Silicon Microphone Tracking Bias Circuit for Offset Reduction
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Solution Overview
Problem
Existing silicon microphones face challenges in achieving optimal noise, linearity, leakage robustness, and overload recovery performance due to high-ohmic pseudo-resistor blocks used in constant charge readout schemes, which can result in significant offset and reduced ADC range.
Innovation Solution
Implementing a tracking biasing scheme with an offset reduction circuit that includes anti-parallel diode-connected MOSFETs for high-value resistors and a switched capacitor circuit to provide stable impedance, ensuring linearity and fast recovery from shock events, while using feedback capacitors to track AC signals and minimize offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high-ohmic pseudo-resistor block is used for SiMic readout amplifier design in a constant charge readout scheme, then the microphone can achieve basic readout functionality, but the noise performance, linearity, leakage robustness, and overload recovery performance are limited
Solution Approach 1:
The readout circuit is segmented into multiple functional blocks: a first readout amplifier for initial signal amplification, a second readout amplifier for further amplification, and a switched capacitor circuit for impedance transformation. This segmentation allows each block to be optimized for specific performance aspects, improving overall noise performance and linearity while distributing the complexity across modular components.
Solution Approach 2:
A switched capacitor circuit is introduced as an intermediary between the pseudo-resistor block and the second readout amplifier. This intermediary transforms the high impedance from the pseudo-resistor to a lower impedance, reducing the loading effect on the first amplifier and improving overall noise performance and linearity without requiring a complete redesign of the readout architecture.
2Stability of the object's composition
If a high-ohmic pseudo-resistor block is used in the readout circuit, then the circuit can operate with constant charge, but the offset increases and ADC range is reduced
Solution Approach 1:
A feedback path is implemented where the output of the second readout amplifier is fed back to its input through a feedback capacitor. This feedback mechanism stabilizes the DC operating point and reduces offset by continuously correcting deviations, allowing the circuit to maintain constant charge operation while achieving better offset specifications and preserving ADC range.
Solution Approach 2:
The impedance transformation circuit dynamically changes the effective impedance seen by the second readout amplifier based on the operating conditions. By transforming the high pseudo-resistor impedance to a lower value under specific operating conditions, the circuit maintains constant charge operation while reducing offset and expanding the usable ADC range.
3Quantity of substance
If anti-parallel MOSFETs operating in deep-triode region are used to create high-value resistors, then extremely high resistance values are achieved, but noise, linearity, leakage robustness, and overload recovery performance are limited
Solution Approach 1:
The switched capacitor circuit acts as an intermediary that transforms the extreme high impedance from the anti-parallel MOSFET pseudo-resistors to a more moderate impedance level. This transformation maintains the benefit of using simple MOSFET-based high-value resistors while eliminating their detrimental effects on noise and linearity, as the second readout amplifier now sees a lower, more manageable impedance.
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 tracking biasing scheme enhances signal-to-noise ratio, improves linearity, and ensures robust leakage performance with fast overload recovery, significantly reducing offset and maintaining signal integrity across varying conditions.
Implementation Method 1
a capacitive MicroElectroMechanical System (MEMS) device for converting sound waves into an analog signal
Implementation Method 2
readout circuitry including an amplifier for amplifying the analog signal
Implementation Method 3
an Analog-to-Digital Converter (ADC) for converting the amplified analog signal into a digital signal
Implementation Method 4
stacking anti-parallel Metal Oxide Silicon Field Effect Transistors (MOSFETs) operating in the deep-triode region to achieve an extremely high value resistor
Implementation Method 5
using feedback capacitors to track AC signals and minimize offset
Data Source
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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.