MEMS Oscillator Driving Circuit With ΣΔ DAC and Passive Harmonic Filtering
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Solution Overview
Problem
Existing MEMS gyroscope driving circuits face challenges in controlling low-power gyroscopes due to high power consumption from filtering higher harmonics and the complexity of analog and digital control systems, which generate spurious vibrational modes and increase costs.
Innovation Solution
A driving circuit that combines digital and analog control using a first-order ΣΔ-type digital-to-analog converter (DAC) with a passive filtering stage, reducing power consumption and complexity by sampling at frequencies lower than the oscillation frequency and minimizing high-frequency noise through low-pass filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an active filtering block is used to filter higher harmonics in the AGC block, then the stability of the feedback loop is improved, but the power consumption increases and the device complexity increases
Solution Approach 1:
The patent replaces the active filtering block (electronic amplifier-based filter) with a passive filtering block (RC low-pass filter). This substitution eliminates the need for powered amplifiers in the filtering stage, thereby reducing power consumption while maintaining the ability to filter higher harmonics and ensure feedback loop stability.
Solution Approach 2:
The patent uses a simple RC low-pass filter configuration with minimal components (resistors and capacitors) instead of complex active filtering circuits. This approach uses inexpensive, simple components that consume no power, effectively replacing the power-hungry active filter while achieving the same filtering function.
2Reliability
If an active filtering block is used to filter higher harmonics in the AGC block, then the stability of the feedback loop is improved, but the device complexity increases
Solution Approach 1:
The patent replaces the complex active filtering block (requiring multiple amplifiers and complex circuitry) with a simple passive RC low-pass filter. This substitution dramatically reduces the structural complexity of the analog driving circuit while maintaining the necessary filtering functionality for feedback loop stability.
Solution Approach 2:
The patent extracts and removes the complex active filtering stage from the circuit, retaining only the essential passive filtering functionality. This extraction eliminates unnecessary complexity while preserving the core function of filtering higher harmonics to maintain feedback loop stability.
3Measurement precision
If a fourth-order ΣΔ DAC is used to generate PDM signal, then the digital control precision is improved, but the device complexity increases and high-frequency noise increases
Solution Approach 1:
The patent merges the DAC order reduction with the filtering function by using a simple first-order ΣΔ DAC combined with a passive low-pass filter. This combination achieves the same effective control precision as a fourth-order DAC would provide, but with much lower complexity by distributing the filtering function across the passive filter rather than requiring high-order digital processing.
Solution Approach 2:
The patent changes the DAC order parameter from fourth-order to first-order, fundamentally simplifying the digital control architecture. This parameter change is compensated by the passive low-pass filter which removes the resulting high-frequency quantization noise, maintaining control precision while reducing complexity.
4Measurement precision
If a fourth-order ΣΔ DAC is used to generate PDM signal, then the digital control precision is improved, but the high-frequency noise increases causing spurious vibrational modes
Solution Approach 1:
The patent converts the high-frequency quantization noise generated by the first-order ΣΔ DAC into a benefit by using it to drive the passive low-pass filter. The filter is designed to attenuate these high-frequency components, effectively using the noise-generating mechanism as part of the overall noise-reduction strategy, thereby eliminating spurious vibrational modes while maintaining control precision.
Solution Approach 2:
The patent introduces a passive low-pass filter as an intermediary between the first-order ΣΔ DAC and the MEMS oscillator. This intermediary component serves as a mediator that removes the harmful high-frequency quantization noise from the PDM signal, allowing the use of a simple first-order DAC without generating spurious vibrational modes.
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 enables efficient control of MEMS gyroscopes with reduced power consumption and complexity, maintaining resonance while minimizing spurious vibrational modes and lowering costs, thus addressing the limitations of prior art.
Implementation Method 1
comprises a first-order ΣΔ-type digital-to-analog converter (DAC)
Implementation Method 2
a passive filtering stage, reducing power consumption and complexity by sampling at frequencies lower than the oscillation frequency and minimizing high-frequency noise through low-pass filtering
Implementation Method 3
MEMS-type gyroscopes are devices of resonant type. Consequently, the use of gyroscopes requires controlling the primary resonator thereof in order to ensure its correct oscillation
Implementation Method 4
this analog control is implemented through an analog driving circuit which generally comprises a capacitance-to-voltage (C2V) converter which senses the displacements of the seismic mass of the gyroscope by capacitive effect
Data Source
Figure 1
Figure 2
Figure 3A~3C
AI summary
Driving circuit (14) for controlling a MEMS oscillator (1), comprising: a digital conversion stage (20) to acquire a differential sensing signal (VSD) indicative of a displacement of a movable mass (7) of the MEMS oscillator (1), and to convert the differential sensing signal (VSD) of analog type into a digital differential signal (Vdiff) of digital type; a processing block (22) to generate a digital control signal (Vctrl,dig) of digital type as a function of the comparison between the digital differential signal (Vdiff) and a differential reference signal (Vref) indicative of a target amplitude of oscillation of the movable mass (7) which causes the resonance of the MEMS oscillator (1); an analog conversion stage (24) comprising a ΣΔ DAC and configured to convert the digital control signal (Vctri,dig) into a PDM control signal (Vctrl,PDM) of analog type; and a filtering stage (26) of low-pass type to, by filtering the PDM control signal (Vctrl,dig), generate a control signal (Vctrl) for controlling the amplitude of oscillation of the movable mass (7) .