Datasheet

Figure 2-1. Sense Connections

VDD

TIME

VSS

OUT

VDD

SNSK

SNS

SENSE

ELECTRODE

Cby

2.4 Sensitivity

2.4.1 Introduction

The sensitivity on the QT1012 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.

2.4.2 Increasing Sensitivity

In some cases it may be desirable to increase sensitivity; for example, when using the sensor with very

thick panels having a low dielectric constant, or when the device is used as a proximity sensor. Sensitivity

can often be increased by using a larger electrode or reducing panel thickness. Increasing electrode size

can have diminishing returns, as high values of Cx will reduce sensor gain.

The value of Cs also has a dramatic effect on sensitivity, and this can be increased in value with the

trade-off of a slower response time and more power. Increasing the electrode's surface area will not

substantially increase touch sensitivity if its diameter is already much larger in surface area than the

object being detected. Panel material can also be changed to one having a higher dielectric constant,

which will better help to propagate the field.

Ground planes around and under the electrode and its SNSK trace will cause high Cx loading and

decrease gain. The possible signal-to-noise ratio benefits of ground area are more than negated by the

decreased gain from the circuit, and so ground areas around electrodes are discouraged. Metal areas

near the electrode will reduce the field strength and increase Cx loading and should be avoided, if

possible. Keep ground away from the electrodes and traces.

2.4.3 Decreasing Sensitivity

In some cases the QT1012 may be too sensitive. In this case gain can easily be lowered further by

decreasing Cs.

2.5 Moisture Tolerance

The presence of water (condensation, sweat, spilt water, and so on) on a sensor can alter the signal

values measured and thereby affect the performance of any capacitive device. The moisture tolerance of

QTouch devices can be improved by designing the hardware and fine-tuning the firmware following the

AT42QT1012

Datasheet

DS40001948A-page 8