QT140 View Datasheet(PDF) - Quantum Research Group

Part Name

Description

Manufacturer

QT140

4 AND 5 KEY QTOUCH™ SENSOR ICs

Quantum Research Group 

Figure 1-4 Open Electrode for Back-Illumination

Figure 1-5 Shielding Against Fringe Fields

Sense

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Sense

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complete the return path. If the circuit ground cannot be

earth grounded by wire, for example via the supply

connections, then a ‘virtual capacitive ground’ may be

required to increase return coupling.

A ‘virtual capacitive ground’ can be created by connecting

the IC's circuit ground to:

(1) A nearby piece of metal or metallized housing;

(2) A floating conductive ground plane;

(3) A larger electronic device (to which its output might be

connected anyway).

Free-floating ground planes such as metal foils should

maximize exposed surface area in a flat plane if possible. A

square of metal foil will have little effect if it is rolled up or

crumpled into a ball. Virtual ground planes are more effective

and can be made smaller if they are physically bonded to

other surfaces, for example a wall or floor.

1.3.5 FIELD SHAPING

The electrode can be prevented from sensing in undesired

directions with the assistance of metal shielding connected

to circuit ground (Figure 1-5). For example, on flat surfaces,

the field can spread laterally and create a larger touch area

than desired. To stop field spreading, it is only necessary to

surround the touch electrode on all sides with a ring of metal

connected to circuit ground. The ring will stop field spreading

from that point outwards.

If one side of the panel to which the electrode is fixed has

moving traffic near it, these objects can cause inadvertent

detections. This is called ‘walk-by’ and is caused by the fact

that the fields radiate from either surface of the electrode

equally well. Again, shielding in the form of a metal sheet or

foil connected to circuit ground will prevent walk-by; putting

an air gap between the grounded shield and the electrode

will help to keep the value of Cx low.

1.3.6 SENSITIVITY

Sensitivity can be altered to suit various applications and

situations on a channel-by-channel basis. The easiest and

most direct way to impact sensitivity is to alter the value of

Cs; more Cs yields higher sensitivity.

1.3.6.1 Alternative Ways to Increase Sensitivity

Sensitivity can also be increased by using bigger electrodes,

reducing panel thickness, or altering panel composition.

Increasing electrode size can have diminishing returns, as

high values of Cx counteract sensor gain; however, Cs can

be increased to combat this up to the rated device limit. Also,

increasing the electrode's surface area will not substantially

increase touch sensitivity if its diameter is already much

larger in surface area than fingertip contact area.

The panel or other intervening material can be made thinner,

but again there are diminishing rewards for doing so. Panel

material can also be changed to one having a higher

dielectric constant, which will help propagate the field

through to the front. Locally adding some conductive material

to the panel (conductive materials essentially have an infinite

dielectric constant) will also help; for example, adding carbon

or metal fibers to a plastic panel will greatly increase frontal

field strength, even if the fiber density is too low to make the

plastic bulk-conductive.

1.3.6.2 Decreasing Sensitivity

In some cases the circuit may be too sensitive. Gain can be

lowered further by a number of strategies: a) making the

electrode smaller, b) making the electrode into a sparse

mesh using a high space-to-conductor ratio (Figure 1-3), or

c) by decreasing the Cs capacitors.

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QT140/150 1.01/1102

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