Optical Proximity Sensor Dynamic Range via Ambient Light Cancellation
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Solution Overview
Problem
Existing optical proximity sensors face challenges in accurately detecting weak AC current signals due to high ambient light interference, leading to increased complexity, reduced dynamic range, and lower power efficiency, as they require complex two-step measurement cycles and high-resolution ADCs to separate AC and DC signals.
Innovation Solution
A differential integrator circuit is employed, which emits AC pulses of infrared light with low or no emission between pulses, using a differential amplifier to cancel out ambient light DC signals, allowing for the detection of small AC current signals with increased dynamic range and sensitivity, reducing the need for high-resolution ADCs and simplifying signal acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a two-step measurement cycle is used to separate AC and DC signals, then measurement precision is improved, but device complexity increases and power efficiency decreases
Solution Approach 1:
The patent employs periodic pulsing of the infrared LED at a known frequency, converting the measurement problem from a complex two-step cycle to a simpler periodic sampling approach. The AC signal is extracted by synchronizing detection with the LED pulse timing, eliminating the need for separate measurement cycles while maintaining precision.
Solution Approach 2:
The patent replaces the mechanical/time-based two-step measurement cycle with an electrical signal processing approach using lock-in amplification and synchronous detection. This substitution reduces operational complexity while maintaining the ability to separate AC and DC signals through electrical rather than temporal separation.
2Adaptability or versatility
If DC current signals are amplified to match AC signal levels, then dynamic range is improved, but measurement precision deteriorates due to loss of resolution for small AC signals
Solution Approach 1:
The patent introduces an intermediary signal processing stage using lock-in amplification that operates at the LED pulse frequency. This intermediary process selectively amplifies only the AC component at the known frequency while rejecting DC and other frequency components, thereby maintaining resolution for small AC signals while achieving wide dynamic range.
Solution Approach 2:
The patent changes the detection parameter from direct amplitude measurement to frequency-based detection. By transforming the signal into the frequency domain through synchronous detection at the LED pulse frequency, the system can resolve small AC signals without being dominated by large DC signals, effectively expanding dynamic range while preserving precision.
3Measurement precision
If high-resolution ADC is used to resolve small AC signals in presence of large DC signals, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces the requirement for high-resolution ADC with a combination of synchronous detection and signal processing techniques. By converting the AC signal to a DC signal through lock-in amplification at the known LED frequency, the system can use standard-resolution ADCs to measure the transformed signal, eliminating the need for expensive high-resolution converters while maintaining precision.
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 enhances the dynamic range and sensitivity of optical proximity sensors, allowing for accurate detection of small AC current signals, reduces complexity, and improves power efficiency by eliminating the need for complex subtraction operations and high-resolution ADCs, enabling more precise distance measurements.
Implementation Method 1
a light detector configured to detect ambient light DC signals and infrared light AC pulses emitted by the light emitter and reflected from an object to be detected towards the light detector
Data Source
AI summary
Various embodiments of an optical proximity sensor and corresponding circuits and methods for measuring small AC signal currents arising from the detection of pulsed AC light signals emitted by a light emitter and reflected from an object to detected in the presence of larger ambient light DC current signals are disclosed. Circuits and corresponding methods are described that improve the dynamic range, sensitivity and detection range of an optical proximity sensor by cancelling the contributions of DC current signals arising from ambient light signals that otherwise would dominate the detected small AC signal currents. The DC signal cancellation occurs in a differential amplifier circuit before small AC signal currents are provided to an analog-to-digital converter. The circuits and methods may be implemented using conventional CMOS design and manufacturing techniques and processes.