Auto-Zeroing Current-Feedback Amplifier for Low-Offset Sensing
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Solution Overview
Problem
Conventional current-feedback instrumentation amplifiers face challenges in reducing input offsets while maintaining high input impedance and avoiding spurious signals, which leads to measurement errors and gain errors when reading high-impedance signal sources.
Innovation Solution
The implementation of an auto-zeroing circuit that periodically switches between amplification and auto-zeroing configurations to null offset currents, combined with a ping-pong architecture and a high-frequency feedforward path to ensure constant offset-free stages and reduce spurious signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If chopping is used to eliminate offset in current-feedback instrumentation amplifiers, then offset is reduced, but spurious AC signals appear at the output and input impedance is reduced
Solution Approach 1:
The patent implements periodic switching between auto-zeroing configuration and amplification configuration. During auto-zeroing phases, offset is measured and compensated; during amplification phases, the compensated signal is amplified. This periodic action eliminates DC offset while minimizing spurious signals through proper timing and configuration management.
Solution Approach 2:
The patent extracts the offset compensation function into a separate auto-zeroing circuit that operates independently from the main amplification path. By measuring and compensating offset in a dedicated configuration, the harmful spurious signals are minimized while still achieving offset reduction in the main signal path.
2Measurement precision
If chopping is used to eliminate offset, then offset is reduced, but input impedance is reduced due to switched capacitive load
Solution Approach 1:
The patent uses periodic switching where the input stage is connected to the auto-zeroing circuit only during brief calibration intervals, and connected to the signal source during amplification intervals. This minimizes the duration of switched capacitive loading, thereby maintaining high input impedance for high-impedance signal sources while still achieving offset compensation.
3Adaptability or versatility
If conventional current-feedback topology is used, then input range includes negative supply rail and output swings rail-to-rail, but offsets of transconductance amplifiers add directly to input voltage
Solution Approach 1:
The patent introduces an auto-zeroing circuit as an intermediary between the input stage and the output stage. This intermediary circuit measures the offset voltages of the transconductance amplifiers and generates compensating signals, thereby eliminating the direct addition of offsets to the input voltage while preserving the rail-to-rail input and output capabilities.
Solution Approach 2:
The patent implements feedback loops that continuously monitor the output of transconductance amplifiers and adjust the input signals to compensate for offset voltages. This feedback mechanism ensures that offset errors are automatically corrected while maintaining the full rail-to-rail operating range of the amplifier.
Data Source
AI summary
An embodiment of the present invention is directed to an instrumentation amplifier. The amplifier includes a first amplification sub-circuit, which includes an input stage for sensing a differential input and generating an intermediate current based thereon, a feedback stage, and an auto-zeroing circuit. The feedback stage is operable to generate a feedback current based on an output voltage of the amplifier. The auto-zeroing circuit is operable to generate a nulling current, which compensates for errors in the intermediate and feedback currents resulting from input offsets in the input and feedback stages. The amplifier further includes a second amplification sub-circuit, an output stage, and a switching circuit. The switching circuit switches the amplifier between first and second configurations. In the first configuration, the first amplification sub-circuit provides a first amplification path for the amplifier. In the second configuration, the second amplification sub-circuit provides a second amplification path for the amplifier.