Common-Mode Current Sensing in Piezoresistive Bridge Feedback
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Solution Overview
Problem
Piezoresistive bridges in MEMS devices, such as micromirrors, require precise determination of current flowing through them to accurately assess temperature and sensitivity, which is challenging due to their temperature sensitivity.
Innovation Solution
A bridge current determination stage that calculates the current through the piezoresistive bridge by determining the common mode feedback current and the current provided by a fully differential operation amplifier, allowing for precise determination of the bridge current and subsequent temperature and sensitivity compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a piezoresistive bridge is used in MEMS devices to sense physical quantities, then the device can measure position and other parameters, but the current through the bridge varies with temperature causing measurement inaccuracies
Solution Approach 1:
The total current measurement is segmented into three separate current components: Ibridge (bridge current), Icm (common mode feedback current), and Iopamp (operational amplifier current). By measuring each component separately and calculating Ibridge = Itotal - Icm - Iopamp, the patent eliminates temperature-induced measurement errors while maintaining accurate current determination.
2Measurement precision
If the total current at the output node is measured directly, then the measurement process is simple, but the current includes multiple components making it impossible to determine the bridge current accurately
Solution Approach 1:
The patent introduces intermediary current sensing circuits that measure the common mode feedback current (Icm) and operational amplifier current (Iopamp) separately. These intermediary measurements act as mediators that enable the calculation of the bridge current by subtracting the intermediary currents from the total current, thereby achieving accurate bridge current determination without directly measuring it.
3Reliability
If temperature compensation is implemented to improve measurement accuracy, then the sensitivity determination becomes more accurate, but additional circuitry and processing are required
Solution Approach 1:
The patent implements feedback by continuously measuring the common mode feedback current and using it to compensate for temperature effects on the bridge current measurement. The measured Icm is fed back into the calculation process to adjust and correct the bridge current determination, ensuring accurate sensitivity measurements despite temperature variations.
Data Source
AI summary
A method and apparatus for sensing a common mode feedback current are provided. The common mode feedback current may flow through a common mode resistive divider of a piezoresistive bridge. A first current mirror mirrors the common mode feedback current and provides a first mirrored common mode current. A current aggregation stage receives the first mirrored common mode current and determines a bridge current of the piezoresistive bridge based on the first mirrored common mode feedback current. A second current mirror may be used to mirror the first current mirror before determining the bridge current.