Pixel Circuit Controller for High Dynamic Range Imaging
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Solution Overview
Problem
Existing pixel circuits face limitations in dynamic range due to limited storage capacity in the capacitive conversion node, leading to saturation at higher light intensities and requiring cumbersome control mechanisms to manage different light intensity modes.
Innovation Solution
A circuit controller that employs correlated double sampling and an additional double sampling technique, using a single exposure of the photo diode to sample reference and signal voltage values, with switches controlling the capacitance to increase storage capacity and cancel systematic errors, allowing for high dynamic range operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the capacitive conversion node storage capacity is increased to handle higher light intensities, then the dynamic range is improved, but the device complexity and control mechanisms become more cumbersome
Solution Approach 1:
The pixel circuit dynamically adjusts the effective storage capacity by switching between two operational modes: a first mode where the capacitive conversion node operates with its full storage capacity for high light intensities, and a second mode where the storage capacity is reduced for low light intensities. This dynamic adaptation allows the circuit to optimize performance across varying light conditions without requiring complex external control mechanisms.
Solution Approach 2:
The circuit employs periodic switching between the two operational modes based on light intensity thresholds. The transfer switch periodically connects or disconnects the photo diode from the capacitive conversion node, creating distinct exposure periods that correspond to different storage capacity configurations. This periodic action enables automatic adaptation to changing light conditions.
2Adaptability or versatility
If mode switching is implemented to handle different light intensities, then the dynamic range is improved, but systematic errors are introduced during sampling
Solution Approach 1:
The circuit performs preliminary sampling of the pixel voltage at the capacitive conversion node before the actual readout operation. By sampling the voltage value while the node is still isolated from the photo diode, the circuit captures a reference state that can be used for correlation double sampling. This preliminary action occurs before any mode switching or charge transfer, ensuring that systematic errors are minimized.
Solution Approach 2:
The circuit uses correlation double sampling where the preliminary sampled voltage is compared with the final readout voltage to eliminate systematic errors. The feedback mechanism subtracts the reference sample from the signal sample, canceling out common-mode noise and systematic errors that arise during the mode switching and charge transfer operations.
3Productivity
If the photo diode is reset with increased reference voltage to enhance depletion, then the charge transfer efficiency is improved, but the reset noise increases
Solution Approach 1:
The circuit extracts the reset noise from the final signal by performing a preliminary sample of the pixel voltage immediately after reset but before charge transfer. This extracted reference value contains the reset noise component, which is then subtracted from the final signal measurement. By taking out the reset noise component separately, the circuit achieves efficient charge transfer while eliminating the harmful reset noise from the output signal.
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 simplifies the control of pixel circuits, enhances dynamic range, and reduces systematic errors, enabling effective operation across varying light intensities without the need for complex mode switching.
Implementation Method 1
a photo diode for accumulating charge carriers upon exposure to incident light
Data Source
Figure 1~2
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AI summary
A pixel circuit comprises a first capacitor,a photo diode and a switch. A voltage source generates a reference voltage to reset the pixel circuit. The pixel circuit is reset for a first reset time period by electrically coupling a cathode of the photo diode and a first capacitor terminal to the voltage source. The cathode is decoupled from the voltage source and the photo diode is exposed to light for an accumulation time period. After the accumulation time period,a first reference voltage is sampled. The cathode is then coupled, via the switch,to the first capacitor terminal for a selected transfer time period, during which a second signal voltage is sampled. After the selected transfer time period, a first signal voltage is sampled with the cathode decoupled. The pixel circuit is then reset for a second reset time period, after which a second reference voltage value is sampled.