Figure 1-1 Standard mode options
1 - OVERVIEW
The QT118H 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 single low cost, non-critical capacitor is
required for operation.
+2.5 to 5
SENSING
ELECTRODE
1
Vdd
2
3
4
7
5
6
OUT
SNS2
GAIN
SNS1
Figure 1-1 shows the basic QT118H circuit using the device,
a conventional output drive and power supply
with
Cs
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 the output state to the host controller.
OPT1
OPT2
2nF - 500nF
Cx
Vss
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
1.1 BASIC OPERATION
8
The QT118H employs short, ultra-low duty cycle bursts of
charge-transfer cycles to acquire its signal. Burst mode
permits power consumption in the low microamp range,
dramatically reduces RF emissions, lowers susceptibility to
EMI, and yet permits excellent response time. Internally the
signals are digitally processed to reject impulse noise, using
Cs is thus non-critical; as it drifts with temperature, the
threshold algorithm compensates for the drift automatically.
A simple circuit variation is to replace Cs with a bare piezo
sounder (Section 2), which is merely another type of
capacitor, albeit with a large thermal drift coefficient. If Cpiezo
is in the proper range, no other external component is
required. If Cpiezo is too small, it can simply be ‘topped up’
with an inexpensive ceramic capacitor connected in parallel
with it. The QT118H 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.
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 QT118H (Figure 1-3). A 14-bit
single-slope switched capacitor ADC includes both the
required QT charge and transfer switches in a configuration
that provides direct ADC conversion. The ADC is designed to
dynamically optimize the QT burst length according to the
rate of charge buildup on Cs, which in turn depends on the
values of Cs, Cx, and Vdd. 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. Conversely, larger values of Cs reduce the rise of
differential voltage across it, increasing available resolution
by permitting longer QT bursts. The value of Cs can thus be
increased to allow larger values of Cx to be tolerated
(Figures 4-1, 4-2, 4-3 in Specifications, rear).
Option pins allow the selection or alteration of several
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 be connected to
either SNS1 or SNS2 with no performance difference. In both
cases the rule Cs >> Cx must be observed for proper
operation. The polarity of the charge buildup across Cs
during a burst is the same in either case.
It is possible to connect separate Cx and Cx’ loads to SNS1
and SNS2 simultaneously, although the result is no different
than if the loads were connected together at SNS1 (or
SNS2). It is important to limit the
The IC is highly tolerant of changes in Cs since it computes
the threshold level ratiometrically with respect to absolute
load, and does so dynamically at all times.
amount of stray capacitance on
Figure 1-2 2-wire operation, self-powered
both terminals, especially if the load
Cx is already large, for example by
minimizing trace lengths and widths
so as not to exceed the Cx load
specification 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.
Multiple touch electrodes can be
used, for example to create a
control button on both sides of an
lq
1
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