Filtered Reference Voltage Circuit With Fast Startup Settling
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Solution Overview
Problem
Existing low-pass filters for voltage references in MEMS accelerometers exhibit excessively long settling times, making them impractical for use in low-frequency and very low-frequency applications due to high noise levels and slow transient states, which hinder precise acceleration measurements.
Innovation Solution
A circuit with a low-pass filter and a switching mechanism that includes a transistor and control circuit to rapidly charge capacitors, reducing the settling time by using a switching circuit with an on-state resistance that is significantly lower than the stage resistance, allowing for a faster settling time constant.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a low-pass filter with a very low cutoff frequency is used to attenuate noise in the reference voltage, then the noise level in the output signal is reduced, but the settling time during transient state becomes extremely long
Solution Approach 1:
The patent applies dynamics by making the resistance value time-variable. A switching circuit changes the resistance value from a first value (during startup) to a second value (during normal operation). This dynamic adjustment allows the filter to achieve both fast settling time and low noise attenuation by adapting its characteristics to different operational phases.
Solution Approach 2:
The patent uses periodic action through a two-phase operation: a first phase with a first resistance value for rapid settling, and a second phase with a second resistance value for noise attenuation. The switching circuit transitions between these phases, enabling the system to benefit from both fast response and low noise performance at different times.
2Object-affected harmful factors
If the resistance value in the low-pass filter is increased to reduce cutoff frequency and attenuate noise, then noise rejection is improved, but the time constant and settling time increase
Solution Approach 1:
The patent makes the resistance value dynamic by using a switching circuit that changes it from a first value to a second value. During the startup phase, the lower resistance value provides a short time constant for fast settling. During normal operation, the higher resistance value provides low cutoff frequency for noise rejection, thus resolving the contradiction between fast response and noise attenuation.
Solution Approach 2:
The patent applies preliminary action by first establishing a fast settling condition with the first resistance value before transitioning to the noise attenuation condition. The switching circuit is controlled to switch from the first resistance value to the second resistance value after a predetermined time, ensuring that the system settles quickly before entering the noise filtering phase.
3Object-affected harmful factors
If a traditional low-pass filter is used for voltage reference filtering, then noise is attenuated, but the filter exhibits a long transient state that prevents full precision operation
Solution Approach 1:
The patent uses dynamics to make the resistance value time-variable, allowing the filter to transition from a fast-settling mode to a low-noise mode. This enables the system to achieve both rapid stabilization and accurate measurements by adapting the filter characteristics to the operational phase, thus resolving the contradiction between noise attenuation and measurement precision.
Solution Approach 2:
The patent implements periodic action through two distinct operational phases: a startup phase with first resistance value for rapid settling, and a measurement phase with second resistance value for noise attenuation. The switching circuit transitions between these phases, allowing the system to achieve both fast response and high measurement precision at appropriate times.
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 circuit significantly reduces the settling time of the filtered reference voltage, enabling faster startup and more precise low-frequency measurements while maintaining low noise levels, suitable for MEMS accelerometers.
Implementation Method 1
a switching circuit comprising a control electrode, two conduction electrodes, one of which is electrically connected to the midpoint, the switching circuit having an on-state resistance between the two conduction electrodes, the value of which is less than the value of the stage resistance
Implementation Method 2
a low-pass filter having a cutoff frequency of less than 1 Hz and electrically connected to the voltage reference and comprising a stage formed by a stage resistance electrically connected at a midpoint to a stage capacitor
Data Source
AI summary
The invention relates to a circuit comprising a voltage reference (R) and a low-pass filter (F) electrically connected to the voltage reference (R). The filter (F) comprises a stage formed by a stage resistance (Re) electrically connected at a midpoint (M) to a stage capacitor (Ce), the stage resistance (Re) and the stage capacitor (Ce) at least partially defining a time constant of the filter and the midpoint (M) carrying the filtered reference voltage (V′ref). The circuit also comprises a transistor (T) and a control circuit (Cde) of the gate of the transistor (T) configured to bias the transistor (T) in conduction when the circuit (1) is turned on, the on-state resistance of the transistor (T) combining with the stage capacitor (Ce) to raise the filtered reference voltage (V′ref) with a settling time constant lower than the filter time constant.