Capacitive MEMS Sensor Read Circuit With Modulation for Low-Frequency Noise
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Solution Overview
Problem
Existing capacitive inertial micro-electromechanical-system (MEMS) sensors face challenges in reducing low-frequency noise and offset disturbances, particularly due to dispersion currents from switches and thermal drifts, which are exacerbated by the use of short-channel MOS transistors and aging components, limiting their performance in portable devices.
Innovation Solution
A discrete-time read device with a modulator stage that modulates the read voltage and a demodulator stage to separate the harmonic content of capacitance variations from disturbance components, combined with a low-pass filter to eliminate high-frequency disturbances, effectively suppressing noise and offset introduced by the charge-voltage converter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If switched-capacitor read circuits are used to reduce power consumption, then energy efficiency improves, but low-frequency noise and offset disturbances increase due to switch dispersion currents and thermal drifts
Solution Approach 1:
The patent applies periodic modulation of the read voltage at a high frequency (e.g., 10-100 kHz) to shift the measurement process from continuous to periodic operation. This modulation technique moves the signal of interest to a higher frequency band where switch dispersion currents and thermal drifts have minimal impact, thereby reducing low-frequency noise while maintaining the low power consumption benefits of switched-capacitor circuits
Solution Approach 2:
The patent introduces a modulator stage as an intermediary between the capacitive sensor and the demodulator. This modulator converts the static or low-frequency capacitance variations into high-frequency modulated signals, effectively transferring the measurement to a frequency range where harmful low-frequency disturbances are suppressed, thus acting as a mediator that isolates the measurement from noise sources
2Length of moving object
If short-channel MOS transistors are used in switches, then device dimensions are reduced, but dispersion currents and thermal drifts are exacerbated
Solution Approach 1:
By modulating the read voltage at high frequency, the patent moves the measurement operation away from the problematic low-frequency regime where short-channel effects are most pronounced. The periodic switching at elevated frequencies reduces the impact of dispersion currents and thermal drifts inherent to short-channel MOS transistors, thereby maintaining signal accuracy despite using compact transistor dimensions
Solution Approach 2:
The patent replaces direct low-frequency capacitive measurement with high-frequency modulated measurement. This substitution transforms the measurement mechanism from one directly susceptible to switch-related disturbances into a frequency-domain measurement where such disturbances are naturally suppressed, effectively substituting a noise-prone direct measurement with a more robust modulated measurement approach
Data Source
AI summary
A read device of a capacitive sensor includes: a signal source supplying an electrical read signal for driving the capacitive sensor; and a discrete-time sense circuit for generating an electrical output signal, correlated to variations of capacitance of the capacitive sensor, in response to variations of the electrical read signal. The device moreover includes: a modulator stage for generating a modulated electrical read signal on the basis of the electrical read signal and supplying the modulated electrical read signal to the capacitive sensor; a demodulator stage, connected to the sense circuit, for demodulating the electrical output signal and generating a demodulated electrical output signal; and a low-pass filtering stage for generating a filtered electrical output signal, on the basis of the modulated electrical output signal.