METHOD AND APPARATUS FOR IMPROVING DYNAMIC RANGE OF A TOUCHSCREEN CONTROLLER

• US 20110261005A1
• Filed: 01/07/2011
• Published: 10/27/2011
• Est. Priority Date: 04/22/2010
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Chan, Patrick, Joharapurkar, Ashutosh Ravindra
• Granted Patent: US 8,599,167 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• 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: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/96062 with tactile or haptic feedback

Abstract

A touchscreen system for increasing the dynamic range of the system comprising a touchscreen coupled to an offset cancellation element and a capacitance measuring element. The offset cancellation element is configured to be dynamically changed in capacitance such that it offsets parasitic and sensor capacitances of the touchscreen sensors thereby leaving only touch event capacitance to be measured by the measuring element. The offset cancellation element is able to adjust to the initial unwanted capacitances of each sensor as well as dynamically adjust to changes in the unwanted capacitance due to the environment. In some embodiments, the offset cancellation element is a capacitance digital-to-analog converter that is controlled by a controller for offsetting the unwanted capacitance. As a result, the touchscreen system is able to utilize a small integrating capacitor thereby lowering cost and improving the dynamic range of the system.

Claims

1. A capacitance-to-voltage converter for use with a capacitive touchscreen having one or more capacitance sensors, the converter comprising:

a. a capacitance offset cancellation element coupled for alleviating effects of parasitic capacitance on signals produced by one or more of the capacitance sensors; and

b. a analog-to-digital converter coupled to the offset cancellation element for converting the signals to digital signals.

2. The converter of claim 1 further comprising a controller coupled to the offset cancellation element for dynamically adjusting the extent to which the offset cancellation element alleviates the effects of the parasitic capacitance based on the digital signals.

3. The converter of claim 1 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

4. The converter of claim 1 further comprising a measuring element coupled between the offset cancellation element and the analog-to-digital converter, wherein the measuring element comprises a integrating capacitor with a capacitance of less than 20 pico-Farads.

5. The converter of claim 2 wherein an offset value of the offset cancellation element is initially set to zero.

6. The converter of claim 5 wherein the controller incrementally increases the offset value from the initial zero value until the digital signals reach a predetermined value.

7. The converter of claim 2 wherein an offset value of the offset cancellation element is initially set to a maximum value such that all of the effects of the parasitic capacitance are alleviated.

8. The converter of claim 7 wherein the controller incrementally decreases the offset value from the initial maximum value until the digital signals reach a predetermined value.

9. The converter of claim 2 wherein the controller averages the value of the digital signals to determine a baseline value.

10. The converter of claim 9 wherein if the baseline reaches a threshold value, the controller adjusts an offset value of the offset cancellation element by a delta value such that the baseline is moved away from the threshold value.

11. The converter of claim 10 wherein the threshold value is an upper threshold and the offset value is increased by the delta value such that the baseline drops below the upper threshold.

12. The converter of claim 10 wherein the threshold value is a lower threshold and the offset value is decreased by the delta value such that the baseline climbs above the lower threshold.

13. The converter of claim 9 wherein when calculating the baseline value, the controller disregards the values of the digital signals when a touch event causes the values to suddenly increase and/or drop for a duration that is less than a predetermined touch period.

14. The converter of claim 10 wherein the controller adjusts the value of the digital signals by the delta value.

15. A touchscreen system comprising:

a. a capacitive touch screen having one or more capacitance sensors; and

b. a capacitance-to-voltage converter comprising;

i. a capacitance offset cancellation element coupled for alleviating effects of parasitic capacitance on signals produced by one or more of the capacitance sensors; and

ii. a analog-to-digital converter coupled to the offset cancellation element for converting the signals to digital signals.

16. The system of claim 15 further comprising a controller coupled to the offset cancellation element for dynamically adjusting the extent to which the offset cancellation element alleviates the effects of the parasitic capacitance based on the digital signals.

17. The system of claim 15 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

18. The system of claim 15 further comprising a measuring element coupled between the offset cancellation element and the analog-to-digital converter, wherein the measuring element comprises a integrating capacitor with a capacitance of less than 20 pico-Farads.

19. The system of claim 16 wherein an offset value of the offset cancellation element is initially set to zero.

20. The system of claim 19 wherein the controller incrementally increases the offset value from the initial zero value until the digital signals reach a predetermined value.

21. The system of claim 16 wherein an offset value of the offset cancellation element is initially set to a maximum value such that all of the effects of the parasitic capacitance are alleviated.

22. The system of claim 21 wherein the controller incrementally decreases the offset value from the initial maximum value until the digital signals reach a predetermined value.

23. The system of claim 16 wherein the controller averages the value of the digital signals in order to determine a baseline value.

24. The system of claim 23 wherein if the baseline reaches a threshold value, the controller adjusts an offset value of the offset cancellation element by a delta value such that the baseline is moved away from the threshold value.

25. The system of claim 24 wherein the threshold value is an upper threshold and the offset value is increased by the delta value such that the baseline drops below the upper threshold.

26. The system of claim 24 wherein the threshold value is a lower threshold and the offset value is decreased by the delta value such that the baseline climbs above the lower threshold.

27. The system of claim 23 wherein when calculating the baseline value, the controller disregards the values of the digital signals when a touch event causes the values to suddenly increase and/or drop for a duration that is less than a predetermined touch period.

28. The system of claim 24 wherein the controller adjusts the value of the digital signals by the delta value.

29. A method of improving dynamic range of a capacitance-to-voltage converter for use with a touchscreen having one or more capacitance sensors, the method comprising:

a. determining an average digital output over a period with a controller, wherein the average digital output is associated with a sensor of the converter; and

b. adjusting an offset value produced by an offset cancellation element of the converter until the digital output of the converter is equal to a predetermined baseline value.

30. The method of claim 29 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

31. The method of claim 30 further comprising initially setting the offset value to zero.

32. The method of claim 31 further comprising incrementally increasing the offset value from the initial zero value until the digital output equals the predetermined baseline value.

33. The method of claim 30 further comprising initially setting the offset value to a maximum value.

34. The method of claim 33 further comprising incrementally decreasing the offset value from the initial maximum value until the digital output equals the predetermined baseline value.

35. The method of claim 30 wherein the adjusting is performed periodically, in response to a user command and/or upon the startup of the touchscreen.

36. The method of claim 29 further comprising:

a. determining a second average digital output over a period with a second measurement element, wherein the second average digital output is associated with a second sensor of the converter;

b. adjusting a second offset value produced by a second offset cancellation element of the converter until the second digital output of the converter is equal to a second predetermined baseline value; and

c. obtaining a differential measurement between the first digital output as adjusted by the first offset value and the second digital output as adjusted by the second offset value.

37. The method of claim 29 wherein the offset cancellation element is sufficiently accurate such that the adjustment is transparent to a user.

38. A method of improving dynamic range in a capacitance-to-voltage converter for use with a touchscreen having one or more capacitance sensors, the method comprising:

a. averaging a digital output associated with a sensor of the converter in order to determine a baseline value;

b. determining a baseline threshold value; and

c. dynamically adjusting an offset value of an offset cancellation element of the converter by a delta value if the baseline value reaches the baseline threshold value such that the baseline value is distanced from the threshold value.

39. The method of claim 38 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

40. The method of claim 39 wherein adjusting the offset value causes the capacitance of the capacitive digital-to-analog converter to increase or decrease.

41. The method of claim 38 wherein the baseline threshold value is an upper threshold of the converter and dynamic adjusting increases the offset value by the delta value such that the baseline value drops below the upper threshold.

42. The method of claim 38 wherein the baseline threshold value is a lower threshold and the dynamic adjusting decreases the offset value by the delta value such that the baseline value climbs above the lower threshold.

43. The method of claim 38 wherein the averaging the digital output comprises disregarding the values of the digital output when a touch event causes the values of the digital output to increase and drop in a time period that is less than a predetermined touch period.

44. The method of claim 38 wherein the baseline threshold value is determined based on an upper or lower limit of a measuring element of the converter.

45. The method of claim 39 further comprising:

a. averaging a second digital output associated with a second sensor of the converter in order to determine a second baseline value;

b. determining a second baseline threshold value;

c. dynamically adjusting a second offset value of a second offset cancellation element of the converter by a second delta value if the second baseline value reaches the second baseline threshold value such that the second baseline value is distanced from the second threshold value; and

d. obtaining a differential measurement between the first digital output as adjusted by the first offset value and the second digital output as adjusted by the second offset value.

46. The method of claim 38 wherein the offset cancellation element is sufficiently accurate such that the dynamic adjustment is transparent to a user.