values 0 and 127 can be easily achieved by both large and
small fingers at the ends.
Other geometries are possible, for example arcs and
semicircles over a small scale (50mm radius max
recommended semicircle, or any radius as a shallow arc).
The strip can be made longer or shorter and with a different
width. The electrode strip should be about 10mm wide or
more, as a rule. Other features of the PCB layout are:
Increasing the Rs values will move the reported ends
‘outwards’. If they are too large, the values 0 and/or 127 will
not be reportable. The Rs resistors can have differing values.
Having well-defined ends is important in most applications, so
that the user can select the absolute minimum and maximum
values (ie OFF, MAX etc) reliably. If the numerical ends
cannot be achieved the user can have difficulty in controlling
the product.
The components are oriented perpendicular to the strip
length so that they do not fracture easily when the PCB is
flexed during bonding to the panel.
The slider end connections should have a symmetrical
layout; note the dummy end trace connected to Rs1 just
below the slider element, to replicate the upper end trace
connected to Rs2. Without this the slider will be
The end zones should be defined to be physically large
enough so that over a wide range of values of Cs, Rslider etc
a usable set of ends are always preserved.
End zone tolerances can be affected by Cs1 / Cs2
capacitance matching and the values of Rs1 and Rs2 if fixed
end-cal is used. See also Section 2.2.
unbalanced and will tend to skew its result to one side.
The ground ring around the slider measures 2mm thick
and is spaced 1mm from the long end traces. The end
traces should be placed as close as possible to the slider
element and be of the thinnest possible trace thickness.
0-ohm 0805 jumpers are used to connect the ground ring
back to circuit ground. These bridge over the two end
traces.
2.4 Power Supply
The usual power supply considerations with QT parts applies
also to the QT401. The power should be very clean and come
from a separate regulator if possible. This is particularly
critical with the QT401 which reports continuous position as
opposed to just an on/off output.
Additional ground area or a ground plane on the PCB’s
rear will compromise signal strength and is to be avoided.
The slider should normally be used in a substantially
horizontal orientation to reduce tracking accuracy
A ceramic 0.1uF bypass capacitor should be placed very
close to the power pins of the IC.
problems due to capacitive ‘hand shadow’ effects. Thinner
panels and an electrode strip on the back of the PCB (so
it has less material to penetrate) will reduce these effects.
Regulator stability: Most low power LDO regulators have
very poor transient stability, especially when the load
transitions from zero current to full operating current in a few
microseconds. With the QT401 this happens when the device
comes out of sleep mode. The regulator output can suffer
from hundreds of microseconds of instability at this time,
which will have a deleterious effect on acquisition accuracy.
‘Handshadow’ effects: With thicker or wider panels an effect
known as ‘handshadow’ can become noticeable. If the
capacitive coupling from finger to electrode strip is weak, for
example due to a narrow electrode strip or a thick, low
dielectric constant panel, the remaining portion of the human
hand can contribute a significant portion of the total
To assist with this problem, the QT401 waits 500µs after
coming out of sleep mode before acquiring to allow power to
fully stabilize. This delay is not present before an acquisition
burst if there is no preceding sleep state.
detectable capacitive load. This will induce an offset error,
which will depend on the proximity and orientation of the hand
to the remainder of the strip. Thinner panels will reduce this
effect since the finger contact surface will strongly domina te
the total signal and the remaining handshadow capacitance
will not contribute significantly to create an error offset.
Use an oscilloscope to verify that Vdd has stabilized to within
5mV or better of final settled voltage before a burst begins.
Slider strips placed in a vertical position are more prone to
handshadow problems than those that are horizontal.
2.5 PCB Layout and Mounting
The E401 PCB layout (Figure 1-3) should be followed if
possible. This is a 1-sided, 144 x 20 x 0.6mm board; the
blank side is simply adhered to the inside of a 2mm thick (or
less) control panel. Thicker panels can be tolerated with
additional positional error due to capacitive ‘hand shadow’
effects and will also have poorer EMC performance.
PCB Cleanliness: All capacitive sensors should be treated
as highly sensitive circuits which can be influenced by stray
conductive leakage paths. QT devices have a basic
resolution in the femtofarad range; in this region, there is no
such thing as ‘no clean flux’. Flux absorbs moisture and
becomes conductive between solder joints, causing signal
drift and resultant false detections or temporary loss of
sensitivity. Conformal coatings will trap in existing amounts of
moisture which will then become highly temperature
sensitive.
This layout uses 18 copper pads connected with 17
intervening series resistors in a chain. The end pads are
larger to ensure a more robust reading of 0 (left) and 127
(right). The finger interpolates between the copper pads (if
the pads are narrow enough) to make a smooth, 0..127 step
output with no apparent stair-casing. A wide ground border
helps to suppress the sense field outside of the strip area,
which would otherwise affect position accuracy.
The designer should specify ultrasonic cleaning as part of the
manufacturing process, and in extreme cases, the use of
conformal coatings after cleaning.
The small electrodes of this PCB measure about 12.5 x
5.2mm. The lateral (eg 5.2mm) dimension of these electrodes
should be no wider than the expected smallest diameter of
finger touch, to prevent stair-casing of the position response.
2.6 ESD Protection
Since the electrode is always placed behind a dielectric
panel, the IC will be protected from direct static discharge.
However even with a panel transients can still flow into the
lQ
5
QT401 R10.04/0505
