Differential Amplifier Offset Cancellation Using Chopping and Auto-Zero
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Solution Overview
Problem
Conventional differential amplifiers suffer from offset voltage errors due to manufacturing asymmetry, making it difficult for circuit designers to uniformly compensate or cancel these errors, which affects the electrical characteristics of the circuit.
Innovation Solution
An offset-free differential amplifier is designed using a chopping switch unit and an auto-zero switch unit, which selectively or simultaneously remove average and absolute offset voltages by controlling switches in response to specific control signals, ensuring accurate voltage amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional differential amplifier is used, then manufacturing simplicity is maintained, but offset voltage error occurs due to manufacturing asymmetry
Solution Approach 1:
The differential amplifier is divided into multiple operational phases (first phase for offset sampling, second phase for signal amplification). During the first phase, the amplifier operates in a non-inverting configuration to sample offset voltage. During the second phase, it operates in inverting configuration for signal amplification. This temporal segmentation allows offset compensation without permanently increasing circuit complexity.
Solution Approach 2:
The offset voltage is sampled and stored in capacitors during a preliminary first phase before the actual signal amplification begins. The stored offset voltage is then subtracted from the input signal in the second phase, effectively compensating for manufacturing asymmetry before it affects the measurement.
2Manufacturing precision
If offset compensation circuit is added, then offset voltage accuracy is improved, but device complexity increases
Solution Approach 1:
The same operational amplifier is used for multiple purposes: offset sampling, offset storage, and signal amplification. The capacitors serve dual functions of storing offset voltage and participating in the amplification process. The switches configure the amplifier for different operations but are part of the same integrated circuit structure.
Solution Approach 2:
The offset compensation function is merged with the signal amplification function in a single integrated operational amplifier circuit. The capacitors that store offset voltage are the same capacitors used in the amplification process, eliminating the need for separate compensation circuits and reducing overall device complexity.
3Manufacturing precision
If auto-zero method is used, then absolute offset is removed, but circuit complexity increases due to additional switches and capacitors
Solution Approach 1:
The differential amplifier operates in periodic cycles, alternating between a first phase for offset sampling and a second phase for signal amplification. Switches are controlled by phase signals that periodically switch the amplifier between these two configurations, enabling continuous offset compensation without requiring complex continuous control circuits.
Data Source
AI summary
The present invention provides an offset free differential amplifier and an offset free method of a differential amplifier capable of removing an offset of a differential amplifier by selectively using at least one of an auto zero method and a chopping method. The offset free differential amplifier includes a differential amplifier; a chopping switch unit that is connected to two input terminals and one output terminal of the differential amplifier in response to a chopping control signal and removes an average offset of the differential amplifier; and an auto-zero switch unit that is connected to the differential amplifier and the chopping switch unit in response to an auto-zero switch control signal and removes an absolute offset of the differential amplifier.