CMOS Column Readout Amplifier Switching for Fast Low-Noise Sensing
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Solution Overview
Problem
CMOS image sensors operating at low illumination levels face a tradeoff between readout time and noise due to the need for low bandwidth amplifiers, which increases noise accumulation during charge transfer.
Innovation Solution
A column readout amplifier with a signal amplifier, filter capacitor, buffer amplifier, and switching network that charges the filter capacitor quickly during the initial phase and disconnects the buffer amplifier when near equilibrium, allowing the capacitive transimpedance amplifier to finish charging without additional noise, thereby reducing readout time without increasing noise levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a low bandwidth amplifier is used to reduce readout noise, then noise levels are improved, but readout time increases due to longer settling time
Solution Approach 1:
The patent applies dynamics by making the amplifier bandwidth adjustable rather than fixed. The system dynamically switches between high bandwidth mode (for fast readout) and low bandwidth mode (for low noise) based on the operational phase. During the reset phase, high bandwidth is used to quickly clear floating diffusion charge, while during the signal readout phase, low bandwidth is engaged to minimize noise, thus resolving the contradiction between speed and noise performance.
Solution Approach 2:
The patent implements periodic action through phased operation cycles. Each readout cycle is divided into distinct phases (reset phase and signal phase) with different bandwidth requirements. The amplifier bandwidth is periodically adjusted to match the current phase demands, allowing the system to achieve both fast settling during reset and low noise during signal acquisition, thereby resolving the time-noise tradeoff.
2Object-generated harmful factors
If the bandwidth of the readout amplifier is reduced to lower noise, then noise is improved, but the time required for the amplifier to settle increases
Solution Approach 1:
The system dynamically adjusts the amplifier bandwidth based on operational requirements. A switching network changes the effective bandwidth from high during reset operations to low during signal readout. This dynamic adjustment allows fast settling during the brief reset phase while maintaining low noise during the extended signal integration phase, resolving the contradiction between settling time and noise performance.
Solution Approach 2:
The patent applies preliminary action by performing a fast reset operation at the beginning of each pixel readout cycle. During this preliminary reset phase, the amplifier operates in high bandwidth mode to quickly clear any residual charge from the floating diffusion, establishing a clean baseline before the low-noise signal acquisition begins. This preliminary fast settling prevents the need for extended settling time during the actual signal measurement.
3Object-generated harmful factors
If a larger filter capacitor is used to reduce noise, then noise levels are improved, but the settling time increases
Solution Approach 1:
The patent resolves the capacitor settling time issue through dynamic bandwidth adjustment. By switching to high bandwidth mode during the reset phase, the system can use a larger filter capacitor for noise reduction without suffering from prolonged settling time. The high bandwidth setting during reset quickly charges the large capacitor, and then the system switches to low bandwidth mode for noise-sensitive signal readout, thus achieving both low noise and fast operation.
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 reduces readout time while maintaining low noise levels by using a larger filter capacitor without extending the settling time, improving the efficiency of low light image sensing.
Implementation Method 1
the signal amplifier charges the first filter capacitor with a first time constant when directly connected to the first filter capacitor
Implementation Method 2
The switching network connects the amplifier signal output to the buffer amplifier input and the buffer amplifier output to the first filter capacitor during a first time period, and connects the amplifier signal output directly to the first filter capacitor during a second time period
Implementation Method 3
a buffer amplifier having a buffer amplifier input and a buffer amplifier output
Data Source
AI summary
A column readout amplifier and imaging array using the same method are disclosed. The column readout amplifier includes a signal amplifier having an amplifier signal output, a first filter capacitor, a buffer amplifier having a buffer amplifier input and a buffer amplifier output, and a switching network. The switching network connects the amplifier signal output to the buffer amplifier input and the buffer amplifier output to the first filter capacitor during a first time period, and connects the amplifier signal output directly to the first filter capacitor during a second time period. The time periods can be of fixed duration or determined by the difference in potential between the input and output of the buffer amplifier. The column readout amplifier can be used in an imaging array to readout columns of pixels.