Sensors
Freescale Semiconductor 13
MC33794
A “capacitor” can be formed between the driving electrode
and the object, each forming a “plate” that holds the electric
charge. Capacitance is directly proportional to the area of the
electrode plates. Doubling the area doubles the capacitance.
Capacitance is also directly proportional to the dielectric
constant of the material between the plates. Air typically has
a dielectric constant of 1 (unity) whereas water can have a
dielectic constant of 80 (which means the capacitance is
roughly 80 times larger). Plastics and glass that are
commonly used in touch panel applications have dielectric
constants greater than unity. A third consideration is that
capacitance is inversely proportional to the distance between
the plates. Doubling the distance between the two plates will
reduce capacitance by four. This property can be exploited in
cases where small distances need to be measured.
From the above, it can be seen that increased detection
sensitivity is a function of the plate size, the dielectric
constant of the material between the plates, and the distance
between them.
The voltage measured at LEVEL is an inverse function of
the capacitance between the electrode being measured and
the surrounding electrodes and other objects in the electric
field surrounding the electrode. Increasing capacitance
results in decreasing voltage. The value of series resistance
(22 k
Ω) was chosen to provide a nearly linear relationship at
120 kHz over a range of 10 pF to 100 pF.
The measured value may change with any change in
frequency, series resistance, driving voltage, the dielectric
constant of the capacitor, or detector sensitivity. These can
change with temperature and time. There are several ways to
compensate for these changes. One method uses the
REF_A and REF_B capacitors. Another method may use
long term averages to set a baseline value.
Using REF_A and REF_B, a typical measurement
algorithm would start by measuring the voltage for two known
value capacitors (attached to REF_A and REF_B). The value
of these capacitors would be chosen to be near the minimum
and maximum values of capacitance expected to be seen at
the electrodes. These reference voltages and the known
capacitance values are then used with the electrode
measurement voltage to determine the capacitance seen by
the electrode. This method can be used to detect short- and
long-term changes due to objects in the electric field and
significantly reduce the effect of temperature-and time-
induced changes.
Another approach is to run long term averaging of the
electrode values. Long term, in this case, may mean several
seconds. These long term averages are then used as a set
point so that short term changes in the field intensity can be
reliably determined. This is typically the method used for
touch panel applications.
The MC33794 does not contain an ADC. It is intended to
be used with an MCU that contains one. Offset and gain have
been added to the MC33794 to maximize the sensitivity over
the range of 0 pF to 100 pF. An 8-bit ADC can resolve around
0.4 pF of change and a 10-bit converter around 0.1 pF.
Higher resolution results in more distant detection of smaller
objects. Due to the relatively slow data access requirements
(approximately 2 ms per electrode), digital over-sampling
techniques can be used to extend the resolution of 8- or 10-
bit converters by 2 or 3 bits.
DC loading on the electrodes should be avoided. A typical
situation where this might occur is if moisture gets in direct
contact between electrodes, or between an electrode and
ground or shield drive. The signal is generated with a DC
offset that is more than half the peak-to-peak level. This
keeps the signal positive above ground at all times. The
detector uses this voltage level as the midpoint for detection.
All signals below this level are inverted and added to all
signals above this level. Loading of the DC level will cause
some of the positive half of the signal to be inverted and
added and will change the measurement.
If it is not possible to assure that the electrodes will always
have a high DC resistance to ground source, a series
capacitor of about 10 nF should be connected between the IC
electrode terminals and the electrodes. This capacitor will
block DC bias voltages to the detector. Note that it is also
advisable to add a DC blocking capacitor in series with the
Shield Driver output as well.
