Auto-Zero Telescopic Op-Amp Offset Compensation Without Bandwidth Loss
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Solution Overview
Problem
Existing offset compensation techniques for operational amplifiers in imaging devices are either bandwidth limited, require additional amplifiers or noise introduction, or struggle with unity-gain stability, leading to inefficiencies in minimizing offset voltage and noise in the readout chain.
Innovation Solution
A differential telescopic operational amplifier with dynamically biased load transistors and auto-zero phase sampling, which reduces offset voltage by dynamically biasing the load transistors and using switches to sample and hold offset voltages for cancellation during the amplification phase, effectively minimizing offset voltage and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If chopper stabilization technique is used to reduce offset voltage, then offset voltage is filtered out, but bandwidth is limited to half of the chopper frequency and large ripple voltages are generated
Solution Approach 1:
The patent employs periodic switching of the operational amplifier between auto-zero phase and amplification phase. During the auto-zero phase, offset voltages are sampled and stored in capacitors. During the amplification phase, the stored offset voltages are subtracted from the input signals. This periodic action allows offset cancellation without the bandwidth limitations of chopper stabilization.
Solution Approach 2:
The patent performs offset voltage sampling and storage in capacitors during the auto-zero phase before the amplification phase begins. This preliminary action of capturing and storing offset voltages allows them to be subtracted during amplification without limiting the signal bandwidth, resolving the contradiction between offset reduction and bandwidth preservation.
2Measurement precision
If conventional auto-zero amplifier with auxiliary amplifier is used, then offset voltage is reduced through gain ratio, but device complexity increases and layout area becomes larger
Solution Approach 1:
The patent combines the offset cancellation function and signal amplification function into a single operational amplifier by time-division multiplexing. The same amplifier is used for both auto-zeroing and signal amplification by switching between phases, eliminating the need for separate auxiliary amplifiers and reducing device complexity while maintaining offset reduction performance.
Solution Approach 2:
The operational amplifier is designed to perform multiple functions: during the auto-zero phase it samples and stores offset voltages, and during the amplification phase it amplifies signals while subtracting the stored offset voltages. This multi-functionality eliminates the need for separate auxiliary amplifiers, reducing device complexity and layout area.
3Measurement precision
If conventional auto-zero amplifier is used, then offset voltage is reduced, but layout area becomes larger than other amplifier designs
Solution Approach 1:
The patent merges the offset cancellation circuitry and signal amplification circuitry into a single operational amplifier structure. By using the same amplifier for both auto-zeroing and signal amplification through phase switching, the layout area is significantly reduced compared to conventional designs that require separate auxiliary amplifiers.
Solution Approach 2:
The patent uses dynamic phase switching to change the operational amplifier's function between auto-zero phase and amplification phase. This dynamic reconfiguration allows a single amplifier structure to perform multiple functions, reducing the static layout area requirements compared to fixed-architecture conventional amplifiers.
4Measurement precision
If correlated double sampling is used to reduce offset voltage, then offset is cancelled through subtraction, but unity-gain stability becomes difficult to achieve
Solution Approach 1:
The patent uses periodic phase switching between auto-zero and amplification phases. During the amplification phase, the operational amplifier operates with high gain to amplify signals while subtracting stored offset voltages. This periodic high-gain operation avoids the unity-gain stability requirements of continuous correlated double sampling, achieving offset cancellation without compromising stability.
Solution Approach 2:
The patent changes the operational amplifier's operating parameters dynamically by switching phases. During amplification, the amplifier operates at high gain with specific bias conditions that ensure stability. This parameter change approach allows effective offset cancellation while maintaining stability, unlike continuous unity-gain operation in conventional CDS.
Data Source
AI summary
Apparatuses and methods for providing offset compensation include a primary amplifier which includes a first output, a second output, a first load input, and a second load input, a first feedback loop connected to the primary amplifier and which includes a first switch located between the first output of the primary amplifier and the first load input, and a first sampling capacitor coupled to the first switch between the first switch and the first load input and a second feedback loop connected to the primary amplifier and which includes a second switch located between the second output of the primary amplifier and the second load input, and a second sampling capacitor coupled to the second switch between the second switch and the second load input.