By John Carey
Increasingly, embedded applica-
tions must interact directly with
their environment and their end
users. Consider the best new
touchscreen phones, in which the
user interface is a large capacitive
sensing screen that differentiates
a flick from a tap and tracks the
motion of your finger but doesn’t
track your ear.
Sensors are at the heart of
these systems. They sense the
environment and user behavior,
enabling the product to respond
in an intuitive but reliable way.
However, the sensor films them-
selves aren’t intelligent. They don’t
even collect data. They only sense.
They aren’t capable of differentiat-
ing between useful and useless
data or discriminating between
the quality of different types of
Truth be told, these sensor films
hardly sense at all. They really just
project an electric field created by
an intelligent capacitive sensing
chip. This type of capacitive sens-
ing is known as projected capaci-
tive technology, and it’s used in the
most advanced capacitive touch-
screen solutions. Figure 1 shows
an example of how a projected
capacitive touchscreen works.
This is not to say that the sen-
sors themselves are not complex.
On the contrary, a capacitive
touchscreen sensor consists of
a large array of indium tin oxide
(ITO) conductors on one or more
layers of glass or polyethylene
terephthalate plastic. Figure 2
presents an example of a touch-
screen sensor construction.
The good optical clarity and
low resistivity of ITO make it the
perfect conductor for creating a
touchscreen. When the ITO sensor
is connected to a capacitive sens-
ing chip with a suitably high SNR,
it can accurately sense minute
changes in capacitance. A finger’s
presence for instance is on the or-
der of a picoFarad (1012 Farads).
is typically accompanied
by background capacitances of
10’s of nanoFarads (109 Farads).
This situation makes the sensing
environment challenging and