CMOS Ambient Light Sensor Dark Current Cancellation
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Solution Overview
Problem
Ambient light sensors based on CMOS photodiodes face non-linearities due to amplifier designs, particularly at high gain values and low light levels, and dark current issues, which limit their dynamic range and introduce noise, reducing their effectiveness.
Innovation Solution
A light sensor design utilizing two identical photodetectors, one covered to generate dark current, connected in series with a cascode current mirror to amplify the light current proportionally while canceling out dark current without amplifying noise, enhancing the linear range and reducing non-linearities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a separate identical photodiode covered by an opaque layer is included to correct for dark current, then dark current correction is achieved, but device complexity increases and additional noise is introduced
Solution Approach 1:
The patent combines the dark current correction function with the main light detection function by using a single photodiode that performs both tasks. The photodiode generates both the light current (when illuminated) and the dark current (when not illuminated), eliminating the need for a separate covered photodiode. This merging approach reduces device complexity while maintaining dark current correction capability.
Solution Approach 2:
The patent extracts the dark current correction function from a separate component and integrates it into the main photodiode operation. By using the photodiode's response to different light conditions (illuminated vs. non-illuminated), the dark current is corrected through signal processing rather than requiring a separate physical component.
2Power
If amplifiers are used to amplify the small signal from the photodiode, then signal amplitude is sufficient for processing, but non-linearities are introduced at high gain values
Solution Approach 1:
The patent performs preliminary action by measuring the dark current separately (when the photodiode is not illuminated) and subtracting it from the total current measurement. This preliminary dark current measurement and subtraction occurs before the final signal processing, allowing the amplifier to work with a corrected signal that has reduced non-linearities and extends the linear dynamic range.
3Reliability
If the dark current is amplified separately and then subtracted, then dark current correction is achieved, but noise associated with dark current is also amplified
Solution Approach 1:
The patent extracts the dark current measurement from the main signal path by taking a separate measurement when the photodiode is not illuminated. This separate dark current measurement is then used to correct the main signal without requiring amplification of the dark current component itself, thereby avoiding the amplification of associated noise.
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
The solution provides a linear output signal that accurately represents light intensity, extending the useful dynamic range and reducing noise, thereby improving the sensor's performance and battery life in mobile devices.
Implementation Method 1
The first photodetector receives a light signal characterized by a light intensity to be measured. The first photodetector generates a light current therethrough proportional to the light intensity
Implementation Method 2
The current mirror is connected to the first node and the output node and generates an output current that is proportional to the light current through the output node when the output node is connected to an external circuit. The current mirror includes a first cascoded current mirror having first and second MOS transistors and a depletion mode MOS transistor.
Data Source
AI summary
A light sensor having first and second photodetectors and a current mirror is disclosed. The first photodetector receives a light signal and generates a light current proportional to the light intensity received by the first photodetector and a dark current that is independent of the light intensity. The second photodetector is connected in series with the first photodetector at a first node. The second photodetector has a covering that prevents light from reaching the second photodetector. The second photodetector generates a current equal to the dark current. The first photodetector is connected to a power source and the second photodetector is connected to an output node. The current mirror is connected to the first node and the output node and generates an output current that is proportional to the light current through the output node when the output node is connected to an external circuit.