Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Increasing Charging Voltage to Improve Sensitivity of Self-Capacitance Measurements

IP.com Disclosure Number: IPCOM000232428D
Publication Date: 2013-Nov-07
Document File: 24 page(s) / 1M

Publishing Venue

The IP.com Prior Art Database

Abstract

Some embodiments may use a sensing method, which may be referred to as the current source method, in which the delta capacitance is determined by charging one or more electrodes of a touch sensor to a first reference voltage, removing a predetermined amount of charge, and measuring the resulting voltage or charge relative to a second reference voltage. Using higher voltages (e.g., 15V or more) during the charging sequence may provide improved sensitivity and immunity to external noises. Furthermore, flying capacitors are used to isolate higher voltage portions of the device from lower voltage portions, which may protect low voltage measurement circuitry from being overloaded by the higher voltages used to charge the touch sensor. Some embodiments may also provide improved sensitivity by measuring the voltage of the capacitive node relative to the reference voltage of the integrator. The measured capacitor may be charged and discharged to bring its voltage to a level substantially similar to the reference voltage of the differential integrator, which may be significantly lower than the initial charging voltage. The voltage change caused by the delta capacitance (which may be relatively small compared to the charging voltage) may therefore register a larger proportional change when measured relative to the lower reference voltage of the integrator. Furthermore, using two different reference voltages for charging and integration may suppress or cancel out stray capacitance. Some embodiments may also use sequential measurements with opposite polarities, which may cancel low frequency noise that might otherwise distort the measured signal.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 8% of the total text.

Page 01 of 24

Increasing Charging Voltage to Improve Sensitivity of Self-Capacitance Measurements

Introduction

    A touch sensor may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) within a touch-sensitive area of the touch sensor overlaid on a display screen, for example. In a touch- sensitive-display application, the touch sensor may enable a user to interact directly with what is displayed on the screen, rather than indirectly with a mouse or touch pad. There are a number of different types of touch sensors, such as (for example) resistive touch sensors, surface acoustic wave touch sensors, and capacitive touch sensors. When an object touches or comes within proximity of the surface of the capacitive touch screen, a change in capacitance may occur within the touch screen at the location of the touch or proximity. This change in capacitance may be referred to as the "delta capacitance," and a controller may process the delta capacitance to determine the object's position relative to the touch screen.

    Certain touch sensors may also detect the presence and/or position of an object hovering near the touch sensor, which may be referred to as "hover detection." Hover detection may be improved by providing more sensitive measurements and reducing the touch sensor's sensitivity to external noises so that relatively small changes in capacitance caused by a hovering object can be more reliably detected. For example, certain touch sensors may provide improved hover detection by taking measurements that cancel out the constant stray capacitance over which the relatively small change in capacitance is detected.

    Capacitive touch sensors may utilize mutual capacitance sensing methods and/or self-capacitance sensing methods. In mutual capacitance implementations, touch sensors may utilize drive and sense electrodes. A drive electrode and a sense electrode may form a capacitive node. The drive and sense electrodes forming the capacitive node may come near each other, but not make electrical contact with each other. Instead, the drive and sense electrodes may be capacitively coupled to each other across a space between them. A pulsed or alternating voltage applied to 1


Page 02 of 24

the drive electrode may induce a charge on the sense electrode, and the amount of charge induced may be susceptible to external influence (such as a touch or the proximity of an object). When an object touches or comes within proximity of the capacitive node, a change in capacitance may occur at the capacitive node, and this "delta capacitance" may be measured to determine the position of the object.

    In touch sensors using self-capacitance sensing methods, a capacitive node may correspond to a single electrode, or a set of multiple connected electrodes. When an object touches or comes within proximity of the capacitive node, a change in self-capacitance may occur at the capacitive node, and this delta capacitance may b...