Capacitive fingerprint sensing device with current readout from sensing elements

1. A capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, said capacitive fingerprint sensing device comprising:

a plurality of sensing elements, each including:

a protective dielectric top layer to be touched by said finger;

an electrically conductive sensing structure arranged underneath said top layer;

a charge amplifier connected to said sensing structure for providing a sensing voltage signal indicative of a change of a charge carried by said sensing structure resulting from a change in a potential difference between said finger and said sensing structure, said charge amplifier comprising:

a first input connected to said sensing structure;

a second input;

an output for providing said sensing voltage signal;

a feedback capacitor connected between said first input and said output; and

at least one amplifier stage between said first and second inputs, and said output; and

a voltage-to-current converter for converting said sensing voltage signal to a sensing current signal indicative of said change of charge carried by said sensing structure,

wherein said fingerprint sensing device further comprises readout circuitry connected to the voltage-to-current converter comprised in each of said sensing elements for receiving sensing current signals from said plurality of sensing elements and providing a representation of said fingerprint pattern based on said sensing current signals.

2. The capacitive fingerprint sensing device according to claim 1 , further comprising excitation signal providing circuitry for providing a time-varying excitation signal to at least one of said finger and said sensing structure for providing said change in potential difference between said finger and said sensing structure.

3. The capacitive fingerprint sensing device according to claim 1 , wherein said readout circuitry comprises at least one current-to-voltage converter.

4. The capacitive fingerprint sensing device according to claim 3 , wherein said current-to-voltage converter comprises a transimpedance amplifier.

5. The capacitive fingerprint sensing device according to claim 4 , wherein a feedback element comprised in said transimpedance amplifier is constituted by a MOS transistor.

6. The capacitive fingerprint sensing device according to claim 2 , wherein:

said charge amplifier is configured in such a way that a potential at said first input substantially follows a potential at said second input; and

said excitation signal providing circuitry is connected to said second input of the charge amplifier for changing a potential at said second input from a first potential to a second potential, to thereby change a potential of said sensing structure, thereby providing said change in potential difference between said finger and said sensing structure.

7. The capacitive fingerprint sensing device according to claim 6 , wherein said amplifier stage comprised in the charge amplifier comprises a sense transistor having a gate constituting said first input,

wherein said sense transistor is formed in a well in a semiconductor substrate, an interface between said well and said substrate being configured in such a way that current can be prevented from flowing between said well and said substrate,

wherein said excitation signal providing circuitry is connected to said well for changing a potential of said well from a third potential to a fourth potential, a difference between said third potential and said fourth potential being substantially equal to a difference between said first potential and said second potential, to thereby reduce an influence of a parasitic capacitance between said sensing structure and said well.

8. The fingerprint sensing device according to claim 7 , further comprising a shielding structure arranged between said sensing structure and said substrate,

wherein said excitation signal providing circuitry is further connected to said shielding plate and configured to change a potential of said shielding plate from a fifth potential to a sixth potential, a difference between said fifth potential and said sixth potential being substantially equal to a difference between said first potential and said second potential.

9. The capacitive fingerprint sensing device according to claim 2 , wherein each of said sensing elements comprises timing circuitry connected to said excitation signal providing circuitry for:

providing a first excitation control signal to said excitation signal providing circuitry for triggering a first change in said potential difference at a first excitation transition time; and

providing a second excitation control signal to said excitation signal providing circuitry for triggering a second change in said potential difference at a second excitation transition time.

10. The capacitive fingerprint sensing device according to claim 1 , wherein each of said sensing elements further comprises:

reset circuitry controllable to discharge the feedback capacitor; and

timing circuitry connected to said reset circuitry for controlling said reset circuitry between a reset state in which said feedback capacitor is discharged, and a measurement ready state in which said feedback capacitor can be charged to allow measurement of said change of the charge carried by said sensing structure.

11. The capacitive fingerprint sensing device according to claim 9 , wherein said timing circuitry comprises at least a first delay element for controlling said sensing element to transition from a first measurement state to a second measurement state at a transition time defined by a first event and a time delay in relation to said first event,

said first delay element having an input for receiving a first signal defining said first event and an output for providing a second signal defining said transition from the first measurement state to the second measurement state.

12. An electronic device comprising:

the fingerprint sensing device according to claim 1 ; and

processing circuitry configured to:

acquire said representation of said fingerprint pattern from the fingerprint sensing device;

authenticate a user based on said representation; and

perform at least one user-requested process only if said user is authenticated based on said representation.

13. A method of sensing a fingerprint pattern of a finger using a capacitive fingerprint sensor comprising a plurality of sensing elements, each including:

a protective dielectric top layer to be touched by said finger;

an electrically conductive sensing structure arranged underneath said top layer; and

a charge amplifier connected to said sensing structure for providing a sensing voltage signal indicative of a change of a charge carried by said sensing structure resulting from a change in a potential difference between said finger and said sensing structure, said charge amplifier comprising:

a first input connected to said sensing structure;

a second input;

an output for providing said sensing voltage;

a feedback capacitor connected between said first input and said output; and

at least one amplifier stage between said first and second inputs, and said output,

wherein said method comprises the steps of:

changing said potential difference between said finger and said sensing structure;

providing, by the charge amplifier, said sensing voltage;

converting, in said sensing element, said sensing voltage to a sensing current; and

receiving, using readout circuitry external to said sensing elements, sensing currents from each of said sensing elements and providing a representation of said fingerprint pattern based on said received sensing currents.