E102 Atmel, E102 Datasheet - Page 3

• Manufacturer Part Number: E102
• Description: KIT EVAL/DEV USING QT102-ISG
• Manufacturer: Atmel
• Series: Quantum, QTouch™r

Specifications of E102

• Sensor Type: Touch, Capacitive
• Sensing Range: 1 Button/Key
• Interface: Logic Level
• Voltage - Supply: 3V
• Embedded: No
• Utilized Ic / Part: QT102
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Sensitivity: -

1 Overview
1.1 Introduction
The QT102 is a single key device featuring a touch on / touch
off (toggle) output with a programmable auto switch-off
capability.
The QT102 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 low cost, noncritical components are required
for operation.
The QT102 employs bursts of charge-transfer cycles to
acquire its signal. Burst mode permits power consumption in
the 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 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 QT102.
1.2 Electrode Drive
Figure 1.1 shows the sense electrode connections (SNS,
SNSK) for the QT102.
For optimum noise immunity, the electrode should only be
connected to the SNSK pin.
In all cases the sample capacitor Cs should be much larger
than the load capacitance (Cx). Typical values for Cx are
5 - 20pF while Cs is usually 1 - 50nF.
Note: Cx is not a physical discrete component on the PCB, it
is the capacitance of the touch electrode and wiring. It is show
in Figure 1.1 to aid understanding of the equivalent circuit.
Increasing amounts of Cx destroy gain, therefore it is
important to limit the amount of load capacitance on both SNS
terminals. This can be done, for example, by minimizing trace
lengths and widths and keeping these traces away from power
or ground traces or copper pours.
The traces and any components associated with SNS and
SNSK will become touch sensitive and should be treated with
caution to limit the touch area to the desired location.
lQ
ELECTRODE
SENSE
Figure 1.1 Sense Connections
Cx
Rs
Cs
3
4
SNSK
SNS
VDD
VDD
VSS
5
2
TIME
OUT
1
6
3
A series resistor, Rs, should be placed in line with SNSK to
the electrode to suppress electrostatic discharge (ESD) and
Electromagnetic Compatibility (EMC) effects.
1.3 Sensitivity
1.3.1 Introduction
The sensitivity of the QT102 is a function of such things as:
1.3.2 Increasing Sensitivity
In some cases it may be desirable to increase sensitivity; for
example, when using the sensor with very thick panels having
a low dielectric constant. Sensitivity can often be increased by
using a larger electrode or reducing panel thickness.
Increasing electrode size can have diminishing returns, as
high values of Cx will reduce sensor gain.
The value of Cs also has a dramatic effect on sensitivity, and
this can be increased in value with the trade-off of slower
response time and more power. 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. Panel material can also be changed to
one having a higher dielectric constant, which will better help
to propagate the field.
Ground planes around and under the electrode and its SNSK
trace will cause high Cx loading and destroy gain. The
possible signal-to-noise ratio benefits of ground area are more
than negated by the decreased gain from the circuit, and so
ground areas around electrodes are discouraged. Metal areas
near the electrode will reduce the field strength and increase
Cx loading and should be avoided, if possible. Keep ground
away from the electrodes and traces.
1.3.3 Decreasing Sensitivity
In some cases the QT102 may be too sensitive. In this case
gain can be easily lowered further by decreasing Cs.
1.4 Recalibration Timeout
If an object or material obstructs the sense electrode the
signal may rise enough to create a detection, preventing
further operation. To stop this, the sensor includes a timer
which monitors detections. If a detection exceeds the timer
setting (known as the Max On-duration) the sensor performs a
full recalibration. This does not toggle the output state but
ensures that the QT102 will detect a new touch correctly. The
timer is set to activate this feature after ~ 30s. This will vary
slightly with Cs.
•
•
•
•
•
•
the value of Cs
electrode size and capacitance
electrode shape and orientation
the composition and aspect of the object to be sensed
the thickness and composition of any overlaying panel
material
the degree of ground coupling of both sensor and
object
QT102_2R3.05_1007