Method and apparatus for improving dynamic range of a touchscreen controller

• US 8,599,167 B2
• Filed: 01/07/2011
• Issued: 12/03/2013
• Est. Priority Date: 04/22/2010
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Joharapurkar, Ashutosh Ravindra, Chan, Patrick
• Primary Examiner(s): Sharifi-Tafreshi, Koosha
• Application Number:
  US12/986,776
• Publication Number:
  US 20110261005A1
• Time in Patent Office: 1,061 Days
• Field of Search: 345/173, 345/174, 178 1801- 1811
• 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 configured to be utilized by a capacitive touchscreen having one or more capacitance sensors, the converter comprising:

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

a analog-to-digital converter coupled to the offset cancellation element for converting the signals to digital signals; and

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, wherein the controller averages the value of the digital signals to determine a baseline value, the controller configured to adjust an offset value of the offset cancellation element by a delta value such that the baseline is moved away from a threshold value when the baseline reaches the threshold value.

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

3. 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.

4. The converter of claim 1, wherein an offset value of the offset cancellation element is initially set to zero.

5. The converter of claim 4 wherein the controller incrementally increases the offset value from the initial zero value until the digital signals reach a predetermined value.

6. The converter of claim 1, 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.

7. The converter of claim 6 wherein the controller incrementally decreases the offset value from the initial maximum value until the digital signals reach a predetermined value.

8. The converter of claim 1, 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.

9. The converter of claim 1, 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.

10. The converter of claim 1, wherein when calculating the baseline value, the controller disregards the values of the digital signals when a touch event causes the values to at least one of an increase or a drop for a duration that is less than a predetermined touch period.

11. The converter of claim 1, wherein the controller adjusts the value of the digital signals by the delta value.

12. A touchscreen system comprising:

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

a capacitance-to-voltage converter comprising;

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

a analog-to-digital converter coupled to the offset cancellation element for converting the signals to digital signals; and

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, wherein an offset value of the offset cancellation element is initially set to at least one of zero or a maximum value such that all of the effects of the parasitic capacitance are alleviated when the offset value is initially set to the maximum value, wherein the controller incrementally increases the offset value from the initial zero value until the digital signals reach a predetermined value when the offset value is initially set to zero or the controller incrementally decreases the offset value from the initial maximum value until the digital signals reach a predetermined value when the offset value is initially set to the maximum value.

13. The system of claim 12 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

14. The system of claim 12 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.

15. The system of claim 12, wherein the controller averages the value of the digital signals in order to determine a baseline value.

16. The system of claim 15, wherein 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 when the baseline reaches a threshold value.

17. The system of claim 16 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.

18. The system of claim 16 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.

19. The system of claim 15, wherein when calculating the baseline value, the controller disregards the values of the digital signals when a touch event causes the values to at least one of an increase or a drop for a duration that is less than a predetermined touch period.

20. The system of claim 16 wherein the controller adjusts the value of the digital signals by the delta value.

21. A method of improving dynamic range of a capacitance-to-voltage converter utilized by a touchscreen having one or more capacitance sensors, the method comprising:

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

at least one of initially setting an offset value of a capacitance offset cancellation element to zero or to a maximum value;

adjusting the offset value produced by the capacitance offset cancellation element of the converter until the digital output of the converter is equal to a predetermined baseline value, wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter; and

at least one of incrementally increasing the offset value from the initial zero value until the digital output equals the predetermined baseline value when the offset value is initially set to zero or incrementally decreasing the offset value from the initial maximum value until the digital output equals the predetermined baseline value when the offset value is initially set to the maximum value.

22. The method of claim 21, wherein the adjusting is performed periodically, in response to at least one of a user command and/or upon a startup of the touchscreen.

23. The method of claim 21 further comprising:

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;

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

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.

24. A method of improving dynamic range in a capacitance-to-voltage converter utilized by a touchscreen having one or more capacitance sensors, the method comprising:

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

determining a baseline threshold value; and

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

25. The method of claim 24 wherein the capacitance offset cancellation element is a capacitive digital-to-analog converter.

26. The method of claim 25 wherein adjusting the offset value causes the capacitance of the capacitive digital-to-analog converter to increase or decrease.

27. The method of claim 24 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.

28. The method of claim 24 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.

29. The method of claim 24 wherein the baseline threshold value is determined based on an upper or lower limit of a measuring element of the converter.

30. The method of claim 25 further comprising:

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

determining a second baseline threshold value;

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

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.