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SI1120 Scheda tecnica(PDF) 9 Page - Silicon Laboratories

Il numero della parte SI1120
Spiegazioni elettronici  DESIGNERS GUIDE
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Produttore elettronici  SILABS [Silicon Laboratories]
Homepage  http://www.silabs.com
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AN442
Rev. 0.1
9
Figure 7. Single Port vs. Dual Port
3.3. Proximity Reflectance Sensitivity
For a short 10 µs pulse, the internal noise floor for both of these products is equivalent to around 0.5 µW/cm2 RMS
(~1.5 µs PWM in high-sensitivity) of radiance at the peak response. This makes the lowest practical pulse
detection threshold around 2 µW/cm2 (12 dB S/N) in low light noise environments. Lower thresholds can be used
with averaging techniques.
An efficient 850 nm infrared LED emitter driven with 400 mA of peak current might output up to 80 mW of infrared.
In a lensed LED package, narrowing the beam width to ±15° might produce 100 mW/sr, which results in an
irradiance of 100 mW/cm2 if the receiving (RX) proximity IC is placed 1 cm directly in front of the transmit (TX)
LED. Note that the dynamic range of noise floor to maximum signal is about 1 to 200,000 or 106 dB (20 x log
200000 = 106 dB).
Typically, the transmit LED is aimed away from the receiving IC, but, if placed next to the IC with an infrared plastic
window only a few milimeters away, the reflected TX levels at the IC may be over 1 mW/cm2. Consequently,
because it is dual-port only, when using the Si1102, the optical coupling from the LED to the receiver must be
below the detect object threshold or below the noise floor. When using an efficient LED and driving with 400 mA,
this may as much as 106 dB or 2 x 105. Generally, this is only possible with a dual-port optical structure in which
the emitting LED is completely isolated and radiates its output through a separate aperture in the product case
different from the receiving window or hole. If the TX LED and IC are close to each other, this may require an
internal optical fence between the two. However, for shorter-range applications (~ 5–10 cm), a single window with
an optical barrier, such as black rubber between TX and receive (the black rubber must be sealed tightly to the
interior surface of the window), may provide sufficient optical isolation. At 5–10 cm, the reflectance from a target
object will be several hundred times higher. Since the criteria for dual port is that the non-target coupling needs to
be less than the target, the stray coupling needs to be less than ~103. Since an LED next to the IC reflecting from a
transparent plastic window probably has about 3% reflectance relative to a mirror (especially if the LED is laterally
spaced by at least twice the distance to the window), then the optical coupling need ony be reduced by another
factor of 30. This can generally be achieved by an optical barrier from the PC board to the inside surface of the
window. Some light will light-pipe internally, but it is usually low enough to allow 5 to 10 cm response.
Transmit LED
Optical block (<10-6 transmissive)
Si1120
internal reflection
Transparent window
Optical block
Transmit LED
Si1120
mode 4
Dual-Port Optical Window
Single-Port Optical Window


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