qt320 Quantum Research Group, qt320 Datasheet - Page 4

• Manufacturer Part Number: qt320
• Description: 2-channel Progammable Advanced Sensor Ic
• Manufacturer: Quantum Research Group

a ‘virtual capacitive ground’ may be required to increase
return coupling.
A ‘virtual capacitive ground’ can be created by connecting the
QT320’s own circuit ground to:
Because the QT320 operates at a relatively low frequency,
about 500kHz, even long inductive wiring back to ground will
usually work fine.
Free-floating ground planes such as metal foils should
maximise 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 1-5). 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 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 QT320, sensitivity can be high enough (depending on
Cx and Cs) that 'walk-by' signals are a concern; if this is a
problem, then some form of rear shielding may be required.
lQ
(1) A nearby piece of metal or metallized housing;
(2) A floating conductive ground plane;
(3) A fastener to a supporting structure;
(4) A larger electronic device (to which its output might be
connected anyway).
IELD
Sense
wire
Figure 1-5 Field Shielding & Shaping
S
HIELDING AND
Unshielded
Electrode
S
HAPING
Sense
wire
Electrode
Shielded
4
1.4 SENSITIVITY ADJUSTMENTS
There are three variables which influence sensitivity
independently for each channel:
There is also a sensitivity dependence of the whole device on
Vdd. Cs and Cx effects are covered in Section 1.2.1.
The threshold setting can be adjusted independently for each
channel from 1 to 16 counts of signal swing (Section 2.2).
Note that sensitivity is also a function of other things like
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 mutual coupling of the sensor circuit and the object (usually
via the local environment, or an actual galvanic connection).
It is advisable to set the sensitivity to the approximate desired
result by changing Cx and Cs first using a signal threshold
fixed at 10. Use the threshold value thereafter to fine-tune
sensitivity.
1.4.1 I
In some cases it may be desirable to greatly increase
sensitivity, for example when using the sensor with very thick
panels having a low dielectric constant, or when sensing low
capacitance objects.
Sensitivity can 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 load will also reduce sensor gain (Figures
5-1 to 5-4). The value of Cs also has a dramatic effect on
sensitivity, and this can be increased in value up to a limit.
Increasing electrode surface area will not substantially
increase sensitivity if its area is already larger than the object
to be 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.
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
1. Cs (sampling capacitor)
2. Cx (unknown capacitance)
3. Signal threshold value
Figure 1-6 Circuit with Csx gain equalization capacitor
NCREASING
S
ENSITIVITY
QT320/R1.03 08/02