QT118H 查看數據表（PDF） - Quantum Research Group

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QT118H

CHARGE-TRANSFER TOUCH SENSOR

Quantum Research Group 

Figure 1-6 Shielding Against Fringe Fields

1.3.5 SENSITIVITY

The QT118H can be set for one of 3 gain levels using option

pin 5 (Table 1-1). This sensitivity change is made by altering

the internal numerical threshold level required for a

detection. Note that sensitivity is also a function of other

things: like the value of Cs, electrode size, 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.

Sen se

wire

Sense

w ire

1.3.5.1 Increasing Sensitivity

In some cases it may be desirable to increase sensitivity

further, for example when using the sensor with very thick

panels having a low dielectric constant.

Sensitivity can often be increased by using a bigger

electrode, reducing panel thickness, or altering panel

composition. Increasing electrode size can have diminishing

returns, as high values of Cx will reduce sensor gain

(Figures 4-1, 4-2). The value of Cs also has a dramatic

U n s h ie ld e d

S hielded

effect on sensitivity, and this can be increased in value (up to

a limit). Also, increasing the electrode's surface area will not

Electrode

Electrode

substantially increase touch sensitivity if its diameter is

already much larger in surface area than the object being

(2) A floating conductive ground plane;

detected. The panel or other intervening material can be

(3) A nail driven into a wall when used with small

made thinner, but again there are diminishing rewards for

electrodes;

doing so. Panel material can also be changed to one having

(4) A larger electronic device (to which its output might be a higher dielectric constant, which will help propagate the

connected anyway).

field through to the front. Locally adding some conductive

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

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.

bonded to other surfaces, for example a wall or floor.

Table 1-1 Gain Setting Strap Options

1.3.4 FIELD SHAPING

The electrode can be prevented from sensing in undesired

directions with the assistance of metal shielding connected

to circuit ground (Figure 1-6). For example, on flat surfaces,

the field can spread laterally and create a larger touch area

Gain

High

Medium

Low

Tie Pin 5 to:

Floating

Pin 6

Pin 7

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 can be on the same or

opposite side from the electrode. The ring will kill 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

1.3.5.2 Decreasing Sensitivity

In some cases the QT118H may be too sensitive, even on

low gain. In this case gain can be lowered further by a

number of strategies: making the electrode smaller, making

the electrode into a sparse mesh using a high

space-to-conductor ratio (Figure 1-4), or by decreasing Cs.

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.

Figure 2-1 Drift Compensation

Again, shielding in the form of a metal sheet or

foil connected to circuit ground will prevent

walk-by; putting a small air gap between the

S ig na l

H ys te res is

grounded shield and the electrode will keep the

value of Cx lower and is encouraged. In the case

of the QT118H, the sensitivity is low enough that

T hr e s h o ld

R eference

'walk-by' should not be a concern if the product

has more than a few millimeters of internal air

gap; if the product is very thin and contact with

the product's back is a concern, then some form

of rear shielding may be required.

Output

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