DEMOMC33941 Freescale Semiconductor, DEMOMC33941 Datasheet - Page 6

• Manufacturer Part Number: DEMOMC33941
• Description: BOARD DEMO FOR MC33941 SENSOR
• Manufacturer: Freescale Semiconductor

Specifications of DEMOMC33941

• Sensor Type: Touch, Capacitive
• Interface: Analog
• Voltage - Supply: 9 V ~ 18 V
• Embedded: No
• Utilized Ic / Part: MC33941
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Sensitivity: -
• Sensing Range: -

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.
electrode are affected by objects in proximity. A “capacitor” is
CAPACITOR MODEL
•
•
•
MC33941
6
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)
The 33941 generates a low radio frequency sine wave with
The amplitude and phase of the sinusoidal wave at the
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
=
k
ε
d
Drive level ~ 5 v p-p
Drive level ~ 5 v p-p
0
Figure 3. Capacitor Model
A
Sine Generator
Sine Generator
(120 KHz)
(120 KHz)
Load Resistor
Load Resistor
(22 K ohms)
(22 K ohms)
Stray Variable
Stray Variable
Capacitance
Capacitance
C
k
Figure 2 . Conceptual Block Diagram
PRINCIPLE OF OPERATION
Virtual Ground
Virtual Ground
d
Voltage Level Proportional to 1/C (voltage divider)
Voltage Level Proportional to 1/C (voltage divider)
Capacitance
Capacitance
increases as
increases as
electrodes move
electrodes move
closer together
closer together
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.
always useful to approach the problem using the capacitor
model.
Electrodes
Electrodes
Object
Object
Detector
Detector
While exploring applications using the E-Field chip, it is
Table 4 Dielectric Constants of Various Materials
Acrylic
Glass
Nylon Plastic
Polyester Film
Flexible Vinyl Film
Air
Water
Ice
Automotive Oil
Dielectric Material
Capacitor Model
Capacitor Model
Detected Signal
Detected Signal
Level Decreases
Level Decreases
with Increasing
with Increasing
Capacitance
Capacitance
Low Pass Filter
Low Pass Filter
Thickness (mil)
84.5
74.5
68
10
9
-
-
-
-
Freescale Semiconductor
2.4-4.5
3.0-5.0
2.8-4.5
7.5
3.2
3.2
2.1
80
k
1
Sensors