Silicon Microphone Offset Reduction Through Tracking-Bias Feedback
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Solution Overview
Problem
Silicon microphones face challenges with limited noise, linearity, leakage robustness, and overload recovery performance due to high-ohmic pseudo-resistor blocks in constant charge readout schemes, which can cause offset issues affecting ADC range and signal quality.
Innovation Solution
Implementing a tracking biasing scheme with a switched capacitor resistor circuit and diode-based resistor circuit in a feedback loop configuration, utilizing anti-parallel diodes and switched capacitors to achieve high impedance and linearity, along with a capacitive feedback loop for signal tracking and offset reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high-ohmic pseudo-resistor block is used in constant charge readout scheme, then the microphone achieves high impedance, but noise performance, linearity, leakage robustness, and overload recovery performance are limited
Solution Approach 1:
The pseudo-resistor block is segmented into multiple stacked anti-parallel MOSFETs operating in deep-triode region. This segmentation allows achieving high impedance while improving noise performance and linearity by distributing the resistance function across multiple devices rather than using a single high-ohmic resistor
Solution Approach 2:
The invention changes the operating parameters by using MOSFETs in deep-triode region instead of traditional resistors. This parameter change enables dynamic control of resistance while maintaining high impedance, thereby improving noise performance, linearity, and overload recovery characteristics
2Measurement precision
If offset is not controlled, then the circuit is simpler, but ADC range is reduced and signal quality deteriorates
Solution Approach 1:
An offset reduction circuit is implemented using feedback mechanism where the output of the amplifier is fed back through a capacitor to the inverting input. This feedback loop automatically compensates for offset voltages, improving signal quality and ADC range without requiring complex manual calibration circuits
Solution Approach 2:
The offset reduction circuit uses the microphone's own output signal to generate the feedback voltage that compensates for offset. This self-service approach eliminates the need for external calibration equipment or complex offset adjustment mechanisms, achieving offset compensation with minimal additional circuitry
Data Source
AI summary
A microphone includes a first resistor circuit for receiving a bias voltage; an offset reduction circuit coupled to the first resistor circuit; a second resistor circuit coupled to the offset reduction circuit; an amplifier coupled to the second resistor circuit configured for receiving an input signal and generating an output signal; and a capacitor circuit coupled to the amplifier, the offset reduction circuit, and the second resistor circuit.