MC33941_08 Datasheet, PDF (6/11 Pages) Freescale Semiconductor, Inc – Electric Field Imaging Device

PRINCIPLE OF OPERATION
The 33941 generates a low radio frequency sine wave with
nominal 5.0 V peak-to-peak amplitude. The frequency is set
by an external resistor and is optimized for 120 kHz. An
internal multiplexer routes the signal to one of the 7 terminals
under control of the ABC input terminals. A receiver
multiplexer simultaneously connected to the selected
electrode and routes its signal to a detector, which converts
the sine wave to a DC level. The DC level is filtered by an
external capacitor, is multiplied and offset to increase
sensitivity. All electrode outputs are grounded internally by
the device when not selected.
The amplitude and phase of the sinusoidal wave at the
electrode are affected by objects in proximity. A “capacitor” is
formed between the driving electrode and the object, each
forming a “plate” that holds the electric charge. The voltage
measured is an inverse function of the capacitance between
the electrode being measured, the surrounding electrodes,
and other objects in the electric field surrounding the
electrode. Increasing capacitance results in decreasing
voltage. The value of the series resistor (22kohm) was
chosen to provide a near linear relationship at 120 kHz over
a range of 10pF to 70pF.
While exploring applications using the E-Field chip, it is
always useful to approach the problem using the capacitor
model.
Drive level ~ 5 v p-p
Voltage Level Proportional to 1/C (voltage divider)
Load Resistor
(22 K ohms)
Detector ^{Low Pass Filter}
Stray Variable
Capacitance
Sine Generator
(120 KHz)
Electrodes
Object
Detected Signal
Level Decreases
with Increasing
Capacitance
Virtual Ground
Capacitance
increases as
electrodes move
closer together
Capacitor Model
Figure 2 . Conceptual Block Diagram
CAPACITOR MODEL
The capacitance measured by the E-Field IC is:
• Proportional to the area of the electrode
• Proportional to the dielectric constant of the material
between the electrodes
• Inversely proportional to the distance between the objects
C = ^{kε 0 A}
C
d
k
d
C=The Capacitance in Farads (F)
A=The area of the plates in square meters (m2)
d=The distance between the plates in meters (m)
k=The dielectric constant of the material separating the plates
0=Is the permittivity of free space (8.85 x 10-12 F/m)
Figure 3. Capacitor Model
MC33941
6
Table 4 Dielectric Constants of Various Materials
Dielectric Material Thickness (mil)
Acrylic
84.5
k
2.4-4.5
Glass
Nylon Plastic
74.5
7.5
68
3.0-5.0
Polyester Film
10
3.2
Flexible Vinyl Film
9
Air
-
2.8-4.5
1
Water
Ice
-
80
-
3.2
Automotive Oil
-
2.1
Sensors
Freescale Semiconductor