Switched-Capacitor DC Restore for Low-Noise MEMS Charge Amplifiers
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Solution Overview
Problem
MEMS sensors suffer from inaccurate force measurements due to manufacturing tolerances, wear, and external stresses, which lead to noise contributions from charge amplifiers and improper phase relationships in switching capacitor feedback networks, resulting in poor signal-to-noise ratios.
Innovation Solution
A MEMS device with a capacitive output structure, a sense amplifier, and a demodulator, utilizing a switching capacitor connected in parallel, where the switching signal is synchronized with the demodulation signal to minimize noise by maintaining a specific phase relationship, such as being in quadrature or twice the frequency of the demodulation signal, thereby reducing period average noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a switching capacitor feedback network is implemented in the charge amplifier, then the impedance can be controlled at different frequencies, but noise is introduced due to improper phase relationships between switching signal and demodulation signal
Solution Approach 1:
The patent applies periodic action by synchronizing the switching capacitor feedback network with the demodulation signal. The switching signal is configured to occur at specific phases (0°, 90°, 180°, or 270°) relative to the demodulation signal, creating a periodic relationship that eliminates noise contributions from the switching capacitor at the demodulator output. This periodic synchronization ensures that the switching capacitor impedance varies predictably with frequency while maintaining proper phase relationships.
Solution Approach 2:
The patent changes the impedance parameter of the feedback network dynamically by using a switching capacitor configuration. The impedance Zfb varies with frequency according to the switching signal phase and duty cycle, allowing the system to achieve high impedance at the demodulation frequency (reducing noise) while maintaining appropriate impedance at other frequencies for proper amplifier operation.
2Stability of the object's composition
If continuous time feedback with MOSFET is used, then the same impedance is provided at DC voltage and driving frequency, but there is a tradeoff between noise and DC biasing
Solution Approach 1:
The patent transitions from static continuous-time feedback to dynamic switched feedback. The feedback impedance becomes time-varying, controlled by the switching signal phase and duty cycle. This dynamic approach allows the system to achieve different impedance values at different frequencies, specifically providing high impedance at the demodulation frequency to reduce noise while maintaining appropriate DC biasing conditions through the virtual ground connection at the inverting input.
Solution Approach 2:
The switching capacitor feedback network operates periodically, with the impedance changing in sync with the demodulation signal. This periodic switching creates frequency-selective impedance characteristics, where the feedback impedance is high at the demodulation frequency (reducing noise) and appropriately matched at other frequencies, eliminating the tradeoff present in continuous-time feedback.
3Reliability
If manufacturing tolerances and wear are present in MEMS sensor, then the relative locations of movable proof masses and fixed electrodes become inaccurate, but the measurement accuracy deteriorates
Solution Approach 1:
The patent employs feedback through the switching capacitor network connected to the charge amplifier. This feedback mechanism compensates for variations in capacitance caused by manufacturing tolerances, wear, and external stresses. The synchronized switching creates a stable reference relationship that helps maintain measurement accuracy despite physical degradation of the MEMS structure over time.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively eliminates period average noise at the demodulator output, maintaining stable output voltage and reducing noise contributions from the switching capacitor, thus enhancing the signal-to-noise ratio and accuracy of MEMS sensor measurements.
Implementation Method 1
a feedback capacitor and a switching capacitor are each connected in parallel between the first node and the output node
Data Source
AI summary
A capacitive sensing charge amplifier, e.g., in a direct current (DC) feedback network, may incorporate a switching capacitor, including a first control switch and a second control switch, which receives a switching signal to charge and discharges the switching signal to a virtual ground of a sense amplifier. Noise incorporated into the output of the sense amplifier (e.g., a MEMS output signal) is filtered by a demodulation signal at a demodulator such that the period average noise at the demodulator output equals zero. The time varying nature of the switching capacitor resistance generally reshapes the system's post-demodulation noise to reduce its low frequency output noise.