Variable Bandwidth Filter for Global Shutter Image Sensor Noise
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Solution Overview
Problem
Global shutter image sensors face challenges in achieving fast settling times with low noise and ripple when implementing a negative NVDD voltage supply due to large line capacitance across numerous pixel array gate terminals, leading to increased noise and reduced performance.
Innovation Solution
A variable bandwidth low pass filter is employed in the NVDD supply circuit, initially providing a large bandwidth for fast settling and then decreasing it to minimize noise and ripple, allowing for simultaneous pixel cell shutdown with reduced noise and improved image sensor performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a fixed bandwidth low pass filter is used in the NVDD supply circuit, then noise and ripple are reduced, but settling time increases
Solution Approach 1:
The patent applies a variable bandwidth low pass filter that dynamically adjusts its bandwidth based on the operational phase. During the reset phase, the filter operates at high bandwidth to enable fast settling of the NVDD voltage. During normal operation, the bandwidth is reduced to minimize noise and ripple. This dynamic adjustment resolves the contradiction by allowing the system to achieve both fast settling and low noise/ripple at different times rather than being constrained by a fixed bandwidth setting.
Solution Approach 2:
The patent changes the bandwidth parameter of the low pass filter from a fixed value to a variable value that can be adjusted between high and low states. This parameter change enables the system to optimize performance for different operational requirements: high bandwidth for fast settling during reset, and low bandwidth for noise reduction during steady-state operation.
2Productivity
If the global shutter switch is turned off to begin normal exposure operation, then pixel cells can capture images, but noise and ripple increase due to large line capacitance
Solution Approach 1:
The patent applies preliminary action by fully charging the NVDD voltage to AVDD level before turning off the global shutter switch. This pre-charging ensures that the large line capacitance associated with numerous pixel array gate terminals is already charged, minimizing voltage droop and reducing noise and ripple when the switch transitions off. This preliminary charging action enables the system to achieve both image capture capability and reduced noise/ripple.
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 enables both rapid settling and low noise in the global shutter switch, enhancing image sensor performance by reducing noise and ripple during the transition from AVDD to NVDD voltage, thereby improving image capture quality.
Implementation Method 1
A variable bandwidth low pass filter is employed in the NVDD supply circuit, initially providing a large bandwidth for fast settling and then decreasing it to minimize noise and ripple
Data Source
AI summary
A pixel cell includes a photodiode to accumulate image charge. A global shutter transistor is coupled to the photodiode to reset the image charge in the photodiode in response to a global shutter control signal. A global shutter control signal generator circuit generates the global shutter control signal to have a first value signal or a second value signal. The first value signal is coupled to turn on the global shutter transistor to reset the photodiode. The second value signal controls the global shutter transistor to be in a low leakage off mode. A supply circuit is coupled to provide the second value signal to the global shutter control signal generator circuit. The supply circuit includes a variable filter circuit coupled to an output of the supply circuit to selectively vary a bandwidth of the second value signal in response to a bandwidth select signal.