QT112-S Atmel, QT112-S Datasheet - Page 3

• Manufacturer Part Number: QT112-S
• Description: SENSOR IC TOUCH/PROXMTY 1CH8SOIC
• Manufacturer: Atmel
• Series: QProx™r
• Type: Capacitiver

Specifications of QT112-S

• Rohs Status: RoHS non-compliant
• Touch Panel Interface: 1, 2-Wire
• Number Of Inputs/keys: 1 Key
• Resolution (bits): 14 b
• Data Interface: Serial
• Voltage Reference: Internal
• Voltage - Supply: 2.45 V ~ 5.25 V
• Current - Supply: 60µA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Output Type: Logic
• Interface: 2-Wire
• Input Type: Logic
• Package / Case: -
• Other names: 427-1007

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 object, however it is
impossible for the sensor to distinguish
between the two touch areas.
1.3 ELECTRODE DESIGN
1.3.1 E
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
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.
If a relatively large electrode surface is desired, and if tests
show that the electrode has more capacitance than the
QT110 can tolerate, the electrode can be made into a sparse
mesh (Figure 1-4) having lower Cx than a solid plane.
Sensitivity may even remain the same, as the sensor will be
operating in a lower region of the gain curves.
1.3.2 K
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
field flows. By implication it requires that the signal ground
and the target object must both be coupled together in some
manner for a capacitive sensor to operate properly. Note that
there is no need to provide actual hardwired ground
connections; capacitive coupling to ground (Cx1) is always
sufficient, even if the coupling might seem very tenuous. For
example, powering the sensor via an isolated transformer
LECTRODE
IRCHOFF
Figure 1-4 Mesh Electrode Geometry
is
’
S
related to electrode
G
C
EOMETRY AND
URRENT
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AW
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R esult
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Figure 1-3 Internal Switching & Timing
Do ne
will provide ample ground coupling, since there is
capacitance between the windings and/or the transformer
core, and from the power wiring itself directly to 'local earth'.
Even when battery powered, just the physical size of the
PCB and the object into which the electronics is embedded
will generally be enough to couple a few picofarads back to
local earth.
1.3.3 V
When detecting human contact (e.g. a fingertip), grounding
of the person is never required. The human body naturally
has several hundred picofarads of ‘free space’ capacitance
to the local environment (Cx3 in Figure 1-5), which is more
than two orders of magnitude greater than that required to
create a return path to the QT110 via earth. The QT110's
PCB however can be physically quite small, so there may be
little ‘free space’ coupling (Cx1 in Figure 1-5) between it and
the environment to complete the return path. If the QT110
circuit ground cannot be earth grounded by wire, for example
via the supply connections, then a ‘virtual capacitive ground’
may be required to increase return coupling.
C
S EN SO R
X1
IRTUAL
Figure 1-5 Kirchoff's Current Law
C ha rg e
Am p
C
Su rro und in g e nv iro nm e n t
S e n se E le ctro de
APACITIVE
G
ROUNDS
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SNS2
SNS1
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s
ELE C TRO DE
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