Capacitive distance sensor

• US 6,362,633 B1
• Filed: 10/27/1998
• Issued: 03/26/2002
• Est. Priority Date: 02/14/1996
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Tartagni, Marco
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): Nguyen, Vincent Q.
• Application Number:
  US09/179,923
• Time in Patent Office: 1,246 Days
• Field of Search: 324/661, 324/662, 324/663, 324/678, 324/686, 324/691, 324/687, 382/124, 382/125, 382/126, 382/127, 361/179, 361/181, 341/33
• US Class Current: 324/662
• CPC Class Codes: G01B 7/004 for measuring coordinates of points
• CPC Class Codes: G01B 7/023 for measuring distance between sensor and object G01B7/082 and G01B7/102 take precedence
• CPC Class Codes: G01B 7/34 for measuring roughness or irregularity of surfaces
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

The distance sensor has a capacitive element (33, 34) in turn having a first plate (23) which is positioned facing a second plate (18) whose distance is to be measured. In the case of fingerprinting, the second plate is defined directly by the skin surface of the finger being printed. The sensor includes an inverting amplifier (13), between the input and output of which the capacitive element (33, 34) is connected to form a negative feedback branch. By supplying an electric charge step to the input of the inverting amplifier, a voltage step directly proportional to the distance being measured is obtained at the output.

Claims

1. A fingerprint sensor comprising:

a semiconductor integrated circuit;

a plurality of sensor cells formed as part of said integrated circuit;

a plurality of capacitor plates within each of said sensor cells, said plurality including a first capacitor plate and a second capacitor plate;

a surface for receiving a finger having a fingerprint pattern thereon; and

an amplifier circuit within each sensor cell, the amplifier circuit having an input terminal connected to the first capacitor plate and an output terminal connected to the second capacitor plate;

a feedback loop from the output of the amplifier to the input of the amplifier on the semiconductor circuit, the feedback loop response being modified based on the presence of a fingerprint being adjacent the surface.

2. The apparatus according to claim 1, further including:

a charge transfer circuit for selectively providing an input charge to an input terminal of the amplifier.

3. The apparatus according to claim 1, further including a reset circuit for placing the output terminal of the amplifier in a selected state at a selected time.

4. The apparatus according to claim 1, further including:

a switch selectively coupling the input terminal to the output terminal in a short circuit connection.

5. The apparatus according to claim 1 wherein the input terminal of the amplifier is a negative input terminal.

6. The apparatus according to claim 1 wherein the amplifier is a single input invertor amplifier.

7. The apparatus according to claim 1 wherein said sensor cells are formed in an array that includes a large number of sensor cells on said semiconductor integrated circuit.

8. The apparatus according to claim 7, further including:

a buffer connected to the output of the sensor cells,

a horizontal scanning stage for enabling the reading of a cell in a selected row; and

a vertical scanning stage for enabling the reading of a cell in a selected column.

9. The apparatus according to claim 1, further including:

a voltage reference source coupled to the input terminal of the amplifier.

10. The apparatus according to claim 9 wherein the voltage reference source is coupled to the input terminal via an input capacitor, the input capacitor having one plate coupled to the reference voltage and the other plate coupled to the input of the amplifier.

11. The apparatus according to claim 10, further including:

a logic circuit having an output terminal of the logic circuit coupled to the sensor cell to provide the reference voltage to the amplifier circuit.

12. The apparatus according to claim 11, further including:

a voltage source in said logic circuit for changing the reference voltage from a first voltage level to a second voltage level at a selected time.

13. An apparatus comprising:

an integrated circuit;

an amplifier formed in said integrated circuit, said amplifier having an input terminal and an output terminal;

a first plate of a capacitor positioned adjacent a top surface of the integrated circuit and being electrically connected to the input terminal of the amplifier;

a second plate of the capacitor positioned adjacent the top surface of the integrated circuit and being electrically connected to the output terminal of the amplifier;

a dielectric layer positioned over the first and second plates of the capacitor; and

a switch for selectively directly connecting or disconnecting the output terminal and the input terminal of the amplifier to or from each other to perform a reset operation.

14. The apparatus according to claim 13, further including a charge source selectively coupled to the input terminal of the amplifier.

15. The apparatus according to claim 13, further including:

a logic circuit for selectively connecting the output of the amplifier to an output circuit to determine changes in the output voltage caused by an object adjacent to the top surface of the integrated circuit.

16. The apparatus according to claim 13, further including:

a plurality of amplifiers having respective first and second capacitor plates and respective switches all formed within said integrated circuit to provide an array of sensor cells.

17. The apparatus according to claim 16, further including:

a logic circuit to enable output from selected ones of said sensor cells to an output buffer.

18. An apparatus for sensing the change in capacitance of plates in a negative feedback loop comprising:

a sensor cell having a first capacitor plate and a second capacitor plate;

an amplifier having an input and an output within the sensor cell, said first capacitor plate being coupled to input and said second capacitor plate being coupled to the output;

a negative feedback loop from the output of the amplifier to the input of the amplifier, said negative feedback loop varying the voltage level at the

output terminal based on variations in the capacitive coupling between said first plate and said second plate when a voltage step change occurs on the input terminal.

19. The apparatus according to claim 18, further including:

switch means for placing the input terminal and the output terminal at the same voltage; and

means for causing said output terminal to reach a value based on the capacitive coupling between said first plate and said second plate as modified by an object placed adjacent to said plates.

20. The apparatus according to claim 18, further including;

a reset circuit means for placing the output of the amplifier at a known voltage prior to sensing.

21. The apparatus according to claim 20 wherein the reset circuit includes a switch for directly connecting the input of the amplifier to the output of the amplifier, both of which are thus coupled to the same selected voltage.

22. The apparatus according to claim 18, further including:

first reference voltage means for placing a first reference voltage on the input terminal of said amplifier at a first selected time; and

second reference voltage means for placing a second reference voltage on the input terminal of said amplifier at a second selected time.

23. The apparatus according to claim 22 wherein the first reference voltage means includes an input capacitor.

24. The apparatus according to claim 22 wherein the first reference voltage means includes a voltage source coupled to an input of the amplifier.

25. A method of sensing variations in capacitive coupling across an array of sensor cells comprising:

coupling an input of each respective sensor cell to a first selected reference voltage;

coupling an amplifier circuit within the sensor cell to the first selected reference voltage;

capacitively coupling the output of the amplifier to the input of the amplifier in a negative feedback capacitive loop;

placing an object adjacent a plurality of sensor cells in the array to change the capacitive coupling of the negative feedback from the output terminal to the input terminal for the amplifier;

inputting a charge to the sensor cell; and

sensing the output voltage of a plurality of sensor cells in the array to determine a respective change in capacitive coupling in a plurality of cells caused by having said object adjacent the respective cell.

26. The method according to claim 25, further including:

resetting the sensor cells to have a selected output voltage on the amplifier output terminal prior to sensing the output voltage to determine the change in capacitive coupling.

27. The method according to claim 25, further including:

setting the outputs of each of said sensor cells to a first voltage level;

setting the inputs of each of said sensor cells to a second voltage level at a selected time prior to sensing the output voltage to determine the change in capacitive coupling.

28. The method according to claim 25, further including:

charging an input capacitor having a first plate connected to an input of the sensor cell to a selected voltage;

changing the voltage on a second plate of said input capacitor to a second voltage causing a rapid change in the voltage applied to the input of the sensor cell.

29. The method according to claim 25 wherein said sensor cells are positioned in an integrated circuit.

30. The method according to claim 25, further including:

scanning the array of sensor cells in a horizontal scan pattern; and

scanning the array of sensor cells in a vertical scan pattern to selectively enable output from one sensor cell at a time.

31. The method according to claim 25 wherein the object causes the output voltage sensed from a first plurality of reference cells to be different from the output voltage sensed in a second plurality of reference cells.