Amplifier Offset Cancellation Under Large Pixel Column Loads
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Solution Overview
Problem
Conventional offset cancellation techniques in amplifiers used in CMOS imagers face difficulties when dealing with large loads from arrays of pixel columns, as they struggle to effectively cancel offset voltages without amplifying them, especially under varying conditions of process, temperature, and voltage.
Innovation Solution
The proposed solution involves a method and system that includes selectively coupling amplifier outputs to inputs during offset cancellation phases, using buffer elements to manage large loads, and employing feedback circuits with reset and gain paths to isolate and cancel amplifier offsets, ensuring that only the input signal is amplified.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional offset cancellation techniques are used in amplifiers with large capacitive loads from pixel columns, then offset voltage cancellation is attempted, but the techniques fail to effectively cancel offset voltages under large loads
Solution Approach 1:
The patent applies preliminary action by performing offset cancellation during a dedicated reset phase before the actual pixel signal amplification. The amplifier offset is canceled by feeding back the amplifier output to its input during this preliminary phase, establishing a zeroed baseline before the large capacitive load of pixel columns is connected for signal amplification.
Solution Approach 2:
The patent segments the amplifier operation into distinct phases: a reset phase for offset cancellation and a signal amplification phase for pixel reading. During the reset phase, the large capacitive load is disconnected from the amplifier input, allowing offset cancellation to proceed without interference from the load. This temporal segmentation resolves the contradiction by isolating the offset cancellation operation from the harmful large load condition.
2Reliability
If amplifier outputs are fed back to inputs for offset cancellation, then offset voltage is canceled, but large capacitive loads on input terminals prevent effective cancellation
Solution Approach 1:
The patent employs dynamic switching to connect and disconnect the large capacitive load from the amplifier input terminal based on operational phase. During the reset phase, switching elements disconnect the pixel column capacitive load from the amplifier input, enabling effective offset cancellation. During the signal amplification phase, the switching elements reconnect the load to amplify the pixel signals. This dynamic reconfiguration resolves the contradiction by adapting the circuit topology to the current operational requirement.
Solution Approach 2:
The patent introduces switching elements as intermediary components between the amplifier input and the large capacitive load of pixel columns. These switches act as mediators that can selectively connect or disconnect the load from the amplifier input based on the operational phase, enabling offset cancellation without the harmful influence of the large load while maintaining the ability to amplify signals when needed.
3Productivity
If offset cancellation is performed with large pixel column arrays, then signal amplification is achieved, but offset voltages are amplified along with the signal
Solution Approach 1:
The patent performs offset cancellation as a preliminary action during the reset phase before signal amplification begins. By zeroing the amplifier offset beforehand and storing the canceled state on reset capacitors, subsequent signal amplification proceeds without amplifying the offset voltage, thus resolving the contradiction between achieving signal amplification and avoiding amplified offset errors.
Solution Approach 2:
The patent employs feedback by connecting the amplifier output back to its input through feedback capacitors during the reset phase. This feedback mechanism allows the amplifier to automatically cancel its own offset voltage by comparing the output with the input and adjusting accordingly, effectively eliminating the offset before signal amplification occurs and preventing offset amplification during the subsequent signal phase.
Data Source
AI summary
Methods, devices, and systems for offset cancellation 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 cancellation phase, an amplifier offset may be canceled 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 cancellation phase, a buffer may use the first amplifier input to drive a first pixel signal. During a different portion of the cancellation phase, the buffer may use the second amplifier input to drive a second pixel signal. To sense the pixel columns during an amplification phase, the first and second pixel signals are coupled to the first and second amplifier inputs, respectively, with the result that the amplifier offset and the buffer offset are cancelled from the amplifier output.