at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 5

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

2.3
2.4
2.4.1
2.4.2
9525B–AT42–09/10
Electrode Drive
Electrode Design
Electrode Geometry and Size
Kirchoff’s Current Law
The internal ADC treats Cs as a floating transfer capacitor; as a result, the sense electrode can
in theory be connected to either SNS1 or SNS2 with no performance difference. However the
electrode should only be connected to pin SNS2 for optimum noise immunity.
In all cases the rule Cs >> Cx must be observed for proper operation; a typical load capacitance
(Cx) ranges from 10-20 pF while Cs is usually around 10-50 nF.
Increasing amounts of Cx destroy gain; therefore it is important to limit the amount of stray
capacitance on both SNS terminals, 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 SNS1 and SNS2 will become touch sensitive
and should be treated with caution to limit the touch area to the desired location.
A series resistor, Rseries, should be placed inline with the SNS2 pin to the electrode to suppress
ESD and EMC effects.
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 QT113B operates 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.
If a relatively large electrode surface is desired, and if tests show that the electrode has more
capacitance than the QT113B can tolerate, the electrode can be made into a sparse mesh (see
Figure
sensor will be operating in a lower region of the gain curves.
Like all capacitance sensors, the QT113B relies on Kirchoff’s Current Law (see
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 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.
2-2) having lower Cx than a solid plane. Sensitivity may even remain the same, as the
AT42QT113B
Figure
2-2) to
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