SELF CAPACITANCE TOUCH SENSING

• US 20150035787A1
• Filed: 10/30/2013
• Published: 02/05/2015
• Est. Priority Date: 07/31/2013
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Shahparnia, Shahrooz, Krah, Christoph H.
• Granted Patent: US 10,061,444 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G06F 1/3262 Power saving in digitizer or tablet
• CPC Class Codes: G06F 2203/04107 Shielding in digitiser, i.e. guard or shielding arrangements, mostly for capacitive touchscreens, e.g. driven shields, driven grounds
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes

Abstract

Self capacitance touch circuits to cancel the effects of parasitic capacitance in a touch sensitive device are disclosed. One circuit can generate a parasitic capacitance cancelation signal that can be injected into touch sensing circuitry to cancel the parasitic capacitance. Another circuit can adjust the phase and magnitude of the parasitic capacitance cancelation signal based on characteristics of channels in the touch sensing circuitry so as to fine tune the parasitic capacitance cancelations. Another circuit can drive a guard plate and touch panel electrodes so as to cancel the parasitic capacitances between the panel and the plate and between the electrodes.

Claims

1. A touch sensitive device comprising:

a cancelation circuit capable of generating a cancelation signal based on parasitic capacitance affecting a self capacitance signal and coupling the cancelation signal to the self capacitance signal so as to cancel the parasitic capacitance from the self capacitance signal; and

a sense circuit capable of receiving the self capacitance signal with the parasitic capacitance canceled therefrom and a stimulation signal, and outputting a touch signal in response to the self capacitance signal with the parasitic capacitance canceled therefrom and the stimulation signal.

2. The device of claim 1, wherein the cancelation circuit comprises:

a voltage source capable of generating the cancelation signal to offset a parasitic current associated with the parasitic capacitance.

3. The device of claim 1, wherein the cancelation circuit comprises:

a voltage source capable of generating first and second voltage signals, the first voltage signal having a first phase and the second voltage signal having a second phase;

a phase adjustment circuit capable of receiving the first and second voltage signals, generating a third voltage signal from the first and second voltage signals, and adjusting a phase of the third voltage signal to be between the first and second phases; and

a magnitude adjustment circuit capable of receiving the third voltage signal, generating the cancelation signal from the third voltage signal, and adjusting a magnitude of the cancelation signal.

4. The device of claim 1, wherein the cancelation circuit comprises:

a first voltage source capable of generating first and second voltage signals, the first voltage signal having a first phase and the second voltage signal having a second phase;

a phase adjustment circuit capable of receiving the first and second voltage signals, generating the cancelation signal from the first and second voltage signals, and adjusting a phase of the cancelation signal to be between the first and second phases;

a second voltage source capable of generating the stimulation signal; and

a magnitude adjustment circuit capable of receiving the stimulation signal and adjusting a magnitude of the stimulation signal;

wherein the cancelation circuit is capable of coupling the phase-adjusted cancelation signal to the self capacitance signal, and

wherein the sense circuit is capable of receiving the magnitude-adjusted stimulation signal.

5. The device of claim 1, wherein the cancelation circuit comprises:

a first adjustment circuit capable of setting a phase of the cancelation signal to a global value; and

a second adjustment circuit capable of adjusting a phase of the stimulation signal.

6. The device of claim 1, wherein the cancelation circuit comprises:

a magnitude adjustment circuit capable of adjusting a magnitude of the cancelation signal; and

a phase adjustment circuit capable of adjusting a phase of the stimulation signal.

7. The device of claim 1, wherein the cancelation circuit comprises:

a voltage source capable of generating the cancelation signal to cancel the parasitic capacitance associated with a guard plate of the device.

8. The device of claim 1, wherein the cancelation circuit comprises:

a numerically controlled oscillator; and

a programmable push/pull current source configured to generate a cancellation signal.

9. The device of claim 1, wherein the sense circuit comprises:

a first buffer capable of receiving the self capacitance signal with the parasitic capacitance canceled therefrom and the stimulation signal, and outputting a buffer signal in response to the self capacitance signal and the stimulation signal;

a second buffer capable of receiving the buffer signal and outputting the touch signal in response to the buffer signal; and

a voltage source capable of powering the first and second buffers in a power-saving mode.

10. The device of claim 1, wherein the sense circuit comprises:

a first buffer capable of receiving the self capacitance signal with the parasitic capacitance canceled therefrom and the stimulation signal, and outputting a buffer signal in response to the self capacitance signal and the stimulation signal;

a second buffer capable of receiving the buffer signal and outputting the touch signal in response to the buffer signal;

a first voltage source capable of powering the first buffer at a first voltage; and

a second voltage source capable of powering the second buffer at a second voltage, the second voltage being substantially lower than the first voltage.

11. The device of claim 1, further comprising:

an amplifier coupled to the sense circuit and capable of supplying a voltage signal to the sense circuit to intermittently boost power to the sense circuit.

12. The device of claim 1, further comprising:

a touch panel capable of generating the self capacitance signal affected by the parasitic capacitance, the self capacitance signal indicative of an object proximate to the panel.

13. The device of claim 1, wherein the cancelation signal and the stimulation signal have independently programmable common mode voltage levels to shift a common mode level at the sense circuit substantially below an input common mode level of the sense circuit.

14. The device of claim 1 being at least one of a mobile phone, a digital player, or a portable computer.

15. A touch sensitive device comprising:

a cancelation circuit capable of generating a cancelation signal based on parasitic capacitance affecting a self capacitance signal, calibrating the cancelation signal, and coupling the calibrated cancelation signal to the self capacitance signal so as to cancel the parasitic capacitance on the self capacitance signal; and

a sense circuit capable of receiving the coupled calibrated cancelation and self capacitance signal and a stimulation signal and outputting an output signal in response to the coupled signal and the stimulation signal,

wherein the cancelation circuit calibrates the cancelation signal so as to minimize the output signal.

16. The device of claim 15, wherein the cancelation circuit comprises:

a voltage source capable of calibrating the cancelation signal by adjusting the cancelation signal to offset a parasitic current associated with the parasitic capacitance.

17. The device of claim 15, wherein the cancelation circuit comprises:

a phase adjustment circuit capable of calibrating the cancelation signal by adjusting a phase of the cancelation signal; and

a magnitude adjustment circuit capable of calibrating the cancelation signal by adjusting a magnitude of the cancelation signal.

18. The device of claim 15, wherein the cancelation circuit comprises:

a phase adjustment circuit capable of calibrating the cancelation signal by adjusting a phase of the cancelation signal; and

a magnitude adjustment circuit capable of calibrating the cancelation signal by adjusting a magnitude of the stimulation signal and causing the phase adjustment circuit to adjust the phase of the cancelation signal in response thereto.

19. The device of claim 15, wherein the cancelation circuit comprises:

a phase adjustment circuit capable of calibrating the stimulation signal by adjusting a phase of the stimulation signal and causing a phase of the cancelation signal to adjust in response thereto; and

a magnitude adjustment circuit capable of calibrating the stimulation signal by adjusting a magnitude of the stimulation signal and causing a magnitude of the cancelation signal to adjust in response thereto.

20. The device of claim 15, wherein the cancelation circuit comprises:

a voltage source capable of calibrating the cancelation signal by adjusting a voltage signal to a guard plate of the device so as to cancel the parasitic capacitance associated with the panel and the guard plate.

21. The device of claim 15, wherein the cancelation circuit comprises:

a voltage source capable of calibrating the cancelation signal by adjusting a voltage signal to a touch electrode of the panel so as to cancel the parasitic capacitance associated with the touch electrode.

22. A method for canceling effects of parasitic capacitance in a touch sensitive device, the method comprising:

generating a self capacitance signal indicative of an object proximate to the panel, the self capacitance signal being affected by a parasitic capacitance;

generating a cancelation signal at a cancelation circuit based on the parasitic capacitance;

coupling the cancelation signal with the self capacitance signal to cancel the parasitic capacitance therefrom; and

generating a touch signal indicative of the proximate object at a sense circuit in response to a first input of the self capacitance signal with the parasitic capacitance canceled therefrom and a second input of a stimulation signal.

23. The method of claim 22, further comprising:

calibrating the cancelation signal at the cancelation circuit based on the parasitic capacitance.

24. A touch sensing system comprising:

a cancelation circuit capable of generating a cancelation signal and using the cancelation signal to cancel parasitic capacitance from a self capacitance signal generated at a touch node; and

a receive circuit capable of receiving the self capacitance signal and a stimulation signal, and outputting a touch signal in response to the self capacitance signal and the stimulation signal.

25. The system of claim 25, wherein a phase of the stimulation signal is the same as a phase of the self capacitance signal;

and

wherein a magnitude of the stimulation signal is the same as a magnitude of the self capacitance signal.