Solid State Imaging Device Voltage Clamping for Smear Noise Reduction
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Solution Overview
Problem
CMOS type solid state imaging devices experience streak-shaped noise (smear) due to potential fluctuations in the horizontal direction when high luminance light is encountered, particularly during the transfer period of pixels, leading to longer settling times for the column amplifier output and changes in current and voltage values, which worsen at high-speed image reading.
Innovation Solution
Incorporating a voltage limiting unit connected to the signal line to prevent voltage from dropping below a certain level, and a controlling unit that makes the voltage limiting unit and signal processing unit non-conductive during the transfer period, thereby stabilizing the signal line voltage and reducing smear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a connecting switch is used to make the vertical signal line and column amplifier non-conductive during transfer period, then rapid fluctuation of column amplifier output is suppressed, but the voltage of the vertical signal line drops to GND level causing current value decrease and IR drop change
Solution Approach 1:
A clip circuit is introduced as an intermediary component between the vertical signal line and the column amplifier. This clip circuit includes a first clip section with a first diode and a second clip section with a second diode, which act as mediators to prevent the signal line voltage from dropping to GND level while still allowing the connecting switch to isolate the column amplifier during the transfer period.
2Object-affected harmful factors
If the connecting switch is turned OFF during transfer period, then smear is reduced, but the settling time of column amplifier output becomes longer
Solution Approach 1:
The clip circuit is activated in advance during the transfer period to maintain the vertical signal line voltage at a predetermined level before the column amplifier reads out the pixel signals. This preliminary voltage maintenance prevents the need for long settling time after the connecting switch is turned ON, as the voltage is already stabilized.
3Use of energy by moving object
If high luminance light enters the device, then photoelectric conversion is enhanced, but potential fluctuation in horizontal direction causes streak-shaped noise
Solution Approach 1:
The clip circuit applies local voltage clamping specifically to the vertical signal line where high luminance light-induced potential fluctuations occur. By maintaining the voltage at a predetermined level only in this specific location, the invention suppresses streak-shaped noise without affecting the overall photoelectric conversion efficiency of the pixel array.
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 configuration effectively suppresses smear occurrence during high luminance conditions, allowing for faster and more reliable image reading by maintaining signal stability and reducing inrush currents.
Implementation Method 1
The voltage limiting unit is connected to a signal line and limits a voltage of the signal line so as not to become a predetermined value or less
Implementation Method 2
The pixel includes a photoelectric conversion unit converting incident light into electric charge
Data Source
AI summary
A voltage limiting unit of a solid state imaging device limits a voltage of a signal line so as not to become a predetermined value or less. A signal processing unit to which a pixel signal is input via a signal line. A controlling unit is disposed between the voltage limiting unit and the signal processing unit on the signal line. Further, the controlling unit causes the voltage limiting unit and the signal processing unit to be non-conductive in a period in which a transfer unit of a pixel transfers the electric charge converted by a photoelectric conversion unit of the pixel to a floating diffusion region of the pixel.