Silicon Microphone Log Amplifier for PGA Clipping and SNR
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Solution Overview
Problem
Silicon microphones face challenges in handling large input signal swings without degrading signal-to-noise ratio (SNR) due to programmable gain amplifier (PGA) clipping, which distorts the input signal.
Innovation Solution
Implementing a logarithmic amplifier system with multiple programmable gain amplifiers having different gains, each driving a summing circuit, and utilizing a switched capacitor or continuous-time resistor circuit to achieve a piecewise linear transfer function, with signal compression in the analog domain and decompression in the digital domain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single programmable gain amplifier (PGA) is used to amplify the analog signal, then the signal can be amplified, but the PGA clips and distorts the input signal when handling large input signal swings
Solution Approach 1:
The single PGA is divided into multiple PGAs with different gain values (e.g., first PGA with gain G1, second PGA with gain G2 where G1 < G2). Each PGA handles a different portion of the input signal range, preventing any single PGA from clipping while maintaining signal integrity across the full dynamic range.
Solution Approach 2:
The system dynamically selects which PGA output to use based on the input signal level. A selector circuit chooses between the outputs of multiple PGAs with different gains, allowing the system to adapt to varying signal conditions and prevent clipping by selecting the appropriate gain stage for the current signal level.
2Reliability
If multiple PGAs with different gains are used to handle large signal swings, then signal clipping is prevented, but the signal-to-noise ratio (SNR) degrades
Solution Approach 1:
Each PGA is optimized for a specific gain level and operates in a localized portion of the signal range. The first PGA with lower gain handles larger signal levels, while the second PGA with higher gain handles smaller signal levels. This local optimization ensures that each PGA operates in its optimal performance region, maintaining high SNR while preventing clipping.
Solution Approach 2:
The system includes a selector circuit that monitors the output levels of multiple PGAs and dynamically selects the appropriate output based on which PGA is not clipping. This feedback mechanism ensures that the output with the best signal quality (highest SNR without clipping) is always selected.
3Adaptability or versatility
If multiple PGAs with different gains are implemented, then the dynamic range is extended, but the system area increases
Solution Approach 1:
Multiple PGAs share common input and output infrastructure. The PGAs share the same input node and their outputs are combined through a selector circuit rather than requiring separate complete signal paths. This multi-functional approach allows the system to handle a wide dynamic range while minimizing the additional area required by reusing shared components.
Data Source
AI summary
A logarithmic amplifier includes programmable gain amplifiers each having a different gain, wherein an input of each of the programmable gain amplifiers is coupled to an input of the logarithmic amplifier; and a summing circuit having inputs coupled to a corresponding output of each of the programmable gain amplifiers and an output coupled to an output of the logarithmic amplifier, wherein the summing circuit generates a logarithmic transfer function having piecewise linear segments.