TWI564815B - Fingerprint sensing device and its fingerprint sensing method - Google Patents

Description

Fingerprint sensing device and fingerprint sensing method thereof

The invention relates to a fingerprint sensing device and method, in particular to a fingerprint sensing device with low parasitic capacitance and a fingerprint sensing method thereof.

1 is a conventional fingerprint sensing device 10 in which a protective layer 12 is provided for a finger to touch and protect a plurality of electrode plates 16a, 16b, 16c below it, and an Electro-Static Discharge (ESD) layer 14 is provided. The ESD protection, the plurality of detecting circuits 18a, 18b, and 18c are respectively connected to the corresponding electrode plates 16a, 16b, and 16c for detecting the capacitance between the electrode plates 16a, 16b, and 16c and the fingers (not shown). The sensing voltage is obtained. Since the fingerprint of the finger is composed of uneven lines, the fingerprint has peaks and valleys. Moreover, the peak of the fingerprint is different from the distance between the valley and the electrode plate, so the sensing voltage generated by the peak and the valley is also different. The fingerprint sensing device 10 can determine that the texture of the corresponding electrode plates 16a, 16b, 16c is a peak or a valley according to the magnitude of the sensing voltage. After obtaining the texture corresponding to all the electrode plates, the fingerprint sensing device 10 can obtain the fingerprint image of the finger.

However, as shown in FIG. 1, parasitic capacitances Cp1a, Cp1b, and Cp1c exist between the electrode plates 16a, 16b, and 16c and the lower conductor. The conductor below the electrode plate includes detection circuits 18a, 18b, 18c, ground terminals, and other conductors. The parasitic capacitances Cp1a, Cp1b, Cp1c affect the sensing of the electrode plates 16a, 16b, 16c. The higher the parasitic capacitances Cp1a, Cp1b, Cp1c, the smaller the dynamic range of the sensing voltage obtained by the measuring electrode plates 16a, 16b, 16c. The more difficult it is to correctly identify whether the detected texture is a peak or a grain. Further, the operational noise of the detecting circuits 18a, 18b, and 18c also interferes with the electrode plates 16a, 16b, and 16c via the parasitic capacitances Cp1a, Cp1b, and Cp1c.

Therefore, a fingerprint sensing device with low parasitic capacitance is what it is.

The object of the present invention is to provide a fingerprint sensing device with low parasitic capacitance and a fingerprint sensing method thereof.

According to the present invention, a fingerprint sensing device includes an electrode plate, a protective layer, a feedback capacitor, a component layer, and a barrier. The protective layer is above the electrode plate for finger touch. The feedback capacitor is coupled to the electrode plate, and the feedback capacitor and the electrode plate are independent components. The component layer is located under the electrode plate, and the component layer includes a plurality of circuit components connected to the feedback capacitor to form a detecting circuit for detecting a capacitance formed between the finger and the electrode plate for determining the corresponding electrode plate Fingerprint. The barrier is between the electrode plate and the component layer. In the excitation mode, the electrode plate and the baffle plate are applied with a first voltage, and in the detecting mode, the electrode plate and the baffle plate are applied with a second voltage.

According to the present invention, a fingerprint sensing device includes an electrode plate, a protective layer, a detecting circuit, a first switch, and a barrier. The protective layer is above the electrode plate for finger touch. In the detecting mode, the detecting circuit detects a capacitance formed between the electrode plate and the finger for determining a fingerprint corresponding to the electrode plate. The first switch is connected between the electrode plate and the detecting circuit. The blocking plate is between the electrode plate and the detecting circuit. During the excitation mode, the first switch is open to disconnect the electrode plate from the detecting circuit, and the electrode plate and the blocking plate are applied with a first voltage. In the detection mode, the first switch is closed to connect the detecting circuit to the electrode plate, and the electrode plate and the blocking plate are applied with a second voltage.

According to the present invention, a fingerprint sensing method includes: disconnecting an electrode plate and a detecting circuit in a driving mode, and applying a first voltage to the electrode plate and a blocking plate; and detecting In the mode, the electrode plate and the detecting circuit are connected and a second voltage is applied to the electrode plate and the blocking plate, so that the detecting circuit detects the capacitance formed between the finger and the electrode plate for judging The fingerprint of the electrode plate should be a peak or a valley. The barrier plate is between the electrode plate and the detecting circuit, and a protective layer is provided above the electrode plate for the finger to touch.

The invention utilizes a baffle plate under the electrode plate to reduce the parasitic capacitance between the electrode plate and other conductors below, to achieve a larger signal dynamic range, and prevent the operation noise of the detecting circuit from interfering with the electrode plate, and During the sensing process, the baffle plate has the same potential as the electrode plate, thereby eliminating the parasitic capacitance effect between the baffle plate and the electrode plate.

10‧‧‧Finger sensing device

12‧‧‧Protective layer

14‧‧‧Electrostatic discharge layer

16a‧‧‧Electrode plate

16b‧‧‧electrode plate

16c‧‧‧electrode plate

16d‧‧‧electrode plate

18a‧‧‧Detection circuit

18b‧‧‧Detection circuit

18c‧‧‧Detection circuit

18d‧‧‧Detection circuit

20a‧‧‧Operational Amplifier

20b‧‧‧Operational Amplifier

20c‧‧‧Operational Amplifier

20d‧‧‧Operational Amplifier

22‧‧‧Finger sensing device

24a‧‧‧Baffle

24b‧‧‧Baffle

24c‧‧‧Baffle

24d‧‧‧Baffle

28‧‧‧Metal plates

30‧‧‧Metal plates

32‧‧‧Component layer

34‧‧‧ fingers

36‧‧‧Toggle switch

Figure 1 is a conventional fingerprint sensing device; 2 is a first embodiment of the fingerprint sensing device of the present invention; FIG. 3 shows the structure of the fingerprint sensing device of FIG. 2; FIG. 4 shows an equivalent circuit of the fingerprint sensing device of FIG. 2 in an excitation mode; An equivalent circuit of the fingerprint sensing device of FIG. 2 in the detection mode is shown; FIG. 6 shows a timing chart of the circuits of FIGS. 4 and 5; and FIG. 7 is a second embodiment of the fingerprint sensing device of the present invention.

2 is an embodiment of the fingerprint sensing device 22 of the present invention. The fingerprint sensing device 10 of FIG. 1 includes a protective layer 12, an electrostatic discharge layer 14, electrode plates 16a, 16b, and 16c, and a detecting circuit 18a. 18b, 18c. In addition, the fingerprint sensing device 22 further includes a baffle plate 24a, 24b, 24c and a switch SWse, SWsp, wherein the baffle plate 24a is between the electrode plate 16a and the detecting circuit 18a, and the baffle plate 24b is at the electrode plate 16b and the detecting circuit 18b. Between the electrode plate 16c and the detecting circuit 18c, one end of the switch SWse is connected to the blocking plate 24a, 24b, 24c, the other end of the switch SWse receives the voltage VR1, and one end of the switch SWsp is connected to the blocking plate 24a, 24b. 24c, the other end of the switch SWsp receives the voltage VR2. In the fingerprint sensing device 22, parasitic between the electrode plates 16a, 16b, 16c and other conductors below (for example, the detecting circuits 18a, 18b, 18c and the ground end) can be used by using the baffles 24a, 24b, 24c. The capacitance is reduced from Cp1a, Cp1b, and Cp1c of FIG. 1 to the remaining Cp1aa, Cp1ba, and Cp1ca, wherein the parasitic capacitances Cp1aa, Cp1ba, Cp1ca, and Cp1da are much smaller than the parasitic capacitances Cp1a, Cp1b, and Cp1c. Since the parasitic capacitance corresponding to the electrode plates 16a, 16b, and 16c is greatly reduced, a large signal dynamic range can be achieved, and a large amount of signals can be obtained. Moreover, the arrangement of the baffles 24a, 24b, 24c helps to improve the operational noise interfering electrode plates 16a, 16b, 16c of the detecting circuits 18a, 18b, 18c. In addition, by controlling the switching of the switches SWse and SWsp, the baffles 24a, 24b, and 24c can have the same potential as the electrode plates 16a, 16b, and 16c, and therefore, during the process of sensing the electrode plates 16a, 16b, and 16c, the baffle plate is blocked. The effects of the parasitic capacitances Cp1ab, Cp1bb, Cp1cb between the 24a, 24b, 24c and the electrode plates 16a, 16b, 16c can also be eliminated.

3 shows an embodiment of the structure of the fingerprint sensing device 22 of FIG. 2. In the embodiment of FIG. 3, only the structure of a set of sensing units is shown based on the convenience of the description. The sensing unit includes an electrode plate 16a, a detecting circuit 18a, a baffle plate 24a, and a switch SW1a connected therebetween. SW2a, SWse, SWsp. As shown in FIG. 3, circuit elements such as switches SW1a, SW2a, SW3a, SWse, SWsp, and operational amplifier 20a are disposed in the element layer 32, and the operational amplifier 20a and the switch SW3a and the feedback capacitor Cfba constitute a detecting circuit 18a. The feedback capacitor Cfba is between the baffle plate 24a and the element layer 32 and directly under the baffle plate 24a. The feedback capacitor Cfba is composed of the metal plates 28 and 30, and is independent of the electrode plate 16a. The baffle plate 24a is between the electrode plate 16a and the element layer 32 and directly under the electrode plate 16a to reduce the parasitic capacitance between the electrode plate 16a and other conductors below (for example, the detecting circuit 18a, the ground end). The element layer 32 includes at least a semiconductor substrate (not shown) for fabricating the components required for the detection circuit 18a. The feedback capacitor Cfba can be fabricated in other ways, such as by two layers of polysilicon in the component layer 32.

When the finger 34 contacts the fingerprint sensing device 22, a capacitance Csa is formed between the finger 34 and the electrode plate 16a. By detecting the capacitance Csa, it can be determined that the fingerprint pattern of the corresponding electrode plate 16a is a peak or a valley (valley) ). In the excitation mode, the switches SW1a, SW3a, and SWsp are closed, and the switches SW2a and SWse are open. At this time, the electrode plate 16a and the baffle plate 24a are both applied with a voltage VR2, and the feedback capacitor Cfba is short-circuited. Therefore, the voltage on the feedback capacitor Cfba is set to 0V. In the detection mode, the switches SW1a, SW3a, and SWsp are open circuits, and the switches SW2a and SWse are closed to respectively connect the electrode plate 16a and the blocking plate 24a to the inverting input terminal of the operational amplifier 20a and the voltage VR1, respectively, due to the operational amplifier The virtual grounding characteristic is applied to the electrode plate 16a. The detecting circuit 18a detects the capacitance Csa to generate the sensing voltage Voa for determining that the texture of the corresponding electrode plate 16a is a peak or a valley. The electric potential of the electrode plate 16a and the baffle plate 24a are the same regardless of the excitation mode or the detection mode, and therefore the effect of the parasitic capacitance Cp1ab between the electrode plate 16a and the baffle plate 24a is eliminated.

4 and 5 are equivalent circuits of the fingerprint sensing device 22 of FIG. 2, wherein FIG. 4 shows the operation in the excitation mode, and FIG. 5 shows the operation in the detection mode. In FIGS. 4 and 5, Csa, Csb, Csc, and Csd are capacitances formed by the fingers and the electrode plates 16a, 16b, 16c, and 16d, wherein the electrode plates 16a, 16b, 16c, and 16d are regarded as capacitances Csa and Csb, respectively. The electrodes on the right side of Csc and Csd are regarded as the electrodes on the left side of the capacitors Csa, Csb, Csc, and Csd. One end of the switch SW1a is connected to the electrode plate 16a and the switch SW2a, and the other end of the switch SW1a receives the voltage VR2. The switch SW2a is connected between the electrode plate 16a and the detecting circuit 18a. Cp1aa is the electrode plate 16a and The parasitic capacitance between the conductors below it. Cp1ab is a parasitic capacitance between the electrode plate 16a and the barrier rib 24a. The detecting circuit 18a includes an operational amplifier 20a, a switch SW3a and a feedback capacitor Cfba. The switch SW3a and the feedback capacitor Cfba are connected in parallel between the inverting input terminal Ina and the output terminal Oa of the operational amplifier 20a, and the operational amplifier 20a is non- The inverting input terminal Ipa receives the voltage VR1, and the capacitor Cp2a is a parasitic capacitance on the inverting input terminal Ina of the operational amplifier 20a. One end of the switch SW1b is connected to the electrode plate 16b and the switch SW2b, and the other end of the switch SW1b receives the voltage VR2. The switch SW2b is connected between the electrode plate 16b and the detecting circuit 18b. Cp1ba is the parasitic capacitance between the electrode plate 16b and the conductor below it. Cp1bb is a parasitic capacitance between the electrode plate 16b and the baffle plate 24b. The detecting circuit 18b includes an operational amplifier 20b, a switch SW3b, and a feedback capacitor Cfbb. The switch SW3b and the feedback capacitor Cfbb are connected in parallel between the inverting input terminal Inb and the output terminal Ob of the operational amplifier 20b, and the operational amplifier 20b is non- The inverting input terminal Ipb receives the voltage VR1, and the capacitor Cp2b is the parasitic capacitance on the inverting input terminal Inb of the operational amplifier 20b. One end of the switch SW1c is connected to the electrode plate 16c and the switch SW2c, and the other end of the switch SW1c receives the voltage VR2. The switch SW2c is connected between the electrode plate 16c and the detecting circuit 18c. Cp1ca is the parasitic capacitance between the electrode plate 16c and the conductor below it. Cp1cb is a parasitic capacitance between the electrode plate 16c and the baffle plate 24c. The detecting circuit 18c includes an operational amplifier 20c, a switch SW3c and a feedback capacitor Cfbc. The switch SW3c and the feedback capacitor Cfbc are connected in parallel between the inverting input terminal Inc and the output terminal Oc of the operational amplifier 20c. The inverting input terminal Ipc receives the voltage VR1, and the capacitor Cp2c is a parasitic capacitance on the inverting input terminal Inc of the operational amplifier 20c. One end of the switch SW1d is connected to the electrode plate 16d and the switch SW2d, and the other end of the switch SW1d receives the voltage VR2. The switch SW2d is connected between the electrode plate 16d and the detecting circuit 18d. Cp1da is the parasitic capacitance between the electrode plate 16d and the conductor below it. Cp1db is a parasitic capacitance between the electrode plate 16d and the baffle plate 24d. The detecting circuit 18d includes an operational amplifier 20d, a switch SW3d and a feedback capacitor Cfbd. The switch SW3d and the feedback capacitor Cfbd are connected in parallel between the inverting input terminal Ind and the output terminal Od of the operational amplifier 20d, and the operational amplifier 20d is non- The inverting input terminal Ipd receives the voltage VR1, and the capacitor Cp2d is the parasitic capacitance on the inverting input terminal Ind of the operational amplifier 20d. Since the barrier plates 24a, 24b, 24c, 24d are provided between the electrode plates 16a, 16b, 16c, 16d and the detecting circuits 18a, 18b, 18c, 18d, the electrode plates 16a, 16b, 16c in Figs. 4 and 5 The parasitic capacitance between 16d and the other conductors below is reduced from Cp1a, Cp1b, Cp1c in Fig. 1 to Cp1aa, Cp1ba, Cp1ca, Cp1da. The parasitic capacitances Cp1aa, Cp1ba, Cp1ca, and Cp1da are much smaller than the parasitic capacitances Cp1a, Cp1b, and Cp1c.

FIG. 6 is a timing chart showing the circuit of FIGS. 4 and 5 when detecting the electrode plate 16a. As shown in the circuit of FIG. 4 and the times t1 to t2 of FIG. 6, when the fingerprint sensing device 22 is in the excitation mode, the switches SW1a, SW3a, SW1b, SW3b, SW1c, SW3c, SW1d, SW3d, and SWsp are closed (on). The switches SW2a, SW2b, SW2c, SW2d, and SWse are open. At this time, the voltage VR2 charges the capacitors Csa, Csb, Csc, and Csd, and the voltages of the feedback capacitors Cfba, Cfbb, Cfbc, and Cfbd are 0V. Due to the virtual grounding characteristics of the operational amplifier, the voltages of the inverting input terminals Ina, Inb, Inc, Ind of the operational amplifiers 20a, 20b, 20c, 20d will be equal to VR1, and at this time the outputs of the operational amplifiers 20a, 20b, 20c, 20d Oa, Ob, Oc, Od are connected to their inverting input terminals Ina, Inb, Inc, Ind, so the sensing voltages Voa, Vob, Voc, Vod will be equal to VR1. In the excitation mode, the potentials at both ends of the parasitic capacitances Cp1ab, Cp1bb, Cp1cb, and Cp1db are all VR2, so the voltages of the parasitic capacitances Cp1ab, Cp1bb, Cp1cb, and Cp1db are 0V. At the end of the excitation mode, as shown at time t2 of FIG. 6, the switches SW1a, SW3a, SW1b, SW1c, SW1d, and SWsp are opened, and the switches SW2a, SW2b, SW2c, SW2d, and SWse are kept in an open state, and the switch SW3b, SW3c and SW3d remain in a closed state.

As shown in the circuit of FIG. 5 and the time t3~t4 of FIG. 6, when the fingerprint sensing device 22 enters the detection mode and detects the fingerprint of the corresponding electrode plate 16a, the switches SW1a, SW3a, SW1b, SW1c, SW1d, SWsp While remaining in the open state, the switches SW2a, SW2b, SW2c, SW2d, and SWse are closed, and the switches SW3b, SW3c, and SW3d are kept in a closed state. At this time, the sensing voltage Voa=VR1-(VR2-VR1)×[(Csa/Cfba)+(Cp1aa/Cfba)], the fingerprint sensing device 22 can determine the size of the capacitor Csa according to the sensing voltage Voa, and further determine the corresponding electrode. The grain of the plate 16a is a peak or a grain. At the end of the detection mode, as shown at time t4 of FIG. 6, the switches SW1a, SW3a, SW1b, SW1c, SW1d, and SWsp are kept in an open state, and the switches SW2a, SW2b, SW2c, SW2d, and SWse are opened, and the switch SW3b, SW3c and SW3d remain in a closed state. In the detection mode, due to the virtual grounding characteristics of the operational amplifier, the potentials at both ends of the parasitic capacitances Cp1ab, Cp1bb, Cp1cb, and Cp1db are all VR1. It can be seen from the above equation of the sensing voltage Voa that the parasitic capacitance Cp1ab between the electrode plate 16a and the baffle plate 24a does not affect the sensing voltage Voa, and the electrode plate 16a and the other conductors below only have a small parasitic capacitance Cp1aa. Therefore, compared with the conventional fingerprint sensing device 10 having a large parasitic capacitance Cp1a, the fingerprint sensing device 22 of the present invention has a large signal dynamic range, and a large output signal amount can be obtained. And it can improve the operation noise of the detection circuit to interfere with the electrode plate. On the other hand, the switch in the figure may be disposed under the electrode plate and the baffle plate, and the present invention can also improve the operation noise of the switches to interfere with the electrode plate.

In FIG. 6, in the process from the excitation mode to the detection mode, the switches SW1a, SW3a, SW1b, SW1c, SW1d, and SWsp are turned on first, and then the switches SW2a, SW2b, SW2c, SW2d, and SWse are closed.

FIG. 5 is an example of measuring the capacitance Csa between the electrode plate 16a and the finger. Those skilled in the art will understand how to apply to measure other electrode plates, and details are not described herein again.

In the fingerprint sensing device 22 of FIG. 2, each of the electrode plates 16a, 16b, and 16c corresponds to one detecting circuit 18a, 18b, and 18c, but in other embodiments, a plurality of electrode plates 16a and 16b may be used. 16c shares a detection circuit 18a as shown in FIG. In Fig. 7, the switch 36 is used to connect the detecting circuit 18a to the electrode plates 18a, 18b or 18c to be detected.

In the embodiment of FIG. 2, FIG. 4, FIG. 5 and FIG. 7, the plurality of blocking plates 24a, 24b, 24c, and 24d share the switches SWse and SWsp. However, in other embodiments, one of the blocking plates may be a corresponding one. Group switches SWse, SWsp.

The above description of the preferred embodiments of the present invention is intended to be illustrative, and is not intended to limit the scope of the invention to the disclosed embodiments. It is possible to make modifications or variations based on the above teachings or learning from the embodiments of the present invention. The embodiments are described and described in the actual application of the present invention in various embodiments.

12‧‧‧Protective layer

14‧‧‧Electrostatic discharge layer

16a‧‧‧Electrode plate

16b‧‧‧electrode plate

16c‧‧‧electrode plate

18a‧‧‧Detection circuit

18b‧‧‧Detection circuit

18c‧‧‧Detection circuit

22‧‧‧Finger sensing device

24a‧‧‧Baffle

24b‧‧‧Baffle

24c‧‧‧Baffle

Claims (8)

A fingerprint sensing device includes: an electrode plate; a protective layer above the electrode plate for finger touch; a feedback capacitor coupled to the electrode plate, wherein the feedback capacitor and the electrode plate are independent An element layer is disposed under the electrode plate, and the component layer includes a plurality of circuit components connected to the feedback capacitor to form a detecting circuit for detecting a capacitance formed between the finger and the electrode plate for judging Corresponding to the fingerprint of the electrode plate; and a barrier plate between the electrode plate and the component layer; wherein, in the excitation mode, the electrode plate and the barrier plate are applied with a first voltage; in the detection mode, The electrode plate and the barrier plate are applied with a second voltage. The fingerprint sensing device of claim 1, wherein the plurality of circuit components comprise: an operational amplifier having an inverting input terminal, a non-inverting input terminal, and an output terminal, wherein the non-inverting input terminal receives the second And a first switch coupled in parallel with the feedback capacitor between the inverting input of the operational amplifier and the output, wherein the first switch is closed in the excitation mode and open in the detection mode . The fingerprint sensing device of claim 2, wherein the component layer further comprises: a second switch having two ends connected to the inverting input terminal of the operational amplifier and the electrode plate, wherein the second switch is in an excitation mode For the open circuit, it is closed in the detection mode; a third switch has one end connected to the second switch and the electrode plate, and the other end receives the first voltage, wherein the third switch is closed in the excitation mode, The detection mode is an open circuit; a fourth switch has one end connected to the blocking plate and the other end receiving the first voltage, wherein the The fourth switch is closed in the excitation mode and is open in the detection mode; and a fifth switch is connected to the blocking plate at one end and the second voltage is received at the other end, wherein the fifth switch is in the excitation mode Open circuit, closed circuit in detection mode. A fingerprint sensing device includes: an electrode plate; a protective layer above the electrode plate for finger touch; and a detecting circuit for detecting a formation between the electrode plate and the finger in the detecting mode a capacitor for determining a fingerprint corresponding to the electrode plate; a first switch connected between the electrode plate and the detecting circuit, wherein during the energizing mode, the first switch is open to open the electrode plate and the Detecting a connection between the circuits, in the detecting mode, the first switch is closed to connect the detecting circuit to the electrode plate; and a blocking plate is disposed between the electrode plate and the detecting circuit; In the excitation mode, the electrode plate and the baffle plate are applied with a first voltage; in the detecting mode, the electrode plate and the baffle plate are applied with a second voltage. The fingerprint sensing device of claim 4, wherein the detecting circuit comprises: an operational amplifier having an inverting input terminal, a non-inverting input terminal and an output terminal, wherein the non-inverting input terminal receives the second voltage a feedback capacitor is connected between the inverting input terminal and the output terminal; a second switch is connected in parallel with the feedback capacitor, wherein the second switch is closed in the excitation mode, and in the detection mode open circuit. The fingerprint sensing device of claim 5, further comprising: a third switch having one end connected to the first switch and the electrode plate, the other end receiving the first voltage, wherein the third switch is closed in the excitation mode An open circuit in the detection mode; a fourth switch having one end connected to the blocking plate and the other end receiving the first voltage, wherein the The fourth switch is closed in the excitation mode and is open in the detection mode; and a fifth switch is connected to the blocking plate at one end and the second voltage is received at the other end, wherein the fifth switch is in the excitation mode Open circuit, closed circuit in detection mode. A fingerprint sensing method includes the following steps: in an excitation mode, disconnecting an electrode plate and a detecting circuit, and applying a first voltage to the electrode plate and a blocking plate; and detecting mode Connecting the electrode plate and the detecting circuit and applying a second voltage to the electrode plate and the blocking plate, wherein the detecting circuit detects a capacitance formed between the finger and the electrode plate for determining the corresponding electrode The fingerprint of the board is a peak or a valley; wherein the blocking plate is between the electrode plate and the detecting circuit, and a protective layer is provided above the electrode plate for the finger to touch. The fingerprint sensing method of claim 7, further comprising: setting, in the excitation mode, a voltage of a feedback capacitor connected between the inverting input terminal and the output terminal of the operational amplifier; and detecting mode And connecting the feedback capacitor and the electrode plate to generate a sensing voltage on the feedback capacitor for determining a fingerprint corresponding to the electrode plate, wherein the sensing voltage is a capacitance formed between the finger and the electrode plate Related.