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SI1120 Arkusz danych(PDF) 10 Page - Silicon Laboratories |
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SI1120 Arkusz danych(HTML) 10 Page - Silicon Laboratories |
10 / 22 page AN442 10 Rev. 0.1 However, it is important to keep in mind that, due to this same very high dynamic range, high optical loss ports are acceptable. A 30 dB (31.6 to 1 attenuation) loss only results in one third the range (up to 15 cm). This means existing holes in product cases not designed for optical use may be acceptable for short-range proximity sensing; so, the receiving IC might be put next to a speaker or microphone hole that even has a screen over it, and the emitting infrared LED might be placed behind a glass display screen. In fact, by attenuating the IC input, there is often not much loss in range over what might arise from worst-case environmental light noise. If given two semi- opaque windows, one of which is more opaque than the other, it is best to place the LED under the less opaque location and let the Si1120 or Si1102 be in the more opaque location. This choice will result in a better signal-to- noise ratio. Users are warned that it is possible to adjust the Si1102 to have high apparent sensitivity where the TX to RX coupling might be as much as 10 to 100 times more than the reflectance from the detected object. This type of adjustment will not be stable since drift in the gain or power supply voltage can cause spurious detects or no detects. The LED has a high temperature coefficient of around 0.4%/C. Consequently, a 10 °C change can cause a 4% decrease in signal. If it is unavoidable that a high optical coupling exists between the TX (LED) and RX (Si1120), the Si1120 may still be used for motion detection applications, even under this somewhat difficult condition. Essentially, motion detection looks for changes in reflection over the last second or so, ignoring longer-term slower changes. For example, the repetitive pulse width (sampled every 100 ms) output would be averaged over the last 10 seconds, and a motion detection event is determined to occur if the average pulse width value over the last second has changed by at least 10 µsec. In high-sensitivity proximity mode (PRX50H), 10 µsec of pulse width corresponds to about 3.3 nW/cm2 of reflectance change. The Si1120 offers two proximity ranges that differ in pulse width sensitivity by 9. The less sensitive range overloads (maximum pulse width) at about ten times higher input levels. In a high-reflectance environment, the lower sensitivity range may need to be used. Although motion detection cannot unambiguously detect the presence or absence of an object unless it moves, it can estimate object distance and possibly direction of motion when the object moves. Because of the fourth-power effect, a 20% change in distance results in a change in measured reflectance of over two to one. Since any motion- detected object must return a reflectance that is more than the change threshold, this means that we should be able to accurately and easily detect object movement. Direction of motion can be valuable but may require multiple LED sources. If the Si1120 is not used for motion but, rather, as a two-port absolute proximity detector, the calibrate mode (OFC) should be used to maximize performance. This mode is implemented internally in the Si1102, but, in the Si1120, it is provided as a separate mode. The calibration essentially measures the zero reflectance value by making proximity measurement with the LED TX driver turned off. If a microcontroller controls multiple LEDs, it is possible to perform this calibration function by disabling the LED and doing a proximity measurement (in either high or low sensitivity). The calibration function is important since the zero reflection offset value can change by a small but significant amount depending on the dc ambient. Because the dc ambient may be changing rapidly, it is important to perform a calibrate measurement immediately before the proximity measurement without delay between the two measurements. The jitter on the calibration pulse is a measure of noise in the environment and, consequently, can be used to set a minimum detect value; this might be set to one to several times the peak-to-peak jitter value in order to set a robust detect threshold higher than the environmental noise. For motion detection, the calibrate function is usually not necessary since the long-term average provides the reference. On the Si1120, the VIRL Ambient Mode has the same Reflectance-Pulse Width transfer function as the PRX400 or PRX50 Proximity Modes. Therefore, it is possible to use the VIRL Ambient Mode (looking for excessive PRX jitter) as a way of debugging the presence of severe environmental noise when the PRX400 or PRX50 operations result in excessive PRX jitter. In the same way, the VIRH Ambient Mode can be used to debug the PRX50H Proximity Mode. |
Podobny numer części - SI1120 |
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Podobny opis - SI1120 |
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