Method and apparatus for improving dynamic range of a touchscreen controller

• US 8,830,207 B1
• Filed: 11/04/2013
• Issued: 09/09/2014
• Est. Priority Date: 04/22/2010
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Joharapurkar, Ashutosh R., Chan, Patrick, Allen, David L., Viswanathan, Natarajan
• Primary Examiner(s): Sharifi-Tafreshi, Koosha
• Application Number:
  US14/070,955
• Time in Patent Office: 309 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 for 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;

an 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 zero, wherein the controller incrementally increases the offset value from the initial zero value until the digital signals reach a predetermined value.

2. 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 an integrating capacitor with a capacitance of less than 20 pico-Farads.

3. The converter of claim 1, wherein the controller averages the value of the digital signals to determine a baseline value.

4. The converter of claim 3, wherein when 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.

5. 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;

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

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 in order to determine a baseline value, wherein when 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.

6. The system of claim 5, 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.

7. The system of claim 5, 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.

8. A method of improving dynamic range in a capacitance-to-voltage converter for 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 when the baseline value reaches the baseline threshold value such that the baseline value is distanced from the threshold value.

9. The method of claim 8, 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.

10. The method of claim 8, 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 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.