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MC33941 Datasheet(PDF) 6 Page - Freescale Semiconductor, Inc |
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MC33941 Datasheet(HTML) 6 Page - Freescale Semiconductor, Inc |
6 / 11 page MC33941 Sensors 6 Freescale Semiconductor 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. 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 Detector Low Pass Filter Voltage Level Proportional to 1/C (voltage divider) Drive level ~ 5 v p-p Load Resistor (22 K ohms) Sine Generator (120 KHz) Detected Signal Level Decreases with Increasing Capacitance Electrodes Capacitance increases as electrodes move closer together Capacitor Model Virtual Ground Stray Variable Capacitance Object Detector Low Pass Filter Voltage Level Proportional to 1/C (voltage divider) Drive level ~ 5 v p-p Load Resistor (22 K ohms) Sine Generator (120 KHz) Detected Signal Level Decreases with Increasing Capacitance Electrodes Capacitance increases as electrodes move closer together Capacitor Model Virtual Ground Stray Variable Capacitance Object 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 Water - 80 Ice - 3.2 Automotive Oil - 2.1 |
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