Current Sense Amplifier Biasing for Wide Common-Mode Rejection
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Solution Overview
Problem
Current sense amplifiers face challenges in achieving effective common mode rejection, especially over a large common mode range, which can be overwhelmed by noise, and are sensitive to device mismatches and polarity, leading to accuracy issues and dead zones in measurement.
Innovation Solution
The proposed system includes a front-end differential amplifier with input resistors and an input common mode biasing circuit using two current sources to generate biasing currents, a floating supply domain, and a common mode feedback circuit, which improves common mode rejection and reduces offset dependence on device matching, making the system immune to polarity and enabling automatic calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional current sense amplifier topologies are used with resistor matching techniques, then offset performance is improved, but common mode rejection degrades significantly as input common mode voltage decreases
Solution Approach 1:
The patent divides the current sense amplifier into separate differential and common-mode signal paths. The differential path handles the sense voltage while the common-mode path separately processes common-mode voltages through dedicated transimpedance amplifiers and subtractors, allowing independent optimization of each path's performance
Solution Approach 2:
The patent introduces intermediate common-mode to differential-mode conversion circuits that convert common-mode voltages into differential signals before processing. This intermediary conversion allows the system to handle common-mode rejection separately from the main differential signal path, preventing degradation at low common-mode voltages
2Reliability
If feedback is used to address resistor mismatch, then common mode rejection is improved, but bandwidth of the sense amplifier is limited
Solution Approach 1:
The patent segments the feedback mechanism into separate common-mode feedback loops that operate independently from the main differential signal path. This allows feedback to be applied specifically for common-mode rejection without introducing bandwidth limitations into the primary signal path
Solution Approach 2:
The patent handles common-mode rejection in a separate dimensional space by converting common-mode voltages to differential-mode signals through intermediate conversion circuits. This dimensional transformation allows common-mode feedback to operate without constraining the bandwidth of the main differential signal path
3Measurement precision
If large devices are used to improve noise performance, then noise is reduced, but device complexity and power consumption increase
Solution Approach 1:
The patent segments the noise filtering function into dedicated common-mode noise rejection circuits that operate in parallel with the main signal path. This allows noise performance to be improved through specialized circuits rather than requiring larger devices in the primary path
Solution Approach 2:
The patent introduces intermediate common-mode filtering stages that act as mediators between the sense amplifier and subsequent processing stages. These intermediary circuits remove common-mode noise components without requiring the main signal path devices to be oversized
Data Source
AI summary
A system may include a front end differential amplifier having two input terminals, two input resistors, each of the two input resistors coupled to a respective one of the two input terminals, and an input common mode biasing circuit for an output stage of the front end differential amplifier, the input common mode biasing circuit comprising two current sources configured to generate currents for biasing the output stage of the front end differential amplifier.