Switched-capacitor band-pass filter of a discrete-time type, in particular for cancelling offset and low-frequency noise of switched-capacitor stages
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Solution Overview
Problem
Existing systems for canceling offset and low-frequency noise in switched-capacitor stages, such as those used in MEMS gyroscopes, face challenges in maintaining precise phase control and introducing phase shifts, especially due to aging and temperature variations, and existing solutions like derivative filters and chopping techniques are complex and ineffective.
Innovation Solution
A switched-capacitor band-pass filter employing a correlated-double-sampling technique with multiple phase signals to differentiate between signal and noise components, using feedback and sampling capacitors to isolate and attenuate undesired components without affecting the signal, and incorporating a reset step to manage phase shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If derivative filters are used to eliminate offset in continuous-time read chains, then offset elimination is improved, but phase shift control deteriorates because the position of singularities depends on self-oscillation frequency which varies with ageing and temperature
Solution Approach 1:
The patent replaces continuous-time derivative filtering with a discrete-time switched-capacitor implementation. The derivative operation is performed through capacitive coupling and switching sequences rather than continuous analog differentiation, allowing the filter characteristics to be defined by switching frequencies and capacitor ratios rather than analog component values that drift with temperature and aging.
Solution Approach 2:
The patent changes the operating regime from continuous-time to discrete-time, and fixes the filter's phase characteristics by tying them to the switching frequency rather than to the variable self-oscillation frequency. The filter response is determined by the switching clock and capacitor network, which remain stable despite variations in the mechanical resonator's natural frequency.
2Measurement precision
If chopping techniques with high-frequency offset modulation are used, then offset modulation is improved, but system complexity increases due to complex signal processing requirements and poor phase shift control
Solution Approach 1:
The patent extracts the offset and low-frequency noise components from the signal path by sampling them separately during dedicated phases. The correlated double sampling mechanism captures the offset voltage during a calibration phase and subtracts it during the signal measurement phase, removing the need for complex modulation and demodulation circuits.
Solution Approach 2:
The patent employs periodic switching sequences with distinct phases for offset sampling and signal measurement. The switching network alternates between connecting the capacitor to the input for signal acquisition and connecting it to reference nodes for offset sampling, creating a time-multiplexed operation that simplifies the overall circuit architecture compared to continuous chopping techniques.
3Object-affected harmful factors
If filtering is applied to eliminate offset and flicker noise, then noise attenuation is improved, but phase shifts are introduced that interfere with the driving loop
Solution Approach 1:
The patent replaces continuous-time filtering that introduces phase lag with a discrete-time switched-capacitor filter where the phase response is determined by the switching clock. The filter is designed to have a zero at DC (eliminating offset) while maintaining a flat phase response at the signal frequency, achieving noise attenuation without loop interference.
Solution Approach 2:
The patent implements a feedback mechanism where the filter's transfer function is designed to have a zero at the frequency of interest. By placing a zero at the signal frequency, the filter compensates for any phase shifts and maintains unity gain and zero phase shift at the operating frequency, ensuring the driving loop remains stable while still attenuating low-frequency noise.
Data Source
Figure 1~1a
Figure 2~2a
Figure 3a~3c
AI summary
A band-pass filter made up by an operational amplifier (2) and by an input circuit. The input circuit is formed by a capacitive filtering element (C1), connected to the input of the operational amplifier; a coupling switch (11), coupled between an input node (10) and the capacitive filtering element; a capacitive sampling element (Cs), coupled between the input of the filter and the input node; and a sampling switch (16), coupled between the input node and a reference-potential line. The coupling switch and the input sampling switch close in phase opposition according to a succession of undesired components sampling and sensing steps, so that the capacitive sampling element forms a sampler for sampling the undesired component in the undesired components sampling step, in the absence of the component of interest, and forms a subtractor of the undesired components from the input signal in the sensing step.