Constant Current Source Stabilizes Output Line Voltage in Column AD Conversion
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Solution Overview
Problem
The column AD conversion method in image capturing apparatuses faces challenges in reducing power consumption while maintaining signal linearity, as the sampling timing varies with signal voltage, leading to linearity degradation when capacitive load operations are employed.
Innovation Solution
An image capturing apparatus and method that incorporates a constant current source to supply a lower current than required for output line driving, coupled with a reset circuit and amplification transistor configuration, to stabilize the output line voltage during AD conversion, thereby reducing power consumption and maintaining signal linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a constant current source circuit is employed in the SF load, then the output line voltage can be stabilized, but power consumption increases
Solution Approach 1:
The patent applies periodic action by controlling the constant current source to operate in intermittent periods rather than continuously. The current source is activated during specific phases (reset period and signal readout period) and deactivated during other phases, thereby stabilizing the output line voltage when needed while reducing overall power consumption.
Solution Approach 2:
The patent changes the operational parameters of the constant current source by adjusting its current value and duty cycle based on different operating conditions. During the reset period, a higher current is applied to quickly stabilize the output line voltage, while during signal readout, a lower current is used to maintain stability with reduced power consumption.
2Use of energy by moving object
If capacitive load operation is employed in output line driving, then power consumption is reduced, but output signal linearity degrades due to SF subthreshold current fluctuations
Solution Approach 1:
The patent introduces a constant current source as an intermediary element between the pixel output and the AD converter. This constant current source acts as a mediator that compensates for the SF subthreshold current fluctuations, thereby maintaining output signal linearity while allowing the system to operate in capacitive load mode for reduced power consumption.
Solution Approach 2:
The patent implements feedback control where the constant current source monitors and responds to output line voltage fluctuations caused by SF subthreshold current. By adjusting the constant current in response to detected variations, the system maintains signal linearity while operating in power-efficient capacitive load mode.
3Use of energy by moving object
If sampling timing varies depending on signal voltage, then power consumption is reduced in capacitive load operation, but output signal linearity cannot be maintained
Solution Approach 1:
The patent applies preliminary action by performing a reset operation before the signal readout phase. During this reset period, the constant current source is activated to stabilize the output line voltage and eliminate the effects of SF subthreshold current fluctuations. This preliminary stabilization ensures that subsequent signal measurements maintain linearity even though sampling timing varies during capacitive load operation.
Data Source
AI summary
An image capturing apparatus includes a pixel array, an AD converter, an output line configured to connect the pixel and the AD converter, a reset unit configured to reset the output line, an amplification transistor configured to amplify a signal from the pixel, a connection unit configured to connect a source of the amplification transistor to the output line, a constant current source, and a control unit configured to, after a voltage of the output line is reset, cause a constant current to flow to the output line and to control to connect a source of the amplification transistor to the output line, wherein the control unit sets a value of the constant current so that the constant current is a lower value than a current required to drive the output line.