qt320 Quantum Research Group, qt320 Datasheet - Page 3

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

1.2.2 C
The PCB traces, wiring, and any components associated with
or in contact with Sa and Sb of either channel will become
touch sensitive and should be treated with caution to limit the
touch area to the desired location.
Multiple touch electrodes can be connected to one sensing
channel, for example to create a control button on both sides
of an object, however it is impossible for the sensor to
distinguish between the two connected touch areas.
1.2.3 B
The acquisition process occurs in bursts (Figure 1-7) of
variable length, in accordance with the single-slope CDC
method. The burst length depends on the values of Cs and
Cx. Longer burst lengths result in higher gains and more
sensitivity for a given threshold setting, but consume more
average power and are slower.
Burst mode operation acts to lower average power while
providing a great deal of signal averaging inherent in the CDC
process, making the signal acquisition process more robust.
The QT method is a very low impedance method of sensing
as it loads Cx directly into a very large capacitor (Cs). This
results in very low levels of RF susceptibility.
1.3 ELECTRODE DESIGN
1.3.1 E
There is no restriction on the shape of the electrodes; in most
cases common sense and a little experimentation can result
in a good electrode design. The QT320 will operate equally
well with long, thin electrodes as with round or square ones;
even random shapes are acceptable. The electrode can also
be a 3-dimensional surface or object. Sensitivity is related to
electrode surface area, orientation with respect to the object
being sensed, object composition, and the ground coupling
quality of both the sensor circuit and the sensed object.
Smaller electrodes will have less sensitivity than large ones.
If a relatively large electrode surfaces are desired, and if tests
show that an electrode has a high Cx capacitance that
reduces the sensitivity or prevents proper operation, the
electrode can be made into a mesh (Figure 1-3) which will
have a lower Cx than a solid electrode area.
lQ
LECTRODE
ONNECTION TO
URST
Figure 1-3 Mesh Electrode Geometry
M
ODE
G
EOMETRY AND
O
PERATION
E
LECTRODES
S
IZE
3
1.3.2 K
Like all capacitance sensors, the QT320 relies on Kirchoff’s
Current Law (Figure 1-4) to detect the change in capacitance
of the electrode. This law as applied to capacitive sensing
requires that the sensor’s field current must complete a loop,
returning back to its source in order for capacitance to be
sensed. Although most designers relate to Kirchoff’s law with
regard to hardwired circuits, it applies equally to capacitive
field flows. By implication it requires that the signal ground
and the target object must both be coupled together in some
manner in order for the sensor to operate properly. Note that
there is no need to provide an actual hardwired ground
connection; capacitive coupling to ground (Cx1) often is
sufficient, even if the coupling might seem very tenuous. For
example, powering the sensor via an isolated transformer will
almost always provide ample ground coupling, since there is
plenty of capacitance between the primary and secondary
windings via the transformer core and from there to the power
wiring itself directly to 'local earth'. Even when battery
powered, just the physical size of the PCB and the object into
which the electronics is embedded is often enough to couple
enough back to local earth.
The implications of Kirchoff’s law can be most visibly
demonstrated by observing the E3B eval board’s sensitivity
change between laying the board on a table versus holding
the board in your hand by it’s batteries. The effect can also be
observed by holding the board only by one electrode, letting it
recalibrate, then touching the battery end; the board will work
quite well in this mode.
1.3.3 V
When detecting human contact (e.g. a fingertip), grounding of
the person is never required, nor is it necessary to touch an
exposed metal electrode. The human body naturally has
several hundred picofarads of ‘free space’ capacitance to the
local environment (Cx3 in Figure 1-4), which is more than two
orders of magnitude greater than that required to create a
return path to the QT320 via earth. The QT320's PCB
however can be physically quite small, so there may be little
‘free space’ coupling (Cx1 in Figure 1-4) between it and the
environment to complete the return path. If the QT320 circuit
ground cannot be grounded via the supply connections, then
IRCHOFF
IRTUAL
Figure 1-4 Kirchoff’s Current Law
C
’
S
APACITIVE
C
URRENT
G
L
ROUNDS
AW
QT320/R1.03 08/02