CMOS Image Sensor Readout Circuit for Power Supply Noise Suppression
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Solution Overview
Problem
CMOS image sensors with column parallel ADCs face issues with analog power supply fluctuations and potential malfunctions due to IR drops and noise during concurrent operations, which can lead to image quality degradation.
Innovation Solution
The implementation of a pixel signal readout circuit with a comparator configuration that includes a first amplifier, a second amplifier with increased gain, and a mirror circuit to form a current mirror, which helps in suppressing analog power supply fluctuations by controlling the operating point and sampling process, thereby preventing malfunctions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If column parallel ADCs are used for high-speed readout, then productivity is improved, but analog power supply fluctuations occur causing malfunctions
Solution Approach 1:
The comparator is divided into two separate amplifiers: a first amplifier for signal comparison and a second amplifier for gain enhancement. This segmentation isolates the high-gain stage from the power supply noise generated during concurrent operations, maintaining analog power supply stability while preserving high-speed readout capability through column parallel ADC architecture.
Solution Approach 2:
A mirror circuit is introduced as an intermediary between the first and second amplifiers. This mirror circuit buffers and isolates the analog power supply from fluctuations generated during concurrent comparator operations, preventing power supply noise from affecting the signal comparison process while enabling high-speed parallel readout.
2Productivity
If current flows through amplifiers during concurrent operations, then productivity is improved, but analog power supply fluctuations increase causing noise
Solution Approach 1:
The amplifier is segmented into two distinct stages: the first amplifier handles signal comparison with minimal current draw, while the second amplifier provides gain enhancement in a separate stage. This segmentation prevents current fluctuations in one stage from directly affecting the power supply of the other, reducing noise generation during concurrent operations.
Solution Approach 2:
The mirror circuit serves as an intermediary that decouples the current flow paths of the two amplifiers. By introducing this buffering stage, current fluctuations during concurrent operations are isolated and prevented from coupling into the analog power supply, thereby reducing noise while maintaining high productivity.
3Measurement precision
If gain is increased to improve signal quality, then measurement precision is improved, but power supply fluctuations are amplified causing malfunctions
Solution Approach 1:
The gain enhancement function is separated from the signal comparison function by placing the second amplifier after the first amplifier in a cascaded configuration. This segmentation allows the first amplifier to perform precise signal comparison with stable gain, while the second amplifier provides additional gain enhancement on the already-compared signal, preventing power supply fluctuations from affecting the critical comparison stage.
Solution Approach 2:
The mirror circuit acts as an intermediary buffer between the amplification stages and the power supply. It isolates the high-gain second amplifier from directly loading the analog power supply, preventing power supply fluctuations from being amplified and causing comparator malfunctions, while still allowing the desired signal quality improvement.
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 reduces analog power supply fluctuations, preventing malfunctions and improving image quality by maintaining a constant current and reducing noise-related issues.
Implementation Method 1
The photodiode 11 photoelectrically converts an incident light into an amount of electric charge (here, it is an electron) corresponding to the amount of the incident light.
Implementation Method 2
Upon application of a drive signal to the gate (transfer gate) of the transfer transistor via a transfer control line LTx, the transfer transistor 12 transfers the electrons photoelectrically converted by the photodiode 11 to the floating diffusion FD.
Implementation Method 3
The amplifier transistor 13 amplifies a potential of the floating diffusion to output a voltage corresponding to the potential to the output (vertical) signal line LSGN.
Implementation Method 4
When a reset signal is supplied to the gate of the reset transistor via a reset control line LRST, the reset transistor 15 resets a potential of the floating diffusion FD to a potential of the power supply line LVDD.
Data Source
AI summary
A solid-state imaging device and a camera system are disclosed. The solid-state imaging device includes a pixel unit and a pixel signal readout circuit. The pixel signal readout circuit includes a plurality of comparators disposed to correspond to a pixel column array, and a plurality of counters. Each counter includes a first amplifier, a second amplifier, and a mirror circuit to from a current mirror in parallel with the second amplifier. The first amplifier includes differential transistors, initializing switches connected between gates and collectors of the differential transistors, and first and second capacitors connected to each of the gates of the differential transistors. The second amplifier includes an initializing switch and a third capacitor. The mirror circuit includes a gate input transistor whose gate is inputted with a voltage sampled by the first amplifier or a voltage sampled by the second amplifier.