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參數(shù)資料
| 型號： | MC34940EGR2 |
| 廠商： | Freescale Semiconductor |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC IMAGING DEVICE ELEC 24-SOICW |
| 標(biāo)準(zhǔn)包裝： | 1,000 |
| 類型： | 圖像傳感器 |
| 輸入類型： | 邏輯 |
| 輸出類型： | 電壓 |
| 電流 - 電源： | 8mA |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 24-SOIC（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 24-SOIC W |
| 包裝： | 帶卷 (TR) |
| 配用： | DEMO1985MC34940E-ND - KIT DEMO FOR MC34940E FAMILY |
Sensors
6
Freescale Semiconductor
MC34940
PRINCIPLE OF OPERATION
The MC34940 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 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 (22 k) was chosen
to provide a near linear relationship at 120 kHz over a range
of 10 pF to 70 pF.
While exploring applications using the E-Field chip, it is
always useful to approach the problem using the 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
Figure 3. Capacitor Model
Detected Signal
Level Decreases
with Increasing
Capacitance
increases as
electrodes move
closer together
Drive level ~ 5 V p-p
Load Resistor
Sine Generator
(120 kHz)
Stray Variable
Capacitance
(22 k
)
Detector
Low Pass Filter
Electrodes
Capacitor Model
Virtual Ground
Object
Voltage Level Proportional to 1/C (voltage divider)
d
A
k
C
0
ε
=
C
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)
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
-
1.0
Water
-
80
Ice
-
3.2
Automotive Oil
-
2.1
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