Image Sensor Readout Switch Timing for Dark Offset Suppression
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Solution Overview
Problem
Recent advancements in image sensors for cameras and smartphones have increased sensitivity and gain, making it insufficient to cancel potential fluctuations in column circuits using only a CMOS configuration with reverse-phase pulses, leading to dark offset and degraded signal quality.
Innovation Solution
A photoelectric conversion apparatus with a control unit that manages the transition of a cut-off switch between OFF and ON states to prevent potential fluctuations in the processing circuit, ensuring high-quality signal acquisition by stabilizing the output from the column amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the switch is kept in the OFF state during the entire read-out period, then potential fluctuation from the floating diffusion is blocked, but the processing circuit cannot properly reset and stabilize its potential
Solution Approach 1:
The read-out period is segmented into multiple phases with different switch states: (1) switch OFF during transfer pulse to block potential fluctuation, (2) switch ON after reset to allow potential stabilization. This temporal segmentation resolves the contradiction by applying different switch states to different functional requirements within the overall read-out period.
Solution Approach 2:
The processing circuit is reset in advance before the actual signal read-out begins. The switch is turned ON during this reset phase to allow the processing circuit to stabilize its potential, and then turned OFF before the transfer pulse to block potential fluctuation. This preliminary resetting action ensures the processing circuit is ready to receive signals without being affected by subsequent potential fluctuations.
2Reliability
If the switch is turned ON during the transfer period, then the processing circuit can stabilize its potential, but potential fluctuation from the transfer transistor directly affects the processing circuit input
Solution Approach 1:
The switch is turned OFF in advance during the transfer period to preemptively block the path through which potential fluctuation would affect the processing circuit. This preliminary protective action prevents the harmful effect before it can occur, resolving the contradiction by prioritizing protection during the critical transfer phase.
Solution Approach 2:
The switch operates periodically with different states corresponding to different operational phases: OFF during transfer periods to block fluctuations, ON during read-out periods to allow signal passage. This periodic switching resolves the contradiction by adapting the switch state to the specific phase requirements.
3Object-affected harmful factors
If multiple switch transitions are performed during read-out, then potential fluctuations are cancelled out, but the control complexity and timing precision requirements increase
Solution Approach 1:
The single switch serves multiple functions: blocking potential fluctuation during transfer, enabling signal read-out during read-out period, and allowing processing circuit reset. By making the switch multi-functional and controlling it according to a unified timing sequence, the patent reduces control complexity compared to using separate switches for each function.
Solution Approach 2:
The switch maintains a continuous operational sequence without idle periods, transitioning smoothly between OFF and ON states according to the read-out timing. This continuous action ensures that potential fluctuation cancellation is achieved through consistent timing control rather than requiring complex intermittent adjustments.
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 offset and ensures high-quality image acquisition by stabilizing the output from the column amplifier, even in high-gain conditions, thereby improving signal quality and frame rate.
Implementation Method 1
a pixel array having a plurality of pixels each including a photodiode
Data Source
AI summary
A photoelectric conversion apparatus comprising: a pixel array; a signal line; a processing circuit; a switch for controlling conduction between the signal line and an input node of the processing circuit; and a control unit. The control unit performs a first transition in which, after resetting of the processing circuit, the switch is transitioned to an OFF state and then, during a period after the floating diffusion is reset during which the pixel signals are read out of the pixels into the signal line, the switch is transitioned at least from the OFF state to an ON state; keeps the switch in the OFF state during a period during which the transfer transistor is performing the transfer; and performs a second transition in which, after the transfer, the switch is transitioned from the OFF state to the ON state.