
TMD2671
DIGITAL PROXIMITY DETECTOR
TAOS144B ? SEPTEMBER 2012
Referring again to Figure 7, the reflected IR LED and the background energy is integrated during the LED On
time, then during the LED Off time, the integrated background energy is subtracted from the LED On time
energy, leaving the IR LED energy to accumulate from pulse to pulse.
After the programmed number of proximity pulses have been generated, the proximity ADC converts and scales
the proximity measurement to a 16-bit value, then stores the result in two 8-bit proximity data (PDATAx)
registers. ADC scaling is controlled by the proximity ADC conversion time (PTIME) which is programmable from
1 to 256 2.73-ms time units. However, depending on the application, scaling the proximity data will equally scale
any accumulated noise. Therefore, in general, it is recommended to leave PTIME at the default value of one
2.73-ms ADC conversion time (0xFF).
For additional information on using the proximity detection function behind glass and for optical system design
guidance, please see available TAOS application notes.
Optical Design Considerations
The TMD2671 device simplifies the optical system design by integrating an IR LED into the package, and also
by providing an effective barrier between the LED and proximity sensor. In addition the package contains
integrated lenses and apertures over both the LED and the sensor, which significantly extends the maximum
proximity detection distance and helps to reduce optical crosstalk.
Although the package integrates an optical barrier between the IR LED and detector, placing the device behind
a cover glass potentially provides another significant path for IR light to reach the detector, via reflection from
the inside and outside faces of the cover glass. Because it is cost prohibitive to use anti-reflection coatings
on the glass, the faces of the glass will reflect significantly (typically on the order of 4% of the light), and it is crucial
that the system be designed so that this reflected light cannot find an efficient path back to the optical detector.
See TAOS Application Note DN28: Proximity Detection Behind Glass for a detailed discussion of optical design
considerations.
Copyright E 2012, TAOS Inc.
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