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 QT160/161, 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.
Figure 1-6 Shielding Against Fringe Fields
Sense
wire
Sense
w ire
1.3.6 S
ENSITIVITY
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.
1.3.6.2 Decreasing Sensitivity
In some cases the QT160 may be too sensitive. In this case
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.
Figure 1-5 Kirchoff's Current Law
C
X2
2 - QT160/QT161 SPECIFICS
2.1 SIGNAL PROCESSING
The QT160 processes all signals using 16 bit math, using a
number of algorithms pioneered by Quantum. The algorithms
are specifically designed to provide for high survivability in
the face of adverse environmental changes.
Se nse E le ctro de
SENSO R
C
X 1
C
X 3
2.1.1 D
RIFT
C
OMPENSATION
A
LGORITHM
Signal drift can occur because of changes in Cx, Cs, and
Vdd over time. If a low grade Cs capacitor is chosen, the
signal can drift greatly with temperature. If keys are subject
to extremes of temperature and humidity, the signal can also
shift. It is crucial that drift be compensated, else false
detections, non-detections, and sensitivity shifts will follow.
Drift compensation (Figure 2-1) is a method that makes the
reference level track the raw signal at a slow rate, only while
no detection is in effect. The rate of reference adjustment
must be performed slowly else legitimate detections can also
be ignored. The IC drift compensates each channel
independently using a slew-rate limited change to the
reference level; the threshold and hysteresis values are
slaved to this reference.
Once an object is sensed, the drift compensation
mechanism ceases since the signal is legitimately high, and
therefore should not cause the reference level to change.
The signal drift compensation is 'asymmetric'; the reference
level drift-compensates in one direction faster than it does in
the other. Specifically, it compensates faster for decreasing
signals than for increasing signals. Increasing signals should
not be compensated for quickly, since an approaching finger
Su rro und ing e nv iro nm en t
Increasing electrode size can have diminishing returns, as
high values of Cx counteract sensor gain. 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.
lQ
4
QT160/161 1.07/0904
QUANTUM [ QUANTUM RESEARCH GROUP ]
