SAR-ADC Radix Error Calibration Using LSB Averaging
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Solution Overview
Problem
Existing Analog-to-Digital Converters (ADCs) face challenges in achieving high-resolution linearity due to capacitor mismatch errors, which can be exacerbated by noise during calibration, leading to significant errors that accumulate in higher-significance bits.
Innovation Solution
The implementation of Least-Significant-Bit (LSB) averaging, where additional minimum-size capacitors are used to average out noise during calibration, reducing noise distribution from ±6 LSB to below ±2 LSB, and correcting the SAR search code to improve calibration accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional calibration methods are used without LSB averaging, then the calibration process is simpler and faster, but noise causes significant measurement errors that jump to different values
Solution Approach 1:
The calibration process is segmented into multiple individual calibration trials, with each trial measuring the capacitance value separately. By performing multiple trials and averaging the results, the patent reduces the impact of random noise on the final measurement, thereby improving measurement precision without fundamentally changing the calibration architecture
Solution Approach 2:
The calibration process is repeated periodically multiple times to gather sufficient measurement data. Each periodic calibration trial contributes to the overall averaged result, allowing the system to overcome noise-induced errors through repeated measurements and statistical averaging
2Stability of the object's composition
If capacitor sizes are increased to reduce noise impact, then measurement stability improves, but device area and manufacturing cost increase
Solution Approach 1:
The patent uses multiple copies of the same capacitor array structure for calibration purposes. By creating replicated capacitor sets that mirror the main signal path capacitors, the system can measure and characterize capacitance values without requiring larger physical capacitors, thus maintaining stability through redundancy rather than size
Solution Approach 2:
The capacitor arrays serve multiple functions: they are used both for normal signal conversion and for calibration measurements. This multi-functionality eliminates the need for separate dedicated calibration capacitors, reducing overall device area while maintaining measurement stability through the use of the same well-matched capacitor structures
3Manufacturing precision
If calibration is performed frequently to maintain accuracy, then linearity improves, but processing time and power consumption increase
Solution Approach 1:
The calibration process is performed preliminarily during manufacturing or system initialization, establishing accurate capacitance values before normal operation begins. This preliminary calibration ensures high linearity for subsequent conversions without requiring frequent recalibration, thereby minimizing time loss during operational phases
Solution Approach 2:
The system performs self-calibration using its own internal capacitor structures and signaling resources. By utilizing existing components for calibration purposes rather than requiring external calibration equipment or separate dedicated calibration circuits, the system maintains accuracy efficiently with minimal additional time or power overhead
Data Source
AI summary
A self-calibrating Analog-to-Digital Converter (ADC) performs radix error calibration using a Successive-Approximation Register (SAR) to drive test voltages onto lower-significant capacitors. The final SAR code is corrected by performing LSB averaging on LSB averaging capacitors and then accumulated, and the measurement repeated many times to obtain a digital average measurement. An ideal radix or ratio of the measured capacitor's capacitance to a unit capacitance of an LSB capacitor is subtracted from the digital average measurement to obtain a measured error that is stored in a Look-Up Table (LUT) with the ideal radix. Radix error calibration is repeated for other capacitors to populate the LUT. During normal ADC conversion, the SAR code obtained from converting the analog input is applied to addresses the LUT, and all ideal radixes and measured errors for 1 bits in the SAR code are added together to generate an error-corrected digital value.