Front-end signal compensation

• US 8,553,004 B2
• Filed: 10/28/2011
• Issued: 10/08/2013
• Est. Priority Date: 01/03/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Hotelling, Steve Porter, Land, Brian Richards
• Primary Examiner(s): Nguyen, Kevin M
• Assistant Examiner(s): NGUYEN, JENNIFER T
• Application Number:
  US13/284,732
• Publication Number:
  US 20120038581A1
• Time in Patent Office: 711 Days
• Field of Search: 345173-179, 178 1801- 1806
• US Class Current: 345/173
• CPC Class Codes: G06F 2203/04104 Multi-touch detection in digitiser, i.e. details about the simultaneous detection of a plurality of touching locations, e.g. multiple fingers or pen and finger
• 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 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: G06F 3/0445 using two or more layers of sensing electrodes, e.g. using two layers of electrodes separated by a dielectric layer
• 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

A touch surface device having improved sensitivity and dynamic range is disclosed. In one embodiment, the touch surface device includes a touch-sensitive panel having at least one sense node for providing an output signal indicative of a touch or no-touch condition on the panel; a compensation circuit, coupled to the at least one sense node, for generating a compensation signal that when summed with the output signal removes an undesired portion of the output signal so as to generated a compensated output signal; and an amplifier having an inverting input coupled to the output of the compensation circuit and a non-inverting input coupled to a known reference voltage.

Claims

1. A front-end signal compensation apparatus, comprising:

a compensation circuit coupled to a sense node, the compensation circuit configured for

receiving a sense signal indicative of a detected touch and including a static portion unaffected by the touch and a dynamic portion affected by the touch, and

generating a compensation signal that, when combined with the sense signal prior to further processing of the sense signal, substantially removes the static portion of the sense signal so as to generate a compensated sense signal; and

a sensing circuit coupled to the compensation circuit, the sensing circuit configured for receiving the compensated sense signal.

2. The front-end signal compensation apparatus of claim 1, wherein the sensing circuit comprises:

an amplifier having an inverting input coupled to the compensation circuit and a non-inverting input coupled to a known reference voltage; and

a feedback capacitor coupled between an output of the amplifier and the inverting input.

3. The front-end signal compensation apparatus of claim 2, wherein the compensation circuit comprises:

a combining circuit coupled to the sense node and the inverting input of the amplifier;

a digital-to-analog converter (DAC) for generating the compensation signal, the DAC coupled to the combining circuit; and

a look-up table, coupled to the DAC, for storing and providing a digital code to the DAC, wherein the compensation signal is generated based on the digital code.

4. The front-end signal compensation apparatus of claim 3, wherein the DAC comprises a digital-to-analog voltage converter (VDAC) and a capacitor coupled to the VDAC.

5. The front-end signal compensation apparatus of claim 3, wherein the DAC comprises a digital-to-analog current converter (IDAC).

6. The front-end signal compensation apparatus of claim 1, wherein an amplitude of the compensation signal is selected to be an average of a maximum and minimum amplitude of the sense signal, wherein the maximum amplitude corresponds to a detected “

no touch”

condition and the minimum amplitude corresponds to a detected “

max touch”

condition.

7. A computer system incorporating a front-end signal compensation apparatus, comprising:

a touch-sensitive device having a sense node configured for providing a sense signal indicative of a detected touch at the device, the sense signal including a static portion unaffected by the touch and a dynamic portion affected by the touch;

a compensation circuit coupled to the sense node and configured for generating a compensation signal for substantially offsetting the static portion of the sense signal prior to further processing of the sense signal so as to generate a compensated sense signal; and

a processor coupled to the compensation circuit and configured to perform one or more functions based on data within the compensated sense signal.

8. The system of claim 7, further comprising:

an amplifier coupled to the compensation circuit and configured to receive and amplify the compensated sense signal with a desired gain; and

an analog-to-digital converter (ADC) coupled to the amplifier and configured to convert an output of the amplifier into a digital signal, wherein the processor receives and processes the digital signal.

9. The system of claim 8, wherein the amplifier comprises:

a charge amplifier having an inverting input configured to receive the compensated sense signal and a non-inverting input connected to a known reference voltage; and

a feedback capacitor coupled to an output of the charge amplifier and the inverting input.

10. The system of claim 7, wherein the compensation circuit comprises:

a combining circuit coupled to the sense node and the inverting input of the charge amplifier;

a digital-to-analog converter (DAC) for generating the compensation signal, the DAC coupled to the combining circuit; and

a look-up table, coupled to the DAC, for storing and providing a digital code to the DAC, wherein the compensation signal is generated based on the digital code.

11. The system of claim 10, wherein the DAC comprises a digital-to-analog voltage converter (VDAC) and a capacitor coupled to the VDAC.

12. The system of claim 10, wherein the DAC comprises a digital-to-analog current converter (IDAC).

13. A method of compensating a sense signal generated by a touch-sensitive device, the method comprising:

generating the sense signal at a sense node of the touch-sensitive device, the sense signal including a static portion unaffected by a touch and a dynamic portion affected by the touch;

generating a compensation signal for offsetting the static portion of the sense signal;

combining the sense signal with the compensation signal prior to further processing of the sense signal to generate a compensated sense signal and

amplifying the compensated sense signal with a desired gain.

14. The method of claim 13, wherein amplifying the compensated sense signal comprises applying the compensated sense signal to an inverting input of a charge amplifier and applying a known reference voltage to a non-inverting input of the charge amplifier.

15. The method of claim 13, further comprising:

storing at least one digital code in a look-up table;

providing the at least one digital code to a digital-to-analog converter (DAC), wherein the compensation signal is generated by the DAC based on the at least one digital code.

16. The method of claim 15, wherein the compensation signal comprises a periodic charge waveform and the DAC comprises an IDAC that generates a periodic current waveform.

17. The method of claim 13, wherein the sense signal has a maximum amplitude in a first state and a minimum amplitude in a second state and the compensation signal is generated so as to have an amplitude which is an average of the maximum and minimum amplitudes of the sense signal.

18. The method of claim 17, wherein the first state represents a “

no touch”

condition associated with the sense node and the second state represents a “

max touch”

condition associated with the sense node.

19. A method of compensating for an undesired portion of an output signal generated by a touch-sensitive device, the method comprising:

receiving a periodic sense signal from a sense node of the touch-sensitive device, the periodic sense signal including a static portion unaffected by a touch and a dynamic portion affected by the touch;

generating a periodic compensation signal having a substantially similar waveform as the periodic sense signal but substantially 180 degrees out of phase with the periodic sense signal;

combining the periodic sense signal and the periodic compensation signal prior to further processing of the periodic sense signal so as to generate a compensated sense signal, wherein the compensated sense signal represents the periodic sense signal with the static portion subtracted; and

providing the compensated sense signal to a charge amplifier.

20. The method of claim 19, wherein the periodic compensation signal is generated by a digital to analog voltage converter (VDAC) and a capacitor coupled to the VDAC.

21. The method of claim 19, wherein the periodic compensation signal is generated by a digital to analog current converter (IDAC).

22. The method of claim 19, wherein the periodic compensation signal has a peak-to-peak amplitude substantially equal to an average of a maximum and minimum peak-to-peak amplitude of the periodic sense signal.

23. A computer system utilizing a touch-sensitive device, comprising:

means for generating a sense signal at a sense node of the touch-sensitive device, the sense signal including a static portion unaffected by a touch and a dynamic portion affected by the touch;

means for generating a compensation signal for offsetting the static portion of the sense signal;

means for combining the sense signal with the compensation signal prior to further processing of the sense signal to generate a compensated sense signal; and

means for amplifying the compensated sense signal with a desired gain.

24. The system of claim 23, further comprising:

means for storing at least one digital code;

means for generating the compensation signal based on the at least one digital code.

25. A circuit for compensating a sense signal generated by a touch-sensitive device, comprising:

means for receiving a periodic sense signal from a sense node of the touch-sensitive device, the sense signal including a static portion unaffected by a touch and a dynamic portion affected by the touch;

means for generating a periodic compensation signal having a substantially similar waveform as the periodic sense signal but substantially 180 degrees out of phase with the periodic sense signal;

means for combining the periodic sense signal and the periodic compensation signal prior to further processing of the sense signal so as to generate a compensated sense signal, wherein the compensated sense signal represents the periodic sense signal with the static portion subtracted; and

means for amplifying the compensated sense signal with a desired gain.