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Generation of Current Pulses for Capacitive Offset Cancellation

IP.com Disclosure Number: IPCOM000242042D
Publication Date: 2015-Jun-15
Document File: 4 page(s) / 295K

Publishing Venue

The IP.com Prior Art Database

Related People

Adam Schwartz: INVENTOR [+5]

Abstract

As displays get larger and thinner, the amount of background capacitance increases. Removing this background capacitance is necessary for good touch and hover performance. The removal of background capacitance amounts to sourcing or sinking a fixed amount of charge equal to CV where C is the background capacitance and V is the amplitude of the sensing signal. A fixed amount of charge can be amplified in a current mirror to obviate the need for a large capacitor.

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Page 01 of 4

Generation of Current Pulses for Capacitive Offset Cancellation

Generation of Current Pulses for Capacitive Offset Cancellation


1. Inventor(s): Adam Schwartz, Zheming Li, Tae-Song Chung, Chunbo Liu, and Steve Lo

2. Synaptics Incorporated, Santa Clara, CA, USA


3. Short Summary

As displays get larger and thinner, the amount of background capacitance increases. Removing this background capacitance is necessary for good touch and hover performance. The removal of background capacitance amounts to sourcing or sinking a fixed amount of charge equal to CV where C is the background capacitance and V is the amplitude of the sensing signal. A fixed amount of charge can be amplified in a current mirror to obviate the need for a large capacitor, as indicated below in figure
1.

Figure 1. Current amplification in generating a large amount of charge

However, this approach has some potential problems in which the charge is concentrated in an impulse of current. First of all, the subsequent stages of current amplification circuitry require a very high gain- bandwidth product to accurately amplify that current impulse. Secondly, due to the aforementioned challenge in accurate amplification, the high surge current makes precise matching between the sink and source current sources difficult. Thirdly, this high surge current creates ripples on the power supply. These ripples in the power supply could introduce unexpected noise in the overall system when it is shared with other precision analog blocks such as ADC.


4. Some Problems Solved

Examples of some of the problems addressed by the invention include generating a fixed amount of charge to provide a more accurate and less noisy method for charge subtraction. Such methods and system implementing such methods have the ability to provide improved touch and proximity sensing.

Copyright © 2015 Synaptics Incorporated, All Rights Reserved.

                                                                 Page: 1 of 4 Information contained in this publication is provided as-is, with no express or implied warranties, including any warranty of merchantability, fitness for any particular purpose, or non-infringement. Synaptics Incorporated assumes no liability whatsoever for any use of the information contained herein, including any liability for intellectual property infringement. This publication conveys no express or implied licenses to any intellectual property rights belonging to Synaptics or any other party. Synaptics may, from time to time and at its sole option, update the information contained herein without notice.

Adam Schwartz

1:M


Page 02 of 4

Generation of Current Pulses for Capacitive Offset Cancellation


5. General Description

First Embodiment - DAC voltage ramp into capacitor

Figure 2 illustrates an embodiment comprising a buffered DAC.

R C

I

Figure 2: DAC ramp generation.

The DAC voltage ramps create current pulses at the output with amplitude ΔV*C. The delivered charge is


(1) Q = ΔV*C*T

Some of the advantages of this design include:

 Can control the si...