The optimal Cs values depend on the
thickness of the panel and its dielectric
constant. Lower coupling to a finger caused
by a low dielectric constant and/or thicker
panel will cause the position result to
become granular and more subject to
position errors. The ideal panel is made of
thin glass. The worst panel is thick plastic.
Granularity due to poor coupling can be
compensated for by the use of larger values
of sample capacitors.
Figure 1-3 E510 PCB Layout
A table of suggested values for no missing
position values is shown in Table 1-2.
Values of Cs smaller than those shown in
the table can cause skipping of position
codes. Code skipping may be acceptable in
many applications where fine position data
is not required. Smaller Cs capacitors have
the advantage of requiring shorter
acquisition bursts and hence lower power
drain.
Larger values of Cs improve granularity at
the expense of longer burst lengths and
the Error bit is high, it means the signal has fallen significantly
below the calibration level when not touched. If this happens
the device could report slightly inaccurate position values
when touched.
hence more average power.
Cs1, Cs2 and Cs3 should be X7R type, matched to within
10% of each other (ie, 5% tolerance) for best accuracy. The
E510 reference layout (Figure 1-3) is highly recommended. If
the Cs capacitors are poorly matched, the wheel accuracy will
be affected and there could also be missing codes.
This condition can self-correct via the drift compensation
process after some time under host control (Section 3.3.3).
Alternatively, the host controller can cause the device to
recalibrate immediately by issuing a calibration command
(Section 3.3.2).
2.3 Rs Resistors
Rs1, Rs2, and Rs3 are low value (typically 4.7K) resistors
used to suppress the effects of ESD and assist with EMC
compliance.
2 Wiring & Parts
2.4 Power Supply
The device should be wired according to Figure 1-1. An
example PCB layout (of the E510 eval board) is shown in
Figure 1-3.
The usual power supply considerations with QT parts applies
also to the QT510. The power should be very clean and come
from a separate regulator if possible. This is particularly
critical with the QT510 which reports continuous position as
opposed to just an on/off output.
2.1 Electrode Construction
The wheel electrode should be a resistive element of about
100K ohms +/-50% between each set of connection points, of
a suitable diameter and width. There are no known diameter
restrictions other than those governed by human factors.
A ceramic 0.1µF bypass capacitor should be placed very
close to the power pins of the IC.
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 QT510 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 negative effect on acquisition accuracy.
The electrode can be made of a series chain of discrete
resistors with copper pads on a PCB, or from ITO (Indium Tin
Oxide, a clear conductor used in LCD panels and touch
screens) over a display. Thick-film carbon paste can also be
used, however linearity might be a problem as these films are
notoriously difficult to control without laser trimming or
scribing.
The linearity of the wheel is governed largely by the linearity
and consistency of the resistive element. Positional accuracy
to within 5% is routinely achievable with good grade resistors
and a uniform construction method.
Table 1-2 Recommended Cs vs. Materials
Thickness,
Acrylic
Borosilicate glass
mm
(
εR =2.8)
10nF
22nF
47nF
100nF
-
(
εR =4.8)
5.6nF
10nF
0.4
0.8
1.5
2.5
3.0
4.0
2.2 Cs Sample Capacitors
Cs1, Cs2 and Cs3 are the charge sensing sample capacitors;
normally they are identical in nominal value. They should be
of type X7R dielectric.
22nF
39nF
47nF
100nF
-
lQ
4
QT510 R6.04/0505
QUANTUM [ QUANTUM RESEARCH GROUP ]
