Switched-Capacitor Op-Amp Offset Cancellation With Hybrid Autozeroing
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Solution Overview
Problem
Operational amplifiers face challenges in designing offset-free circuits due to temperature drift, manufacturing imprecision, and layout errors, with existing techniques like autozeroing and chopping having limitations such as residual noise and power consumption issues.
Innovation Solution
A hybrid technique combining autozeroing and chopping is employed in switched capacitor operational amplifier circuits, where an autozeroing capacitor captures offset during an autozeroing frequency interval, and the sampled input signal is chopped and de-chopped at higher frequencies to effectively cancel offsets without modifying the switched capacitor network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If autozeroing technique is used to cancel offset, then offset cancellation is improved, but residual noise and power consumption increase
Solution Approach 1:
The patent combines autozeroing and chopping techniques into a hybrid offset cancellation circuit. The autozeroing capacitor captures offset during a first time interval, then the captured offset is chopped at a second frequency during a second time interval. This merging of two techniques allows the circuit to achieve better offset cancellation while reducing residual noise compared to using autozeroing alone.
Solution Approach 2:
The patent implements periodic offset capture and chopping operations. The autozeroing capacitor periodically captures offset at a first frequency, and the captured offset is periodically chopped at a second frequency. This periodic action allows the circuit to maintain low residual noise while continuously canceling offset over time.
2Object-generated harmful factors
If chopping frequency is increased to reduce residual noise, then noise reduction is improved, but power consumption increases
Solution Approach 1:
The patent uses periodic chopping at a second frequency that is higher than the first frequency but optimized to balance noise reduction with power consumption. The chopping occurs periodically during the second time interval, allowing the circuit to reduce residual noise while avoiding continuous high-frequency operation that would consume excessive power.
Solution Approach 2:
The patent changes the frequency parameter of the chopping operation to a second frequency that is optimized for the specific application. By adjusting this frequency parameter, the circuit achieves effective noise reduction while maintaining reasonable power consumption levels.
3Measurement precision
If larger components are used to reduce offset, then offset cancellation is improved, but device area increases
Solution Approach 1:
The patent uses dynamic offset cancellation techniques (autozeroing and chopping) instead of static component sizing. The autozeroing capacitor dynamically captures offset, and the chopping switches dynamically modulate the captured offset. This dynamic approach achieves effective offset cancellation without requiring larger passive components, thereby maintaining compact device area.
Data Source
AI summary
A system has an output and receives an input signal. An operational amplifier has an input, an output and an offset at the input. A switched capacitor network samples the input signal at a switched capacitor frequency. An autozeroing capacitor connected to the input of the operational amplifier captures the offset during an offset capture interval according to an autozeroing frequency. Chopping and autozeroing switches, connected between the autozeroing capacitor and the switched capacitor network, chop the sampled input signal according to a chopping frequency and autozero the captured offset according to the autozeroing frequency. De-chopping switches, connected between the output of the operational amplifier and the output of the system, operate on the output of the operational amplifier at the chopping frequency to chop the autozeroed captured offset and de-chop the chopped sampled input signal processed by the operational amplifier.