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9542G–AT42–03/10

AT42QT1011

2.

Overview of the AT42QT1011

2.1

Introduction

of detecting near-proximity or touch, making it ideal for implementing touch controls.

With the proper electrode and circuit design, the self-contained digital IC will project a touch or

proximity field to several centimeters through any dielectric like glass, plastic, stone, ceramic,

and even most kinds of wood. It can also turn small metal-bearing objects into intrinsic sensors,

making them responsive to proximity or touch. This capability, coupled with its ability to

self-calibrate, can lead to entirely new product concepts.

The QT1011 is designed specifically for human interfaces, like control panels, appliances, toys,

lighting controls, or anywhere a mechanical switch or button may be found. It includes all

hardware and signal processing functions necessary to provide stable sensing under a wide

variety of changing conditions. Only a single low-cost capacitor is required for operation.

2.2

Basic Operation

Figure 1-1 on page 2 shows a basic circuit.

The QT1011 employs bursts of charge-transfer cycles to acquire its signal. Burst mode permits

power consumption in the microamp range, dramatically reduces RF emissions, lowers

susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally

processed to reject impulse noise, using a “consensus” filter which requires four consecutive

confirmations of a detection before the output is activated.

The QT switches and charge measurement hardware functions are all internal to the QT1011.

2.3

Electrode Drive

For optimum noise immunity, the electrode should only be connected to SNSK.

In all cases the rule Cs>>Cx must be observed for proper operation; a typical load capacitance

(Cx) ranges from 5-20 pF while Cs is usually about 2-50 nF.

Increasing amounts of Cx destroy gain, therefore it is important to limit the amount of stray

capacitance on both SNS terminals. This can be done, for example, by minimizing trace lengths

and widths and keeping these traces away from power or ground traces or copper pours.

The traces and any components associated with SNS and SNSK will become touch sensitive

and should be treated with caution to limit the touch area to the desired location.

A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress ESD and

EMC effects.

2.4

Sensitivity

2.4.1

Introduction

The sensitivity on the QT1011 is a function of things like the value of Cs, electrode size and

capacitance, electrode shape and orientation, the composition and aspect of the object to be

sensed, the thickness and composition of any overlaying panel material, and the degree of

ground coupling of both sensor and object.