AN442

Rev. 0.1

9

Figure 7. Single Port vs. Dual Port

3.3. Proximity Reflectance Sensitivity

(~1.5 µs PWM in high-sensitivity) of radiance at the peak response. This makes the lowest practical pulse

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

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,

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

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

Si1120

internal reflection

Transparent window

Optical block

Transmit LED

Si1120

mode 4

Dual-Port Optical Window

Single-Port Optical Window