QT118H-S ETC, QT118H-S Datasheet - Page 4

• Manufacturer Part Number: QT118H-S
• Description: CHARGE-TRANSFER TOUCH SENSOR
• Manufacturer: ETC

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.
1.3.4 F
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
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
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 a small air gap between the
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
'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.
lq
(2) A floating conductive ground plane;
(3) A nail driven into a wall when used with small
(4) A larger electronic device (to which its output might be
Figure 1-6
Sen se
wire
electrodes;
connected anyway).
IELD
Unshielded
Electrode
S
HAPING
Shielding Against Fringe Fields
S e nse
w ire
Electrode
S hielded
T hre sh old
Ou tpu t
1.3.5 S
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.
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
effect on sensitivity, and this can be increased in value (up to
a 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 the object being
detected. 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.
1.3.5.2 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
space-to-conductor ratio (Figure 1-4), or by decreasing Cs.
electrode
Table 1-1 Gain Setting Strap Options
Figure 2-1 Drift Compensation
ENSITIVITY
Medium
S ig na l
Gain
High
Low
into
R efer ence
a
sparse
mesh
H ys te res is
Tie Pin 5 to:
Floating
Pin 6
Pin 7
using
a
high
3