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A 3-in-1 optical sensor typically refers to a sensor that integrates an infrared emitter, proximity sensing photodiode, ambient light sensor (ALS), a signal processing IC and standard I2C interface. Vishay’s VCNL4020 is just such a sensor. The ALS is most commonly used in mobile phones to automatically optimize the LCD backlight based on ambient light. The proximity sensor is used to turn off the LCD and disable the touch sensor when you are talking on the phone. With the phone to your ear, there is no need for the LCD to be on and a great deal of power can be saved by turning it off. In a recent article published in Electronic Design (http://bit.ly/1079pnO), Tamara Schmitz describes some of the trade offs of mounting a 3-in-1 sensor on the main PCB versus the more expensive but typical mounting on a flexible circuit. The sensor is mounted on a flexible circuit so it can be positioned within 0.2 mm of the glass surface.
Impact of Surrounding Components on a PCB Mounted 3-in-1 Sensor
Tamara Schmitz’s article describes the main benefits of mounting the sensor on the PCB: reduced component count, elimination of an expensive flexible circuit and connector, and simplified assembly. One of the drawbacks she describes is internal reflection.
“The distance created by increasing the gap also causes an increase in internal reflection; the greater the distance, the larger the amount of internal reflection. Many sensors aren’t prepared to deal with a large amount of internal reflection. The internal reflection could reduce the proximity signal range or saturate it completely.”
The air gap is defined as the distance from the top of the sensor to the bottom surface of the glass or window. What is being described as internal reflection in the article is the infrared light reflecting off the inside surface of the glass or window. This is minimal compared to the reflection of light from surrounding components. When a 3-in-1 sensor is moved from its isolated position on the flexible circuit pedestal and mounted on the main printed circuit board, it will be surrounded by lots of other components. Any thought of adding foam around the sensor to eliminate reflections from surrounding components is counterproductive to the effort of reducing cost and component count.
Our testing shows that the reflection of the infrared light off of the surrounding components can increase the proximity offset by up to 25,500 counts. Counts are equivalent to the amount of light being received by the proximity photodiode that was emitted by the sensors infrared emitter; the more light received, the higher the counts. Anything but a 16-bit ADC would overflow or saturate. In other words, an 8-bit (256 counts), 10-bit (1024 counts), or 12-bit (4096 counts) proximity sensor will likely never be able to be moved to the main PCB. It would be saturated and couldn’t sense an object.
Screen Shot of VCNL4020 Demo Kit Interface Showing Offset of Surrounding Parts
Upper window shows offset with ~40,000 counts still available for object detection
The VCNL4020 has 16-bit proximity resolution and, despite the large offset due to reflection off of surrounding components, has more than enough counts available for proximity sensing. Something to consider when you are ready to eliminate that 30 cent flexible circuit and connector.