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PDF MC34940 Data sheet ( Hoja de datos )
| Número de pieza | MC34940 | |
| Descripción | Electric Field Imaging Device | |
| Fabricantes | Freescale Semiconductor | |
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Freescale Semiconductor
Technical Data
Electric Field Imaging Device
The 34940 is intended for cost-sensitive applications where non-contact
sensing of objects is desired. When connected to external electrodes, an
electric field is created. The 34940 detects objects in this electric field. The IC
generates a low-frequency sine wave, which is adjustable by using an external
resistor and is optimized for 120 kHz. The sine wave has very low harmonic
content to reduce harmonic interference. The 34940 also contains support
circuits for a microcontroller unit (MCU) to allow the construction of a two-chip
E-field system.
Features
• Supports up to 7 Electrodes
• Shield Driver for Driving Remote Electrodes Through Coaxial
• High-Purity Sine Wave Generator Tunable with External Resistor
• Response Time Tunable with External Capacitor
• Can support up to 28 touch pad sensors
• Pb-Free and RoHS compliant
Typical Applications
• Appliance Control Panels and Touch Sensors
• Linear and Rotational Sliders
• Spill Over Flow Sensing Measurement
• Refrigeration Frost Sensing
• Industrial Control and Safety Systems Security
• Proximity Detection for Wake-Up Features
• Touch Screens
• Garage Door Safety Sensing
• PC Peripherals
• Patient Monitoring
• Point of Sale Terminals
• Size Detection
• Liquid Level Sensing
ORDERING INFORMATION
Device Name
Temperature
Range
Drawing
MC34940EG/R2
0 to 90°C
98ASB42564B
Package
SOICW-24
Document order number: MC34940
Rev 2.0, 2/2006
34940
ELECTRONIC FIELD
IMAGING DEVICE
24 LEAD (PB-FREE)
SOICW
98ASB42344B
DGND
N/C
SHIELDEN
C
B
A
LEVEL
LPCAP
ROSC
VDDCAP
VPWR
VCCCAP
N/C
E7
E6
E5
E4
E3
E2
E1
TEST
GND
SHIELD
AGND
Figure 1. Pin Connections
© Freescale Semiconductor, Inc., 2006. All rights reserved.
1 page  
PRINCIPLE OF OPERATION
The 34940 generates a low radio frequency sine wave with
nominal 5.0 V peak-to-peak amplitude. The frequency is set
by and 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 routes its signal to a detector, which converts the
sine wave to a DC level. The DC level is filtered by and
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
Load Resistor
(22 K ohms)
Voltage Level Proportional to 1/C (voltage divider)
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 3. 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
d
C
kd
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 4. Capacitor Model
Table 4 Dielectric Constants of Various Materials
Dielectric Material Thickness (mil)
k
Acrylic
84.5 2.4-4.5
Glass
74.5 7.5
Nylon Plastic
68 3.0-5.0
Polyester Film
10 3.2
Flexible Vinyl Film
9
2.8-4.5
Air - 1
Water
- 80
Ice - 3.2
Automotive Oil - 2.1
Sensors
Freescale Semiconductor
MC34940
5
5 Page | ||
| Páginas | Total 9 Páginas | |
| PDF Descargar | [ Datasheet MC34940.PDF ] | |
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