Optical Proximity Sensing With Longer Off-Phase Noise Averaging
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Solution Overview
Problem
Conventional optical proximity sensors face challenges in improving the signal-to-noise ratio (SNR) under high ambient light conditions, particularly due to shot noise from the photodiode, especially when used in smartphones with OLED displays where the sensor is placed behind the screen, limiting the duration the light source can be switched on without causing display distortion.
Innovation Solution
The optical proximity sensor employs a measurement circuit that differentiates between light measurements taken with the light source on and off, with the off-phase lasting longer than the on-phase, utilizing a capacitor arrangement to average noise and increase signal energy, thereby enhancing the SNR. This is achieved by using a photodiode and a light source like a LED or VCSEL, with the measurement circuit comprising an integrator, sum and hold circuit, and capacitors that operate differently during on and off phases to optimize signal processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the light source is switched on for a longer duration to improve signal measurement, then the signal energy increases, but display distortion occurs in behind-OLED configurations
Solution Approach 1:
The patent applies periodic action by implementing alternating on-phase and off-phase measurement cycles. The light source is switched on for a limited duration (e.g., 100-500 microseconds) to capture reflected light signals, then switched off for longer off-phase periods (e.g., 500-2000 microseconds) to allow display recovery and prevent distortion. This periodic switching enables continuous proximity detection while maintaining display integrity in behind-OLED configurations.
2Measurement precision
If the off-phase measurement duration is extended to reduce shot noise, then the signal-to-noise ratio improves, but the measurement cycle time increases
Solution Approach 1:
The patent applies partial action by performing multiple off-phase measurements (e.g., 2-8 measurements) during each off-phase period and averaging the results to reduce shot noise. The off-phase duration is set to 2-8 times longer than the on-phase duration, allowing sufficient time for noise reduction through averaging without excessively extending the total measurement cycle. This approach achieves improved signal-to-noise ratio while maintaining responsive proximity detection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces noise, particularly shot noise, and improves the SNR by a factor of 2 compared to conventional sensors, allowing for more accurate proximity detection even under high ambient light conditions without significantly increasing power consumption or capacitor area, while maintaining display integrity.
Implementation Method 1
a photodiode (205) to receive optical light
Implementation Method 2
a light source (110) which is configured to emit light during an on-phase of the light source. The light source may be a LED (light-emitting diode)
Data Source
AI summary
An optical proximity sensor comprises a photodiode, a light source configured to emit light and a measurement circuit coupled to the photodiode. The measurement circuit is configured to measure light received by the photodiode in a first phase when the light source is turned off and in a second phase when the light source is turned on. The measurement circuit determines the difference between the light measured in the first and second phases, wherein the first phase for off measurement is longer than the second phase for on measurement.