discharge protection.

8. The sensing method as claimed in claim 4, wherein the conductor is a protection electrode adjacent to the electrode plate to be measured and the protect electrode is used to provide an electrostatic discharge protection.

9. The sensing method as claimed in claim 1, wherein
in the step (a), a fifth voltage is supplied to an isolation electrode plate and the isolation electrode plate is formed under the electrode plate to be measured; and
in the step (b), the fifth voltage is stopped supplying and a sixth voltage is supplied to the isolation electrode plate.

10. The sensing method as claimed in claim 9, wherein a difference calculated by subtracting the sixth voltage from the fifth voltage is equal, larger or smaller than a difference calculated by subtracting the second voltage from the first voltage.

11. The sensing method as claimed in claim 4, wherein
in the step (a), a fifth voltage is supplied to an isolation electrode plate and the isolation electrode plate is formed under the electrode plate to be measured; and
in the step (b), the fifth voltage is stopped supplying and a sixth voltage is supplied to the isolation electrode plate.

12. The sensing method as claimed in claim 11, wherein a difference calculated by subtracting the sixth voltage from the fifth voltage is equal, larger or smaller than a difference calculated by subtracting the second voltage from the first voltage.

13. A sensing circuit to sense an electrode plate to be measured of a fingerprint sensor, comprising:
a read-out circuit to read out a measurement result of the electrode plate to be measured;
a first switching unit coupled to a first voltage and a second voltage and having a first node connected to the electrode plate to be measured and a second node connected to the read-out circuit, wherein the first voltage minus the second voltage leaves a non-zero value and the first voltage is larger or smaller than the second voltage;
a second switching unit to connect a conductor to a third voltage or a fourth voltage, wherein the third voltage minus the fourth voltage leaves a non-zero value and when the first voltage is larger than the second voltage and the third voltage is larger than the fourth voltage, or when the first voltage is smaller than the second voltage and the third voltage is smaller than the fourth voltage;
a control unit controlling the first and second switching units, wherein
in a first phase, the control unit controls the first and second units to connect the first node to the first voltage, to connect the second node to the second voltage, to disconnect the first node to the second node, and to connect the conductor to the third voltage;
in a second phase, the control unit controls the first and second switching units to respectively disconnected the first, second and third nodes to the first, second and third voltages, to connect the first node to the second node, and to control the second switching unit to couple the conductor to the fourth voltage.

14. The sensing circuit as claimed in claim 13, wherein a difference calculated by subtracting the fourth voltage from the third voltage is equal, larger or smaller than a difference calculated by subtracting the second voltage from the first voltage.

15. The sensing circuit as claimed in claim 13, wherein after the second phase, in a third phase, the control unit controls the first and second switching units to couple the first node to the first voltage, to disconnect the first node to the second node, and to connect the conductor to the third voltage and after the third phase the control unit performs the same control as in the second phase.

16. The sensing circuit as claimed in claim 13, wherein the first switching unit comprises:
a first switch to couple the first voltage to the first node;
a second switch to couple the second voltage to the second node; and
a third switch between the first and second nodes; wherein
in the first phase, the first and second switches are turned on and the third switch is turned off; and
in the second phase, the first and second switches are turned off and the third switch is turned on.

17. The sensing circuit as claimed in claim 13, wherein the second switching unit comprises:
a fourth switch to couple the third voltage to the conductor; and
a fifth switch to couple the fourth voltage to the conductor; wherein
in the first phase, the fourth switch is turned on and the fifth switch is turned off; and
in the second phase, the fourth switch is turned off and the fifth switch is turned on.

18. The sensing circuit as claimed in claim 13, wherein the first voltage is equal to the third voltage and the second voltage is equal to the fourth voltage.

19. The sensing circuit as claimed in claim 13, wherein the conductor is a first electrode plate adjacent to the electrode plate to be measured and the first electrode plate is used to sense a fingerprint.

20. The sensing circuit as claimed in claim 13, wherein the fingerprint sensor further comprises multiple second electrode plates to sense a fingerprint and the sensing circuit further comprises:
multiple third switching units to respectively connect the second electrode plates to the third voltage or the fourth voltage, wherein
in the first phase, the control unit controls the third switching units to couple the second electrode plates to the third voltage; and
in the second phase, the control unit controls the third switching units to couple the second electrode plates to the fourth voltage.

21. The sensing circuit as claimed in claim 20, wherein the third switching unit further comprises:
a sixth switch to couple to the first voltage to the second electrode plate; and
a seventh switch to couple to the fourth voltage to the second electrode plate; wherein
in the first phase, the sixth switch is turned on and the seventh switch is turned off; and
in the second phase, the sixth switch is turned off and the seventh switch is turned on.

22. The sensing circuit as claimed in claim 13, wherein the conductor is a protection electrode adjacent to the electrode plate to be measured and the protect electrode is used to provide an electrostatic discharge protection.

23. The sensing circuit as claimed in claim 22, further comprising an electrostatic discharge (ESD) protection circuit coupled between the protection electrode and the second switching unit.

24. The sensing circuit as claimed in claim 23, wherein the ESD protection circuit comprises:
a first diode having an anode connected to the protection electrode and a cathode connected to a voltage with a high electric potential;
a second diode having a cathode connected to the anode of the first diode and the protection electrode, and an anode of the second diode connected to ground; and
a resistor element having a node connected to a connection node of the first and second diodes, and another node connected to the second switching unit.

25. The sensing circuit as claimed in claim 13, wherein the read-out circuit has a buffer having a single input connected to the second node.

26. The sensing circuit as claimed in claim 13, wherein the read-out circuit comprises:
a differential circuit having a non-inverting input connected to the second node and an output connected to the control unit; and
a grounded capacitor connected to an inverting input of the differential circuit and coupled to the second voltage through a eighth switch, wherein
in the first phase, the eighth switch is turned on; and
in the second phase, the eighth switch is turned off.

27. The sensing circuit as claimed in claim 16, wherein the read-out circuit comprises:
a differential circuit having a non-inverting input connected to the second node and an output connected to the control unit; and
a grounded capacitor connected to an inverting input of the differential circuit and coupled to the second voltage through a eighth switch, wherein
in the first phase, the eighth switch is turned on; and
in the second phase, the eighth switch is turned off.

28. The sensing circuit as claimed in claim 13, the read-out circuit comprises:
a differential circuit having a non-inverting input connected to the second node, an output connected to the control unit and an inverting input connected to a reference voltage, and the reference voltage is equal to the second voltage.

29. The sensing circuit as claimed in claim 13, wherein
in the first phase, the control unit further supplies a fifth voltage to an isolation electrode plate, wherein the isolation electrode plate is formed under the electrode plate to be measured; and
in the second phase, the control unit further stops supplying the fifth voltage and supplies a sixth voltage to the isolation electrode plate, wherein the fifth voltage minus the sixth voltage leaves a non-zero value.

30. The sensing circuit as claimed in claim 29, wherein a difference calculated by subtracting the sixth voltage from the fifth voltage is equal, larger or smaller than the difference calculated by subtracting the second voltage from the first voltage, or when the first voltage is smaller than the second voltage, the fifth voltage is smaller than the sixth voltage.

31. The sensing circuit as claimed in claim 29, further comprising a fourth switching unit comprising:
a ninth switch to couple the fifth voltage to the isolation electrode plate; and
a tenth switch to couple the sixth voltage to the isolation electrode plate; wherein
in the first phase, the ninth switch is turned on and the tenth switch turned off; and
in the second phase, the ninth switch is turned off and tenth switch turned on.

32. The sensing circuit as claimed in claim 15, wherein
in the first phase, the control unit further supplies a fifth voltage to an isolation electrode plate, wherein the isolation electrode plate is formed under the electrode plate to be measured; and
in the second phase, the control unit further stops supplying the fifth voltage and supplies a sixth voltage to the isolation electrode plate, wherein the fifth voltage minus the sixth voltage leaves a non-zero value.

33. The sensing circuit as claimed in claim 32, wherein a difference calculated by subtracting the sixth voltage from the fifth voltage is equal, larger or smaller than the difference calculated by subtracting the second voltage from the first voltage, or when the first voltage is smaller than the second voltage, the fifth voltage is smaller than the sixth voltage.

34. The sensing circuit as claimed in claim 32, further a fourth switching unit comprising:
a ninth switch to couple the fifth voltage to the isolation electrode plate;
a tenth switch to couple the sixth voltage to the isolation electrode plate; wherein
in the first phase, the ninth switch is turned on and the tenth switch turned off; and
in the second phase, the ninth switch is turned off and tenth switch turned on.