Chopper-Stabilized Amplifier Offset Correction With Low Ripple Noise
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Solution Overview
Problem
Chopper stabilized operational and instrumentation amplifiers face challenges with input offset and noise, particularly in high precision applications, where existing offset correction methods can introduce chopper-induced noise and parasitic capacitance issues, affecting accuracy and system performance.
Innovation Solution
The integration of a sample-and-hold circuit in the offset correction path, combined with autozeroing and frequency-compensation techniques, reduces chopper-ripple noise and parasitic capacitance effects, ensuring a linear frequency characteristic and improved offset cancellation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If chopper stabilized amplifiers are used to reduce input offset, then offset cancellation is improved, but chopper-induced noise is introduced
Solution Approach 1:
The chopper amplifier is divided into separate functional blocks: an input chopper that converts DC offset to AC, an amplifier stage, and an output chopper that converts AC back to DC. This segmentation allows the noise from the output chopper to be filtered separately from the useful offset correction signal, resolving the contradiction between offset cancellation and noise introduction.
Solution Approach 2:
A low-pass filter is introduced as an intermediary element between the output chopper and the signal path. This filter mediates by passing the low-frequency offset correction signal while blocking the high-frequency chopper-induced noise, thus maintaining offset cancellation performance while eliminating the harmful noise component.
2Measurement precision
If offset correction is applied in the signal path, then input offset is reduced, but parasitic capacitance effects increase
Solution Approach 1:
The offset correction function is extracted from the main signal path and implemented through a separate feedback loop. By taking the offset correction path out of the critical signal path, parasitic capacitance effects are minimized while maintaining the offset correction functionality through the integrator-based feedback mechanism.
3Measurement precision
If high gain is used in the chopper amplifier path, then offset cancellation is enhanced, but chopper noise is amplified
Solution Approach 1:
The chopper amplifiers operate at periodic switching frequencies, converting DC offset to AC signals. This periodic action allows the use of frequency-selective filtering to separate the offset correction signals from the noise, enabling high gain operation without proportionally amplifying the chopper noise.
Solution Approach 2:
An integrator-based feedback loop is employed to control the offset correction process. The feedback mechanism automatically adjusts the correction signal level based on the actual offset present, preventing excessive gain from amplifying noise while maintaining effective offset cancellation through the integrated feedback action.
Data Source
AI summary
Chopper stabilized amplifiers combining low clock noise and linear frequency characteristics. The chopper stabilized amplifiers are used in offset correction circuitry, with the output of the chopper stabilized amplifiers being integrated by an integrator. The integrator operates on alternate cycles, with a sample and hold circuit sampling the integrator output when the integrator is not integrating, with the output of the sample and hold being coupled to the main amplification path to cancel offset after at least some amplification is achieved. Autozeroing of amplifiers in the offset correction circuitry is also disclosed. The invention is applicable to operational amplifiers and instrumentation amplifiers.