at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 6

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

2.4.3
2.4.4
6
AT42QT113B
Virtual Capacitive Grounds
Field Shaping
Figure 2-2.
When detecting human contact (for example, 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
than that required to create a return path to the QT113B via earth. The QT113B‘s PCB however
can be physically quite small, so there may be little “free space” coupling (Cx1 in
between it and the environment to complete the return path. If the QT113B 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.
A virtual capacitive ground can be created by connecting the QT113B‘s own circuit ground to:
Free-floating ground planes such as metal foils should maximize exposed surface area in a flat
plane if possible. A square of metal foil will have little effect if it is rolled up or crumpled into a
ball. Virtual ground planes are more effective and can be made smaller if they are physically
bonded to other surfaces, for example a wall or floor.
The electrode can be prevented from sensing in undesired directions with the assistance of
metal shielding connected to circuit ground (see
field can spread laterally and create a larger touch area than desired. To stop field spreading, it
is only necessary to surround the touch electrode on all sides with a ring of metal connected to
circuit ground; the ring can be on the same or opposite side from the electrode. The ring will kill
field spreading from that point outwards.
• A nearby piece of metal or metallized housing;
• A floating conductive ground plane;
• Another electronic device (to which its output might be connected anyway).
Kirchoff’s Current Law
C
SENSO R
X1
Su rro und ing e nviro nm e nt
Sense E le ctrode
Figure
2-2), which is more than two orders of magnitude greater
C
X2
Figure
C
2-3). For example, on flat surfaces, the
X3
9525B–AT42–09/10
Figure
2-2)