1 - OVERVIEW
The QT110 is a digital burst mode charge-transfer (QT) sensor
designed specifically for touch controls; it includes all hardware
and signal processing functions necessary to provide stable
sensing under a wide variety of changing conditions. Only a
few low cost, non-critical discrete external parts are required for
operation.
Figure 1-1 shows the basic QT110 circuit using the device,
with a conventional output drive and power supply
connections. Figure 1-2 shows a second
configuration using a
common power/signal rail which can be a long twisted pair from
a controller; this configuration uses the built-in pulse mode to
transmit output state to the host controller (QT110 only).
1.1 BASIC OPERATION
The QT110 employs low duty cycle bursts of charge-transfer
cycles to acquire its signal. Burst mode permits power
consumption in the low microamp range, dramatically reduces
EMC problems, and yet permits excellent response time.
Internally the signals are digitally processed to reject impulse
noise, using a 'consensus' filter which requires four
consecutive confirmations of a detection before the output is
activated.
The QT switches and charge measurement hardware functions
are all internal to the QT110 (Figure 1-3). A single-slope
switched capacitor ADC includes both the required QT charge
and transfer switches in a configuration that provides direct
ADC conversion. Vdd is used as the charge reference voltage.
Larger values of Cx cause the charge transferred into Cs to
rise more rapidly, reducing available resolution; as a minimum
resolution is required for proper operation, this can result in
dramatically reduced apparent gain.
The IC is highly tolerant of changes in Cs since it computes the
signal threshold level ratiometrically. Cs is thus non-critical and
can be an X7R type. As Cs changes with temperature, the
internal drift compensation mechanism also adjusts for the drift
automatically.
Piezo sounder drive: The QT110 can drive a piezo sounder
after a detection for feedback. The piezo sounder replaces or
augments the Cs capacitor; this works since piezo sounders
are also capacitors, albeit with a large thermal drift coefficient.
If C
piezo
is in the proper range, no additional capacitor is
required. If C
piezo
is too small, it can simply be ‘topped up’ with a
ceramic capacitor in parallel. The QT110 drives a ~4kHz signal
across SNS1 and SNS2 to make the piezo (if installed) sound a
short tone for 75ms immediately after detection, to act as an
audible confirmation.
Option pins allow the selection or alteration of several other
special features and sensitivity.
1.2 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as a
direct result, the sense electrode can in theory be connected to
either SNS1 or SNS2 with no performance difference.
However, the noise immunity of the device is improved by
connecting the electrode to SNS2, preferably via a series
resistor Re (Figure 1-1) to roll off higher harmonic frequencies,
both outbound and inbound.
In order to reduce power consumption and to assist in
discharging Cs between acquisition bursts, a 470K series
resistor Rs should be connected across Cs (Figure 1-1).
The rule Cs >> Cx must be observed for proper operation.
Normally Cx is on the order of 10pF or so, while Cs might be
10nF (10,000pF), or a ratio of about 1:1000.
It is important to minimize the amount of unnecessary stray
capacitance Cx, for example by minimizing trace lengths and
widths and backing off adjacent ground traces and planes so
as keep gain high for a given value of Cs, and to allow for a
larger sensing electrode size if so desired.
The PCB traces, wiring, and any components associated with
or in contact with SNS1 and SNS2 will become touch sensitive
and should be treated with caution to limit the touch area to the
desired location.
1.3 ELECTRODE DESIGN
1.3.1 E
LECTRODE
G
EOMETRY
AND
S
IZE
There is no restriction on the shape of the electrode; in most
cases common sense and a little experimentation can result in
a good electrode design. The QT110 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.
1.3.2 K
IRCHOFF
’
S
C
URRENT
L
AW
Like all capacitance sensors, the QT110
relies on Kirchoff’s Current Law (Figure
1-5) 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
LQ
2 QT110 R1.04/0405
Figure 1-1 Standard mode options
SENSING
ELECT RODE
Cs
Rs
2nF - 500nF
3
46
5
1
+2.5 ~ +5
72
OUT
OPT1
OPT2
GAIN
SNS1
SNS2
Vss
Vdd
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
C
x
8
R
E
Figure 1-2 2-wire operation, self-powered
10µF
1N4148
n-ch Mosfet
CMOS
LOGIC
3.5 - 5.5V
1K
Twisted
pair
C
s
8
OUT
OPT1
OPT2
GAIN
SNS1
SNS2
Vss
Vdd
3
46
5
1
72
R
s
SENSING
ELECTRODE
C
x
R
E
