Ambient Light Sensor Chopping Circuit for Dark Current Cancellation
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Solution Overview
Problem
Ambient light sensors face limitations in measuring low light levels due to high dark current and longer auto zero times, which restrict their ability to accurately detect low light conditions, especially under dark glass where larger photodiodes are required for sensitivity, leading to increased leakage currents.
Innovation Solution
The use of a sensor arrangement with two sets of chopping switches and a chopping technique that applies different polarities at various stages of the integration cycle to cancel out dark current, allowing for accurate low light measurement with reduced auto zero time and lower noise requirements, thereby enhancing the sensitivity and accuracy of light sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a larger photodiode area is used to increase sensitivity under dark glass, then light detection sensitivity is improved, but leakage current increases
Solution Approach 1:
The patent extracts and removes the harmful leakage current component from the measurement signal by implementing a dark current cancellation mechanism. The system separately measures the dark current (leakage current) component and subtracts it from the total current measurement, effectively removing the harmful effect of leakage current while preserving the useful light detection signal.
Solution Approach 2:
The patent changes the operational parameters of the photodiode by applying reverse bias voltage and controlling the integration time. By adjusting the reverse bias voltage, the system optimizes the trade-off between sensitivity and leakage current. The integration time is also adjusted to allow sufficient signal accumulation while limiting the accumulation of leakage current over time.
2Measurement precision
If traditional auto zero operation is used, then offset voltage cancellation is achieved, but measurement time increases
Solution Approach 1:
The patent performs preliminary dark current measurement and cancellation during the normal measurement integration period rather than requiring a separate auto-zero phase. The system measures the dark current level at the beginning of the integration period and uses this information to cancel offset voltages throughout the measurement, eliminating the need for dedicated auto-zero time.
Solution Approach 2:
The patent merges the dark current cancellation function with the normal light measurement function. Instead of treating them as separate operations requiring distinct time periods, the system combines both functions into a single integrated measurement process where dark current characterization and light signal acquisition occur simultaneously, thereby eliminating additional time overhead.
3Measurement precision
If photodiode area is increased to compensate for dark glass attenuation, then sensitivity is improved, but dark current increases
Solution Approach 1:
The patent converts the harmful dark current into a useful measurement reference. By deliberately measuring the dark current level under the same operating conditions as the light signal, the system transforms the harmful leakage current into a beneficial reference value that enables accurate offset cancellation and improves the overall measurement accuracy, especially in low-light conditions.
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 cancels dark current for each measurement, enabling accurate low light level detection and reducing auto zero time, thus improving the overall performance of ambient light sensors by eliminating residual offset voltages and noise, allowing for precise light measurement across a full temperature range.
Implementation Method 1
Some ambient light sensors include semiconductor junctions to generate a sensor signal to detect incident light. The light-to-electron conversion is typical for semiconductor junctions.
Implementation Method 2
The ADC is operable to perform a chopping technique in response to a first clock signal (CLK1)... applying a first phase of chopping at the first set of chopping switches and the second set of chopping switches, where the first phase of chopping is applied at a first polarity, and applying, at a halfway timestamp... a second phase of chopping... where the second polarity is opposite of the first polarity.
Data Source
AI summary
A sensor arrangement for light sensing for light-to-frequency conversion. The sensor arrangement includes a photodiode, an analog-to-digital converter (ADC) operable to perform a chopping technique in response to a first clock signal (CLK1), and convert a photocurrent (IPD) into a digital comparator output signal (LOUT). The ADC includes a sensor input coupled to the photodiode, an output for providing the digital comparator output signal (LOUT), an integrator including an integrator input coupled to the sensor input and operable to receive an integrator input signal, a first set of chopping switches coupled to a first amplifier, a second set of chopping switches electrically coupled to an output of the first amplifier and electrically coupled to input terminals of a second amplifier, and an integrator output providing an integrator output signal (OPOUT).