Open-Loop VCO ADC Calibration for Stable Digital Microphone Sensitivity
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Solution Overview
Problem
Open-loop VCO-based ADC architectures in digital microphones suffer from sensitivities that are dependent on sampling frequency, process corners, and temperature, leading to instability and potential audible artifacts.
Innovation Solution
Implement a startup calibration and digital temperature compensation method using a first and second modulation path, a summing circuit, a difference circuit, and a temperature compensation circuit to adjust sensitivity and compensate for temperature changes, ensuring stability and preventing audible artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If open-loop VCO-based ADC architecture is used, then power consumption and integrated circuit area are reduced, but sensitivity becomes dependent on sampling frequency, process corners, and temperature
Solution Approach 1:
The patent applies preliminary action by performing calibration measurements during a startup calibration mode before normal operation begins. The system measures the actual sensitivity of the VCO-based ADC architecture under current process and temperature conditions, then stores calibration data that will be used to compensate for sensitivity variations during normal operation. This preliminary characterization allows the system to account for process corners and temperature effects without adding complexity to the main signal path.
Solution Approach 2:
The patent implements parameter changes by using a temperature compensation circuit that adjusts the sensitivity parameter based on detected temperature variations. The system changes the operating parameters (gain/sensitivity) of the ADC architecture dynamically to compensate for temperature-induced drift, thereby maintaining sensitivity stability across different temperature conditions while preserving the low power consumption benefits of the open-loop VCO-based design.
2Area of stationary object
If open-loop VCO-based ADC architecture is used, then integrated circuit area is reduced, but sensitivity becomes dependent on process corners and temperature
Solution Approach 1:
The system performs preliminary calibration measurements during startup to characterize the actual sensitivity of the VCO-based ADC under specific process conditions. By measuring and storing calibration data before normal operation, the system compensates for process corner variations without requiring a more complex (and larger) circuit design. This preliminary action allows the compact architecture to maintain manufacturing precision through software-based compensation.
Solution Approach 2:
The patent implements feedback by using the calibration data obtained during startup to adjust and compensate for sensitivity variations caused by process corners and temperature. The system continuously references the stored calibration information and applies appropriate corrections during normal operation, creating a closed-loop compensation mechanism that maintains sensitivity consistency despite the inherent variations in semiconductor manufacturing processes.
3Reliability
If temperature compensation is implemented during normal operation, then sensitivity stability is improved, but device complexity increases
Solution Approach 1:
The patent reduces device complexity by performing the complex calibration measurements during a preliminary startup phase rather than continuously during normal operation. The calibration data is captured once at startup under known conditions and then reused during normal operation, avoiding the need for continuous complex measurement circuits while still maintaining sensitivity stability through the stored calibration information.
Solution Approach 2:
The system applies self-service by using the VCO-based ADC architecture itself to generate the calibration data during startup calibration mode. The same hardware components that will be used during normal operation are utilized to measure and characterize their own sensitivity, eliminating the need for separate external calibration equipment or additional complex calibration circuits. The system calibrates itself using its own resources.
Data Source
AI summary
A digital microphone includes a first modulation path having an input for receiving an analog input signal and an output for generating a first digital signal; a second modulation path having an input for receiving the analog input signal and an output for generating a second digital signal; a summing circuit having a first input for receiving the first digital signal, a second input for receiving the second digital signal, and an output for generating a digital calibration path signal; and a difference circuit having a first input for receiving the first digital signal, a second input for receiving the second digital signal, and an output for generating a digital signal path signal.