CMOS Imager Amplifier Offset Compensation via Phase Segmentation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional amplifier circuits in CMOS imagers face challenges in offset voltage compensation, especially when dealing with large loads from arrays of pixel columns, as existing methods struggle to effectively cancel offset voltages due to imbalances and variations, leading to amplified offset in the output signal.
Innovation Solution
The proposed solution involves a pixel amplifier circuit with a buffer element and feedback capacitors that precharge and subtract offset voltages during specific phases, allowing for effective offset compensation even under large capacitive loads, using switches and capacitors to isolate and cancel the amplifier offset during the gain phase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional offset cancellation techniques are used, then offset voltage can be canceled in small-load scenarios, but the techniques fail to effectively cancel offset when large capacitive loads are present
Solution Approach 1:
The patent applies preliminary action by performing offset sampling before the actual signal amplification. The offset is sampled during a dedicated offset sampling phase when the pixel columns are disconnected from the amplifier inputs, allowing the offset to be captured and stored on capacitor banks before the large capacitive load is connected. This pre-sampled offset is then subtracted from the amplified signal to achieve accurate offset cancellation under large load conditions.
Solution Approach 2:
The patent segments the amplification process into distinct phases: an offset sampling phase where only the amplifier offset is captured, and a signal amplification phase where the actual pixel signals are amplified. This segmentation is achieved by controlling switches to connect or disconnect pixel columns from the amplifier inputs at different times. By separating the offset measurement from the signal amplification, the system can accurately handle large capacitive loads that would otherwise prevent effective offset cancellation.
2Device complexity
If a single amplifier serves many pixel columns, then device complexity is reduced, but the amplifier input terminals experience large capacitive loads that complicate offset cancellation
Solution Approach 1:
The patent uses preliminary action to sample the amplifier offset before connecting the large array of pixel columns to the amplifier. During the offset sampling phase, switches disconnect the pixel columns, presenting a small load to the amplifier inputs. The offset is captured under these favorable conditions and stored on capacitor banks. Later, during signal amplification, the pre-sampled offset is subtracted from the amplified signals, achieving accurate offset cancellation despite the large capacitive load from many pixel columns.
Solution Approach 2:
The patent introduces intermediary capacitor banks that serve as mediators between the amplifier and the large array of pixel columns. These capacitor banks store the sampled offset voltage and provide it back during signal amplification. The switches act as intermediaries to control the connection between pixel columns and amplifier inputs, enabling the system to transition between low-load offset sampling mode and high-load signal amplification mode, thereby simplifying the overall device architecture while maintaining offset cancellation capability.
Data Source
AI summary
Methods, devices, and systems for offset compensation in an amplifier are disclosed, wherein the amplifier inputs may be exposed to large loads from an array of pixel columns coupled in parallel. During a sampling phase, an amplifier offset may be sampled by selectively coupling a first amplifier output to a first amplifier input and a second amplifier output to a second amplifier input. During a portion of the sampling phase, the first amplifier output may be buffered to a first storage element. During a different portion of the sampling phase, the second amplifier output may be buffered to a second storage element. To sense the pixel columns during an amplification phase, the first storage element and the second storage element are coupled to the first and second amplifier inputs, respectively, with the result that the amplifier offset is canceled from the amplifier output.