Capacitive Sensor Amplifier With Adaptive Feedback Bandwidth
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Solution Overview
Problem
Existing amplifier circuits for capacitive sensors, such as MEMS microphones, face challenges in quickly compensating for the rate of change of bias voltage during the settling phase, leading to extended startup times and increased noise levels due to fixed bandwidth settings that do not adapt to the changing voltage levels.
Innovation Solution
An adaptive feedback loop control mechanism in the amplifier circuit that dynamically adjusts the bandwidth based on the sensed rate of change of the bias voltage, allowing for minimal required bandwidth during settling and reducing noise and phase delay, while ensuring the amplifier can compensate for leakage effects and quick startup.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the bandwidth of the DC servo loop is increased to compensate for the rate of change of bias voltage during settling, then the settling time is reduced, but the noise level of the amplified output signal increases and phase delay increases
Solution Approach 1:
The patent applies dynamics by making the bandwidth of the DC servo loop variable rather than fixed. The bandwidth is dynamically adjusted based on the operating state of the capacitive sensor - wider bandwidth during settling phase to reduce settling time, and narrower bandwidth during normal operation to minimize noise and phase delay. This is achieved through a control circuit that automatically adjusts the bandwidth parameter in response to detected conditions.
Solution Approach 2:
The patent changes the bandwidth parameter of the DC servo loop based on the operating state. During the settling phase, the bandwidth is increased to compensate for rapid changes in bias voltage. During normal operation, the bandwidth is reduced to minimize noise amplification and phase delay. This parameter change resolves the contradiction by optimizing bandwidth for each specific operational phase.
2Loss of time
If the bandwidth of the DC servo loop is increased to compensate for the rate of change of bias voltage during settling, then the settling time is reduced, but the phase delay increases
Solution Approach 1:
The patent makes the bandwidth dynamic, adjusting it according to the operational phase. During settling, high bandwidth reduces phase delay impact by quickly establishing the correct DC level. During normal operation, low bandwidth minimizes phase delay on the signal path. This dynamic adjustment resolves the contradiction between fast settling and minimal phase delay.
Solution Approach 2:
The bandwidth parameter is changed based on the operational state of the system. A first bandwidth value is used during settling to minimize settling time, and a second, narrower bandwidth value is used during normal operation to minimize phase delay. This parameter switching resolves the contradiction by optimizing for the dominant concern in each phase.
3Device complexity
If a fixed bandwidth is used in the DC servo loop, then the circuit design is simplified, but the amplifier cannot quickly compensate for the rate of change of bias voltage during settling
Solution Approach 1:
The patent introduces dynamics to the bandwidth parameter, transitioning from a fixed to a variable bandwidth DC servo loop. The control circuit automatically adjusts the bandwidth based on the operational phase, providing fast compensation during settling while maintaining manageable complexity through automated control logic rather than complex manual tuning.
Data Source
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AI summary
An amplifier circuit (AC) for amplifying an output signal (OS) of a capacitive sensor (M) comprises a first input terminal (AIN) to receive the output signal (OS) of the capacitive sensor (M) and a second input terminal (BIN) to receive a bias voltage (Vbias) of the capacitive sensor (M). The amplifier circuit (AC) comprises an amplifier (A) for amplifying the output signal (OS) and a control circuit (CF) arranged in a feedback loop (FL) of the amplifier (A) being configured to control a DC voltage level at an input connection (A1) of the amplifier (A). A bias voltage sensing circuit (BVS) senses a change of the level of the bias voltage (Vbias) at the second input terminal (BIN) and changes the bandwidth of the feedback loop (FL) in dependence on the sensed change of the level of the bias voltage (Vbias).