Input Common-Mode Feedback Circuit for Capacitive Sensing
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Solution Overview
Problem
Existing capacitive sensing systems face challenges in maintaining the input common-mode voltage, leading to signal strength reduction and unwanted distortions due to mismatched capacitors and high pass corner limitations, which affect the time constant and transfer gain, especially when modulated biasing voltages are applied.
Innovation Solution
An input common-mode feedback circuit utilizing adaptive conductance elements and an error amplifier forms a negative feedback loop to inhibit input common-mode voltage shifts, maintaining a stable dc operating point and reducing charge leaks, with adaptive conductance elements switching between high and low impedance states to restore the input common mode voltage quickly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If engineering-tuned capacitor combinations are used to maintain input common-mode voltage, then stability is improved, but device complexity increases
Solution Approach 1:
The patent implements an input common-mode feedback circuit that continuously monitors the input common-mode voltage and adjusts the common-mode voltage generation circuit accordingly. This feedback mechanism automatically maintains voltage stability without requiring complex pre-tuned capacitor combinations, thereby resolving the contradiction between stability and device complexity.
Solution Approach 2:
The common-mode voltage generation circuit is designed to self-adjust based on feedback signals, eliminating the need for external engineering tuning of capacitor values. The circuit autonomously maintains proper common-mode voltage levels, reducing design complexity while ensuring stability.
2Reliability
If high pass corner limitations are applied to reduce charge leaks, then reliability is improved, but frequency response deteriorates at low frequencies
Solution Approach 1:
The patent employs dynamic adjustment of the common-mode voltage that adapts to different signal frequencies. The feedback circuit selectively suppresses charge leaks at DC and low frequencies while maintaining proper operation at higher frequencies, thus resolving the contradiction between reliability and frequency response characteristics.
3Measurement precision
If modulated biasing voltages are applied to enhance signal detection, then sensitivity is improved, but input common-mode voltage stability deteriorates
Solution Approach 1:
The feedback circuit continuously monitors the input common-mode voltage even when modulated biasing voltages are applied for sensing. It detects deviations caused by modulation and generates corrective signals to maintain stability, thereby allowing both high sensitivity measurement and voltage stability to coexist.
Data Source
AI summary
A fully differential sensing apparatus and an input common mode feedback circuit are provided. The input common mode feedback circuit includes a common mode error amplifier and a plurality of adaptive conductance elements. Each adaptive conductance element behaves with a low impedance characteristic when its anode voltage is greater than its cathode voltage by a positive threshold voltage or, on the contrary, when the anode voltage of such an adaptive conductance element is lower than its cathode voltage by a negative threshold voltage, the adaptive element also behaves with a low impedance characteristic; otherwise the aforementioned adaptive conductance element behaves with a high impedance characteristic. The common mode error amplifier and a plurality of such adaptive conductance elements form a negative feedback loop to effectively maintain the input common voltage of a fully differential input amplifier, which can be used for a fully differential sensing apparatus.