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SI1102 Datenblatt(PDF) 11 Page - Silicon Laboratories |
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SI1102 Datenblatt(HTML) 11 Page - Silicon Laboratories |
11 / 22 page AN442 Rev. 0.1 11 3.4. Light Noise The low-cost silicon photodiode used in the proximity sensor has a peak response in the near infrared but also has a significant response in the visible light region unless blocked by a filter. Consequently, both the visible and infrared characteristics of the environment can limit the performance of these proximity sensors. Light in the environment is measured in terms of either Lux, which is illuminance (visual intensity), or in W/m2, which is irradiance (power per area). One of the topics relevant to ambient light and noise issues pertaining to reflectance proximity sensors is the Luminous Efficacy of Radiation (LER). The LER is the ratio of luminous flux to radiant flux. As a reference, direct sunlight at noon provides about 100 mW/cm2 of irradiance (received power) or 110 klux of illuminance (visual intensity), usually the maximum continuous light level that a proximity sensor will encounter. Indoor lighting levels are typically less than 1 klux or about 230µW/cm2 if the light is from efficient lighting (with no infrared like fluorescent), but, if the 1 klux is from incandescent lights, it will produce about 1.5 mW/ cm2 silicon weighted response (relative to the mW/cm2 for a source at the silicon diode peak response) due to their high infrared output. Sunlight has about one-fourth to one-fifth the silicon diode infrared response per lux as an incandescent bulb. The silicon photodiodes used in the Si1102 and Si1120 tend to have a broad response from about 350 to 1000 nm, with a peak around 830 nm. Light noise in the environment arises principally from shot noise, 120 Hz or 100 Hz ac mains light modulation, and high-frequency electronic ballasts used in fluorescent lights (especially compact fluorescents or CFLs). The behavior of the noise sources is different in the visible versus infrared wavelength bands. Shot noise on a photodiode is proportional to the square root of the current or background ambient level. Generally, the largest shot noise occurs in direct sunlight (~100 mW/cm2), which, for both devices, is equivalent to an RMS noise of about 3 µW/cm2. A tenfold reduction to 10 mW/cm2 (shade) will reduce this noise to about 1 µW/ cm2. Below 2 mW/cm2, the shot noise is less than the noise floor. Generally, indoor lighting is less than 1 klux and for it to exceed 1.5 mW/cm2 requires either daylight, window lighting, or bright incandescent lighting. Bright fluorescent lighting is much less than these levels. Consequently, shot noise has little effect on sensitivity for most indoor applications unless the proximity devices are directly under strong incandescent lighting. Even in direct sunlight, the increase in noise floor only reduces detection range by less than twofold. The peak-to-peak ripple of incandescent or halogen light sources at 120/100 Hz is about 10% of the average value, but, since incandescent or halogen contains significant power in the infrared region (eight times more infrared than visible light) and since the photodiodes (used for proximity sensing) respond primarily to infrared light, shot noise is a dominant noise source when the proximity detectors are under direct incandescent or halogen light. Fluorescent lights can actually have much higher light ripple percentage than this for both standard ballasts, but virtually the entire ripple is in the visible band. Most high-frequency inverter ballasts filter the input rectified voltage to reduce 120 Hz/100 Hz ripple or visible flicker but may still have peak-to-peak mains ripple comparable to incandescent ballasts. Of course, any other type of lighting powered from the ac mains, whether white LED, industrial sodium lights, mercury vapor, etc., will all typically have these 120/100 Hz components. In addition, the high-frequency ballasts on fluorescents typically use 20 kHz (long tubes) to 40 kHz (CFLs) inverters, which generate a triangular current waveform that, when folded, causes a 2x frequency light modulation of the ultraviolet light from the mercury vapor plasma discharge. The actual peak-to-peak visible light ripple at 40 kHz or 80 kHz is much less (<5%) than the ultraviolet ripple due to the time constant of the phosphors used on the inside of the tube. Although the mercury plasma mostly generates ultraviolet, it produces a small amount of visible and infrared radiation directly, which can leak past the phosphor. The peak-to-peak percentage modulation of the infrared component can be quite large, although the absolute value is typically low. Without infrared filtering, CFL noise is typically about 5 µW/cm2 to 10 µW/cm2 in most CFL illuminated environments. Consequently, for shorter range applications where detect thresholds are above 20 µW/cm2, infrared filtering is not necessary. For best performance, it is recommended that infrared filtering be available for the Si1120 if the ALS function is not used. For the Si1102, infrared filtering is necessary. Although infrared filtering of the receiver IC can remove visible light inverter noise, it generally has little effect on shot noise or 120/100 Hz infrared from incandescent lights. However, both the Si1102 and Si1120 electrically filter 120/100 Hz noise to reduce its level by 50 dB, or 316 to 1. On the Si1120, 120/100 Hz noise can be mitigated for motion sensing by sampling exactly every 50 ms since there are exactly six cycles of 120 Hz or five cycles of 100 Hz. Similarly, to improve sensitivity and reduce the effects of 120/100 Hz, multi-strobe integration periods should be in exact multiples of 50 ms. |
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