Method of and system for compensating for injection gradient in a capacitive sensing circuit array

• US 6,819,784 B1
• Filed: 04/04/2000
• Issued: 11/16/2004
• Est. Priority Date: 04/04/2000
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Upek, Inc. (Apple Inc.)
• Inventors: Sabatini, Marco, Raynal, Frederic, Gozzini, Giovanni
• Primary Examiner(s): Mariam, Daniel
• Assistant Examiner(s): GORADIA, SHEFALI DINESH
• Application Number:
  US09/542,440
• Time in Patent Office: 1,687 Days
• Field of Search: 382/124-127
• US Class Current: 382/124
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

A fingerprint sensor includes an array comprising a plurality of capacitive pixel cells. Each of the capacitive pixel cells includes a pair of metal plates and a reset transistor. The reset transistor includes a gate, a source connected to one of the metal plates and a drain connected to the other of the metal plates. A reset buffer generates a reset signal. A regenerator is associated with each reset transistor of each capacitive pixel cell. The regenerator regenerates the reset signal received from the reset buffer to eliminate injection gradient over the array.

Claims

1. A fingerprint sensor, which comprises:

a capacitive pixel cell, said capacitive pixel cell including a pair of metal plates and a reset transistor, said reset transistor including a gate, a source connected to one of said metal plates, and a drain connected to the other of said metal plates;

an inverter including an input and an output, said output being connected to said gate of said reset transistor; and

,

a reset buffer having an output connected, through a common line, to the input of said inverter.

2. The fingerprint sensor as claimed in claim 1, including:

a high-gain inverter having an input and an output, said input of said high-gain inverter being connected to one of said metal plates and said output of said high-gain inverter being connected to the other of said metal plates; and

,

a charge buffer having an output operably connected to the input of said high-gain inverter.

3. The fingerprint sensor as claimed in claim 2, including:

an input capacitor connected between the output of said charge buffer and the input of said high-gain inverter.

4. A method of capturing a fingerprint image, which comprises the steps of:

providing a plurality of capacitive pixel cells,

each of said capacitive pixel cells comprising a pair of metal plates and a reset transistor, said reset transistor including a gate, a source connected to one of said metal plates and a drain connected to the other of said metal plates;

generating a reset signal to said plurality of pixel cells; and

,

regenerating said reset signal to reset the reset transistor of each of said pixel cells.

5. A fingerprint sensor, which comprises:

a plurality of capacitive pixel cells, each of said capacitive pixel cells including a pair of metal plates and a reset transistor, said reset transistor including a gate, a source connected to one of said metal plates and a drain connected to the other of said metal plates;

an inverter connected to the gate of each reset transistor;

a reset buffer including an output connected to each inverter.

6. The fingerprint sensor as claimed in claim 5, wherein each capacitive pixel cell includes:

a high-gain inverter having an input and an output, said input of said high-gain inverter being connected to one of said metal plates and said output of said high-gain inverter being connected to the other of said metal plates; and

,

a charge buffer having an output operably connected to the input of said high-gain inverter.

7. The fingerprint sensor as claimed in claim 6, wherein each capacitive pixel cell further includes:

an input capacitor connected between the output of said charge buffer and the input of said high-gain inverter.

8. A fingerprint sensor, which comprises:

a plurality of capacitive pixel cells, each of said capacitive pixel cells including a pair of metal plates and a reset transistor, said reset transistor including a gate, a source connected to one of said metal plates, and a drain connected to the other of said metal plates; and

, means for generating a reset signal;

means for re-generating said reset signal at each of said of said capacitive pixel cells.

9. The fingerprint sensor as claimed in claim 8, wherein said means for regenerating said reset signal includes a plurality of inverters, each of said inverters including an input and an output, respective ones of the plurality of inverters each having an output connected to a corresponding gate of each said reset transistor.

10. The fingerprint sensor as claimed in claim 9, wherein each of the plurality of capacitive pixel cells further comprises a high-gain inverter connecting members of the pair of metal plates and said means for generating said reset signal includes a reset buffer having an output connected to an input of the high-gain inverter of each of said capacitive pixel cells.

11. The fingerprint sensor of claim 9, further comprising a second plurality of inverters, ones of the second plurality of inverters connected in series with corresponding ones of the plurality of inverters.

12. The fingerprint sensor of claim 9, wherein said means for generating an input signal is configured to provide an inverted input signal to each of the plurality of inverters.

13. The fingerprint sensor of claim 9, wherein said reset transistor of each of said capacitive pixel cells comprises a p-channel transistor.

14. The fingerprint sensor as claimed in claim 8, wherein each capacitive pixel cell further comprises

a charge buffer having an output operably connected to the input of said high-gain inverter of each capacitive pixel cell.

15. The fingerprint sensor as claimed in claim 14, wherein each capacitive pixel cell further includes an input capacitor connected between the output of said charge buffer and the input of said high-gain inverter.