CMOS Column Readout Amplifier With Switched Buffer Noise Control
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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 and readout.
Innovation Solution
A column readout amplifier with a signal amplifier, a filter capacitor, a buffer amplifier, and a switching network that connects the signal amplifier output to the filter capacitor during an initial phase to accelerate charging, and then disconnects the buffer amplifier to reduce noise and increase readout speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a low bandwidth amplifier is used to reduce readout noise, then noise is reduced, but readout time increases
Solution Approach 1:
The readout process is divided into two distinct phases: a first time period where the buffer amplifier is connected to accelerate charging of the filter capacitor, and a second time period where the buffer amplifier is disconnected to reduce noise. This temporal segmentation allows the system to optimize for speed during charging and for noise reduction during readout, resolving the contradiction between readout time and noise.
Solution Approach 2:
The switching network dynamically changes the circuit configuration based on the operational phase. During the first time period, the buffer amplifier is connected to the filter capacitor to provide accelerated charging. During the second time period, the buffer amplifier is disconnected to minimize its noise contribution. This dynamic reconfiguration allows the system to achieve both fast readout and low noise performance.
2Object-generated harmful factors
If a larger filter capacitor is used to reduce noise, then noise is reduced, but readout time increases
Solution Approach 1:
The buffer amplifier is connected during the first time period to pre-charge the filter capacitor before the actual readout phase. This preliminary action prepares the capacitor with the necessary charge, allowing a larger capacitor to be used for noise reduction without significantly increasing the critical readout time during the second time period when the buffer amplifier is disconnected.
3Productivity
If the buffer amplifier is connected during the entire readout period, then charging is accelerated, but noise increases
Solution Approach 1:
The readout period is segmented into two distinct time periods with different buffer amplifier connection states. During the first time period, the buffer amplifier is connected to accelerate charging of the filter capacitor. During the second time period, the buffer amplifier is disconnected to eliminate its noise contribution during the critical readout phase. This segmentation resolves the contradiction between charging speed and noise.
Solution Approach 2:
The buffer amplifier is periodically connected and disconnected during the readout process. It is connected during the initial charging phase and then disconnected during the readout phase. This periodic action allows the system to benefit from accelerated charging when needed while avoiding noise during the readout phase.
Data Source
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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 rime 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.