Front-end signal compensation

• US 20080158178A1
• Filed: 01/03/2007
• Published: 07/03/2008
• Est. Priority Date: 01/03/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Hotelling, Steve Porter, Land, Brian Richards
• Granted Patent: US 8,049,732 B2
• Time in Patent Office: Days
• 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 touch surface device, comprising:

a touch-sensitive panel having a sense node for providing an output signal indicative of a touch or no-touch condition on the panel;

a compensation circuit, coupled to the 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 generate a compensated output signal; and

a sensing circuit coupled to the compensation circuit for receiving the compensated output signal.

2. The touch surface device of claim 1 wherein the sensing circuit comprises:

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; and

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

3. The touch surface device of claim 1 wherein the compensation circuit comprises:

a summing circuit having first and second inputs and an output, wherein the first input is coupled to the sense node and the output is coupled to the inverting input of the amplifier;

a digital-to-analog converter (DAC) for generating the compensation signal, the DAC having an output coupled to the second input of the summing circuit; and

a look-up table, coupled to an input of 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 touch surface device of claim 3 wherein the DAC comprises a digital-to-analog voltage converter (VDAC) and a capacitor coupled to an output of the VDAC.

5. The touch surface device of claim 3 wherein the DAC comprises a digital-to-analog current converter (IDAC).

6. The touch surface device of claim 1 wherein the compensation circuit comprises:

an inverter for inverting a drive signal applied to the touch-sensitive panel so as to generate an inverted drive signal;

at least one non-touch-sensitive node within the touch-sensitive panel, wherein the inverted drive signal is applied to the at least one non-touch-sensitive node, thereby generating the compensation signal at the at least one non-touch-sensitive node; and

a sense electrode coupled to the sense node and the at least one non-touch-sensitive node, wherein the sense electrode provides the compensated output signal to the sensing circuit.

7. The touch surface device of claim 6 wherein the sensing circuit comprises:

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; and

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

8. The touch surface device of claim 6 wherein the sense node comprises a mutually capacitive node formed at an intersection between a drive electrode and the sense electrode and the at least one non-touch-sensitive node is formed at an intersection of a compensation electrode, coupled to an output of the inverter, and the sense electrode.

9. The touch surface device of claim 8 wherein the at least one non-touch-sensitive node comprises a plurality of non-touch-sensitive nodes formed along opposite edges of the touch-sensitive panel.

10. The touch surface device of claim 8 wherein the at least one non-touch-sensitive node is configured to provide a total mutual compensation capacitance (C_COMP) that is equal to an average of a maximum and minimum mutual capacitance (C_SIG) provided by the sense node, wherein the maximum mutual capacitance corresponds to a “

no touch”

condition and the minimum mutual capacitance corresponds to a “

max touch”

condition at the sense node.

11. The touch surface device of claim 1 wherein an amplitude of the compensation signal is selected to be an average of a maximum and minimum amplitude of output signal provided by the sense node, wherein the maximum amplitude corresponds to a “

no touch”

condition and the minimum amplitude corresponds to a “

max touch”

condition at the sense node.

12. A computer system incorporating a touch surface device, comprising:

a touch-sensitive panel comprising at least one sense node configured to generate an output signal indicative of touch or no-touch condition on the panel;

a compensation circuit coupled to the at least one sense node and configured to generate a compensation signal for offsetting an undesired portion of the analog output signal so as to generate a compensated output signal; and

a processor coupled to compensation circuit and configured to receive and process the compensated output signal and perform one or more functions based on data provided by the compensated output signal.

13. The system of claim 12 further comprising:

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

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

14. The system of claim 13 wherein the amplifier comprises:

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

a feedback capacitor having one lead connect to an output of the charge amplifier and another lead connected to the inverting input.

15. The system of claim 14 wherein the compensation circuit comprises:

at least one summing circuit having first and second inputs and an output, wherein the first input is coupled to the at least one sense node and the output is coupled to the inverting input of the amplifier;

a digital-to-analog converter (DAC) for generating the compensation signal, the DAC having an output coupled to the second input of the summing circuit; and

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

16. The system of claim 15 wherein the DAC comprises a digital-to-analog voltage converter (VDAC) and a capacitor coupled to an output of the VDAC.

17. The system of claim 15 wherein the DAC comprises a digital-to-analog current converter (IDAC).

18. The system of claim 12 wherein the compensation circuit comprises:

an inverter for inverting a drive signal applied to the touch-sensitive panel so as to generate an inverted drive signal;

at least one non-touch-sensitive node within the touch-sensitive panel, wherein the inverted drive signal is applied to the at least one non-touch-sensitive node, thereby generating the compensation signal at the at least one non-touch-sensitive node; and

a sense electrode coupled to the at least one sense node and the at least one non-touch-sensitive node, wherein the sense electrode provides the compensated output signal to the sensing circuit.

19. The system of claim 18 wherein the at least one sense node comprises a mutually capacitive node formed at an intersection between at least one drive electrode and the sense electrode and the at least one non-touch-sensitive node is formed at an intersection of at least one compensation electrode, coupled to an output of the inverter, and the sense electrode.

20. The system of claim 19 wherein the at least one non-touch-sensitive node is configured to provide a total mutual compensation capacitance (C_COMP) that is equal to an average of a maximum and minimum mutual capacitance (C_SIG) provided by the sense node, wherein the maximum mutual capacitance corresponds to a “

no touch”

condition and the minimum mutual capacitance corresponds to a “

max touch”

condition at the sense node.

21. A method of compensating an output signal generated by a touch surface device, the method comprising:

generating the output signal at a sense node of the touch surface device;

generating a compensation signal for offsetting an undesired portion of the output signal;

summing the output signal with the compensation signal to generate a compensated signal; and

amplifying the compensated signal with a desired gain.

22. The method of claim 21 wherein the act of amplifying comprises applying the compensated signal to an inverting input of a charge amplifier and applying a known reference voltage to a non-inverting input of the charge amplifier.

23. The method of claim 21 wherein the sense node comprises a capacitive sensing node formed at an intersection between a drive electrode and a sense electrode.

24. The method of claim 21 wherein the output signal comprises a periodic output signal having a predetermined frequency and the compensation signal is generated so as to have a frequency substantially equal to the predetermined frequency but substantially 180 degrees out of phase with the periodic output signal.

25. The method of claim 21 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.

26. The method of claim 25 wherein the compensation signal comprises a periodic charge waveform and the DAC comprises a VDAC, the method further comprising providing an output signal of the VDAC to a compensation capacitor coupled to an output of the VDAC.

27. The method of claim 25 wherein the compensation signal comprises a periodic charge waveform and the DAC comprises an IDAC that generates a periodic current waveform.

28. The method of claim 21 further comprising:

providing a first periodic drive signal to a drive electrode of the touch surface device so as to generate a periodic output signal at the sense node;

providing a second periodic drive signal to at least one non-touch-sensitive electrode corresponding to at least one non-touch-sensitive node of the touch surface device so as to generate at least one periodic compensation signal at the at least one non-touch-sensitive node, wherein the second periodic drive signal is generated by inverting the first periodic signal; and

coupling a sense electrode to the sensing node and the at least one non-touch-sensitive node, wherein the sense electrode provides the compensated signal comprising the sum of the periodic output signal and the at least one periodic compensation signal to an amplifier.

29. The method of claim 21 wherein the output 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 output signal.

30. The method of claim 29 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.

31. A method of compensating for an undesired portion of an output signal generated by an electronic device, the method comprising:

receiving a periodic output signal from an output node of the electronic device;

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

summing the periodic output signal and the periodic compensation signal so as to generate a compensated signal, wherein the compensated signal substantially represents the periodic output signal with an undesired portion subtracted from the periodic output signal;

providing the compensated signal to a first input of an amplifier; and

providing a known reference voltage to a second input of the amplifier.

32. The method of claim 31 wherein the electronic device comprises a touch surface device, the output node comprises a capacitive sensing node and the periodic output signal is generated by the capacitive sensing node.

33. The method of claim 32 wherein the periodic compensation signal is generated by at least one non-touch-sensitive node on the touch surface device, wherein a first periodic drive signal applied to the capacitive sensing node is substantially 180 degrees out of phase with a second drive signal applied to the at least one non-touch sensitive node.

34. The method of claim 33 wherein the touch surface device comprises a plurality of electrode rows and a plurality of electrode columns oriented substantially orthogonally with respect to the electrode rows, each intersection of an electrode row with a respective electrode column comprising a capacitive sensing node, wherein a change in magnitude of the periodic output signal represents a touch condition associated with at least one capacitive sensing node.

35. The method of claim 31 wherein the periodic compensation signal is generated by a digital to analog voltage converter (VDAC) and a capacitor coupled to an output of the VDAC.

36. The method of claim 31 wherein the periodic compensation signal is generated by a digital to analog current converter (IDAC).

37. The method of claim 31 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 output signal.

38. A computer system utilizing a touch surface device, comprising:

means for generating the output signal at a sense node of the touch surface device;

means for generating a compensation signal for offsetting an undesired portion of the output signal;

means for summing the output signal with the compensation signal to generate a compensated signal; and

means for amplifying the compensated signal with a desired gain.

39. The system of claim 38 further comprising:

means for storing at least one digital code;

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

40. The system of claim 38 further comprising:

means for providing a first periodic drive signal to a drive electrode of the touch surface device so as to generate a periodic output signal at the sense node;

means for providing a second periodic drive signal to at least one non-touch-sensitive electrode corresponding to at least one non-touch-sensitive node of the touch surface device so as to generate at least one periodic compensation signal at the at least one non-touch-sensitive node, wherein the second periodic drive signal is generated by inverting the first periodic signal; and

means for coupling a sense electrode to the sensing node and the at least one non-touch-sensitive node, wherein the sense electrode provides the compensated signal comprising the sum of the periodic output signal and the at least one periodic compensation signal to an amplifier.

41. A circuit for compensating an output signal generated by an electronic device, comprising:

means for receiving a periodic output signal from an output node of the electronic device;

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

means for summing the periodic output signal and the periodic compensation signal so as to generate a compensated signal, wherein the compensated signal substantially represents the periodic output signal with an undesired portion subtracted from the periodic output signal; and

means for amplifying the compensated signal with a desired gain.