sx8654evk Semtech Corporation, sx8654evk Datasheet - Page 24

• Manufacturer Part Number: sx8654evk
• Description: Haptics Enabled 4/5-wire Resistive Touchscreen Controller With Proximity Sensing
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
The challenge of capacitive sensing is to detect this relatively small variation of C
for a few percents only) and differentiate it from environmental noise (C
environment characteristics like temperature, etc). For this purpose, the chip integrates an auto offset
compensation mechanism which dynamically monitors and removes the C
C
In first order, C
A
and the sensor.
d
When performing proximity sensing the dielectric relative permittivity is roughly equal to that of the air as the
overlay is relatively thin compared to the detection distance targeted. Typical permittivity of some common
materials is given in the table below.
From the discussions above we can conclude that the most robust and efficient design will be the one that
minimizes C
5.2.2
Rev 1 – 25
ε
ε
User
r
0
is the distance between the two electrodes hence the proximity distance between the user and the system.
is the common area between the two electrodes hence the common area between the user’s finger/palm/face
is the dielectric relative permittivity.
is the free space permittivity and is equal to 8.85 10e-12 F/m (constant)
only. See § 5.2.5 for more details.
AFE Block Diagram
th
Env
July 2011
User
value and variations while improving C
can be estimated by the formula below:
Figure 28 – Analog Front-End Block Diagram
Acrylic Glass
Material
Glass
Wood
FR4
Air
C
User
Resistive Touchscreen Controller with Proximity Sensing
User
24
.
ε
0
ε
d
r
Typical
A
8
5
3
2
1
ε
r
Env
Env
also slowly varies together with the
component to extract and process
Sensor
SX8657/SX8658
Haptics Enabled 4/5-Wire
(C
User
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usually contributes