sx8675iwltrt Semtech Corporation, sx8675iwltrt Datasheet - Page 25

• Manufacturer Part Number: sx8675iwltrt
• Description: Haptics Enabled Multitouch 4/5-wire Resistive Touchscreen Controller With Proximity Sensing
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
5.2
5.2.1
Capacitive sensing is the art of measuring a small variation of capacitance in a noisy environment.
mentioned above, the chip’s proximity sensing interface is based on capacitive sensing technology. In order to
illustrate some of the user choices and compromises required when using this technology it is useful to
understand its basic principles.
To illustrate the principle of capacitive sensing we will use the simplest implementation where the sensor is a
copper plate on a PCB but the exact same principles apply if the sensor is the touchscreen’s top plate.
The figure below shows a cross-section and top view of a typical capacitive sensing implementation. The sensor
connected to the chip is a simple copper area on top layer of the PCB. It is usually surrounded (shielded) by
ground for noise immunity (shield function) but also indirectly couples via the grounds areas of the rest of the
system (PCB ground traces/planes, housing, etc). For obvious reasons (design, isolation, robustness …) the
sensor is stacked behind an overlay which is usually integrated in the housing of the complete system. When the
touchscreen is used for sensing the overlay corresponds to the thin and flexible protection film covering the top
panel.
Rev 1 – 25
When the conductive object to be detected (finger/palm/face, etc) is not present, the sensor only sees an
inherent capacitance value C
with ground areas.
When the conductive object (finger/palm/face, etc) approaches, the electrical field around the sensor will be
modified and the total capacitance seen by the sensor increased by the user capacitance C
phenomenon is illustrated in the figure below.
Analog Front-End (AFE)
Capacitive Sensing Basics
th
July 2011
Figure 30 – Proximity Effect on Electrical Field and Sensor Capacitance
PCB copper
Figure 29 – Typical Capacitive Sensing Implementation
Env
created by its electrical field’s interaction with the environment, in particular
Resistive Touchscreen Controller with Proximity Sensing
PCB dielectric
Overlay
Ground
Sensor
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SX8674/SX8675/SX8676
Haptics Enabled Multitouch 4/5-Wire
Top view
Cut view
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