Switched-Capacitor Charge Amplifier for Low-Noise Capacitive Sensing
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Solution Overview
Problem
Capacitive acceleration sensors face challenges with high noise levels due to thermal noise from switches and noise folding, which complicates the conversion of output signals into voltage, especially in battery-operated devices where low power consumption is crucial.
Innovation Solution
A charge amplifier circuit with a dual-stage amplifier configuration and a switched capacitor arrangement that includes integration and feedback capacitors, along with alternating switch control, to cancel thermal noise and achieve precise settling over multiple clock cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a precise settling to the correct output value is required in a sampled system, then an excessive bandwidth with respect to the sampling frequency is needed, but this increases noise folding
Solution Approach 1:
The amplifier is divided into two stages: a first amplifier stage with high bandwidth for precise settling, and a second amplifier stage with limited bandwidth. The switched capacitor arrangement samples the output of the first stage and processes it in the second stage, separating the settling function from the output function. This segmentation allows the first stage to achieve precise settling without contributing its full noise to the final output.
Solution Approach 2:
The noise contribution from the first amplifier stage is extracted and prevented from being folded into the sampled output signal. By using the switched capacitor arrangement to sample and process the signal in a second stage with controlled bandwidth, the harmful noise folding effect is eliminated while preserving the settling precision achieved in the first stage.
2Ease of operation
If conventional designs are used to convert output signal to voltage, then thermal noise from switches (kT/C noise) is added to the signal, but this increases overall noise level
Solution Approach 1:
The switched capacitor arrangement acts as an intermediary between the first amplifier stage and the second amplifier stage. It samples the output of the first stage during a specific phase and transfers it to the second stage, allowing the signal conversion to proceed while the dual-stage architecture manages the noise contributions from the switching operations.
Solution Approach 2:
The switched capacitor arrangement operates periodically with alternating phases: in the first phase, it samples the output of the first amplifier stage; in the second phase, it processes the signal in the second amplifier stage. This periodic operation allows for controlled noise management during signal conversion.
3Use of energy by moving object
If battery-operated devices use low power consumption design, then amplifier power is reduced, but this compromises noise performance
Solution Approach 1:
The power consumption is segmented across two amplifier stages, each optimized for different functions. The first stage operates with higher bandwidth and power to achieve precise settling, while the second stage operates with lower power and limited bandwidth for signal processing. This segmentation allows the overall system to achieve low noise performance without requiring the entire amplifier to consume high power continuously.
Solution Approach 2:
The switched capacitor arrangement enables periodic operation where amplification and signal processing occur in alternating phases. This allows the amplifier to consume power only when needed for each function, reducing overall power consumption while maintaining noise performance through the coordinated operation of both stages.
Data Source
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AI summary
The present disclosure relates to a charge amplifier circuit (10) for a capacitive sensing arrangement (80), the charge amplifier circuit (10) comprising: - an first amplifier stage (20) comprising a first amplifier input (22), a second amplifier input (23) connectable to the capacitive sensing arrangement (80), and comprising a first amplifier output (24) and a second amplifier output (25), - a second amplifier stage (30) comprising a first amplifier input (32), a second amplifier input (33), a first amplifier output (34) and a second amplifier output (35), and - a switched capacitor arrangement (40) between the first amplifier stage (20) and the second amplifier stage (30), the switched capacitor arrangement (40) comprising a first integration capacitor (42), a second integration capacitor (43), a number of first switches (66, 67) and a number of second switches (68, 69).