MEMS Resonant Oscillator Drive Circuit With ΣΔ DAC Amplitude Control
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Solution Overview
Problem
Existing analog and digital driving circuits for MEMS oscillators, such as gyroscopes, face challenges in controlling low-power devices due to high power consumption and complexity, particularly from filtering higher harmonics and the use of active filters and high-frequency analog-to-digital conversion.
Innovation Solution
A driving circuit that includes a digital conversion stage to acquire and convert differential sensing signals into digital signals, processing circuitry to generate a digital control signal based on a comparison with a reference signal, an analog conversion stage with a ΣΔ-type DAC to convert the digital control signal into a PDM control signal, and a low-pass filtering stage to generate a control signal for controlling the amplitude of oscillation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If an active filtering block is used to filter higher harmonics in the AGC block, then the oscillation stability is improved, but the power consumption increases and the circuit complexity increases
Solution Approach 1:
The harmful higher harmonics are extracted and removed from the sensing signal through a dedicated filtering block that operates in parallel with the AGC block, allowing the main AGC path to remain simple and low-power while still achieving stable oscillation control
Solution Approach 2:
A passive filtering block is introduced as an intermediary element between the demodulation stage and the AGC block to eliminate higher harmonics without requiring active components, thus reducing power consumption while maintaining oscillation stability
2Measurement precision
If a fourth-order ΣΔ DAC is used to generate PDM signal for digital control, then the control precision is improved, but the device complexity and cost increase
Solution Approach 1:
The system changes the control parameter from high-resolution digital codes to pulse density modulation (PDM) signals, which can be generated with simpler circuitry while maintaining effective control precision through the inherent noise-shaping properties of ΣΔ modulation
3Measurement precision
If high-frequency ADC is used for analog-to-digital conversion in the AGC block, then the sampling accuracy is improved, but the power consumption increases
Solution Approach 1:
The system uses periodic sampling at the demodulation frequency fdem rather than continuous high-frequency sampling, reducing the overall power consumption while maintaining sufficient sampling accuracy for AGC operation through synchronous detection techniques
4Measurement precision
If synchronous demodulation is used to extract amplitude information at fdem=fdr, then the measurement accuracy is improved, but the generation of higher harmonics increases
Solution Approach 1:
The higher harmonics generated by synchronous demodulation are not treated as errors to be corrected but are instead managed by directing them through a passive filter that removes only the problematic components while preserving the useful amplitude information for AGC control
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 proposed solution reduces power consumption and complexity by using a ΣΔ DAC and passive filtering, effectively controlling the amplitude of oscillation in MEMS oscillators while minimizing the generation of higher vibrational modes.
Implementation Method 1
an analog conversion stage (24) electrically coupled to the processing circuitry and configured to convert the digital control signal into a PDM control signal of analog type, the analog conversion stage including a digital-to-analog converter (DAC) of ΣΔ type
Implementation Method 2
a filtering stage of low-pass type electrically coupled to the analog conversion stage and configured to, by filtering the PDM control signal, generate a control signal for controlling the amplitude of oscillation of the movable mass
Implementation Method 3
a movable mass (7) elastically coupled to the substrate and controllable through the control signal to oscillate in resonance with respect to the substrate
Data Source
AI summary
A driving circuit for controlling a MEMS oscillator includes a digital conversion stage to acquire a differential sensing signal indicative of a displacement of a movable mass of the MEMS oscillator, and to convert the differential sensing signal of analog type into a digital differential signal of digital type. Processing circuitry is configured to generate a digital control signal of digital type as a function of the comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillation of the movable mass which causes the resonance of the MEMS oscillator. An analog conversion stage includes a ΣΔ DAC and is configured to convert the digital control signal into a PDM control signal of analog type. A filtering stage of low-pass type, by filtering the PDM control signal, generates a control signal for controlling the amplitude of oscillation of the movable mass.