Charge Amplifier DC Feedback Sampling for Low Phase Error
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Solution Overview
Problem
Conventional charge amplifiers face challenges in reducing phase error between input charge signals and output voltage signals, particularly due to AC components passing through the DC feedback loop, and struggle to implement large resistors on chip while providing suitable leakage current and signal-to-noise ratio.
Innovation Solution
Incorporating a sampler in the DC feedback loop that samples the AC signal with a DC offset to block AC components, allowing for a smaller resistor implementation on chip and reducing phase error, while maintaining suitable signal levels and signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large resistor is used in the DC feedback loop to reduce cut-off frequency and phase error, then phase error is reduced, but it is difficult to implement on chip while providing suitable leakage current
Solution Approach 1:
The feedback loop is segmented into AC feedback path (through capacitor) and DC feedback path (through resistor), allowing each path to be optimized independently. The DC path uses a smaller on-chip resistor combined with a sampler to achieve the equivalent effect of a large resistor without the manufacturing difficulties.
Solution Approach 2:
A sampler is introduced as an intermediary component in the DC feedback loop. The sampler periodically captures the output voltage and holds it, enabling the use of a smaller resistor while maintaining the low cut-off frequency effect that would otherwise require a large resistor.
2Measurement precision
If a large capacitance is used in the capacitive feedback loop to reduce cut-off frequency, then phase error is reduced, but signal level decreases resulting in low signal-to-noise ratio
Solution Approach 1:
The feedback mechanism is segmented into AC coupling (through capacitor) and DC feedback (through resistor with sampler), allowing the capacitive feedback loop to use smaller capacitance values without compromising phase error performance. The DC path handles the low-frequency stabilization separately.
Solution Approach 2:
The invention changes the operating parameters by introducing a sampler that enables the DC feedback resistor to have a much smaller value while achieving the same cut-off frequency effect. This parameter change allows smaller capacitor values in the AC feedback path, maintaining signal level and SNR.
3Device complexity
If AC components pass through the DC feedback loop, then the circuit is simpler, but phase error increases
Solution Approach 1:
The feedback loop is segmented into AC and DC paths. The DC feedback path includes a sampler that blocks AC components while allowing DC feedback, separating the frequency domains handled by each feedback path and reducing phase error.
Solution Approach 2:
The sampler operates periodically, opening and closing at specific intervals. During the closed state, it allows DC feedback while blocking AC components. This periodic action effectively filters AC components from the DC feedback path without requiring complex continuous filtering circuitry.
Data Source
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AI summary
A charge amplifier circuit (100) for use with a MEMS gyroscope operating at a carrier frequency or other device provides an improved DC feedback loop. The charge amplifier circuit includes a first amplifier (110) having a first amplifier input (112) and a first amplifier output (114). A first capacitor (162) is coupled in parallel to the first amplifier, and a DC feedback loop (130) is coupled in parallel to the first amplifier and in parallel to the first capacitor. The DC feedback loop includes a sampler (140) coupled in series to a first resistor (132). The sampler samples an AC signal (142) with a DC offset at the first amplifier output and drives a DC voltage substantially equal to the DC offset at the sampler output so that substantially all AC components of the AC signal at the carrier frequency are blocked in the DC feedback loop.