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MC33941R2 Arkusz danych(PDF) 7 Page - Freescale Semiconductor, Inc |
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MC33941R2 Arkusz danych(HTML) 7 Page - Freescale Semiconductor, Inc |
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7 / 11 page  MC33941 Sensors Freescale Semiconductor 7 FEATURES SHIELD DRIVER A shield driver is included to minimize the electrode signal along wires. This circuit provides a buffered version of the returned AC signal from the electrode. Since it has nearly the same amplitude and phase as the electrode signal, there is little or no potential difference between the two signals, thereby canceling out any electric field. In effect, the shield drive isolates the electrode signal from external virtual grounds. A common application is to connect the Shield Driver to the shield of a coax cable used to connect an electrode to the corresponding electrode terminal. Another typical use is to drive a ground plane that is used behind an array of touch sensor electrodes in order to cancel out any virtual grounds that could attenuate the AC signal. TUNABLE FREQUENCY The 33941 offers 3 operating frequencies. In addition to the default frequency of 120 kHz, the 33941 has also been characterized to work in two other frequencies (240 kHz and 60 kHz) for applications with specific needs. These frequencies are tunable by attaching a 20k and 82k resistor at ROSC respectively. If a wider capacitance range is needed, simply change the ROSC resistor value to 82k to have the signal generator operate at 60 kHz which will extend the capacitance range to 150pF as seen on Figure 4. The figure also shows that one can achieve higher sensitivity at lower capacitances by setting the ROSC resistor value to 20k. All resistor values listed above are for 5% tolerance resistors. ADJUSTABLE RESPONSE TIME The rectified sine wave is filtered by a Low Pass Filter formed by and internal resistor and an external capacitor attached to LP_CAP. The value of the external capacitor is selected to allow the designer to optimize the balance between noise and settling time. A typical value for the external capacitor is 10nF and in practice it will have a response time of 1.5ms. If faster response time is required a 1nF capacitor can be used and it will have response times <200uS. Please note that reducing the LP_CAP capacitor value increases noise accordingly. Figure 4. Output Voltage vs. Capacitance at 3 Discrete Frequencies Output Voltage vs Capacitance at 3 Discrete Frequencies 0 0.5 1 1.5 2 2.5 3 3.5 4 0 20 40 60 80 100 120 140 160 Capacitance (pF) 120 kHz 240 kHz 60 kHz |
Podobny numer części - MC33941R2 |
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Podobny opis - MC33941R2 |
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