Split-Capacitor SAR ADC Calibration Using n+1 Weight Measurements
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Solution Overview
Problem
Conventional split capacitor SAR ADCs are difficult to calibrate due to the non-functionality of the Least Significant Bit (LSB) weight in comparator decisions, making it challenging to estimate the LSB weight, which is crucial for pipeline ADCs.
Innovation Solution
A novel SAR ADC architecture with a calibration scheme that involves n+1 calibration measurements to determine the weights of sampling cells for each bit, allowing for facilitated estimation of the LSB weight by controlling switch circuits to adjust decision thresholds and sweep calibration signals, enabling accurate LSB weight determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional split capacitor SAR ADC architecture is used, then analog implementation advantages are achieved, but calibration difficulty increases due to inability to estimate LSB weight
Solution Approach 1:
The patent applies preliminary action by performing calibration measurements before normal ADC operation. The calibration process uses n+1 measurements to determine sampling cell weights in advance, storing these weights for use during subsequent conversions. This preliminary calibration enables the ADC to compensate for capacitor weight variations without affecting normal operation.
Solution Approach 2:
The patent implements feedback by using comparator decisions from calibration measurements to determine sampling cell weights. The calibration process feeds back weight information to correct for capacitor variations, and this weight information is used to adjust subsequent conversion operations, creating a closed-loop calibration system.
2Ease of manufacture
If conventional split capacitor SAR ADC is used, then manufacturing advantages are obtained, but measurement precision of LSB weight deteriorates
Solution Approach 1:
The patent uses feedback from comparator decisions in calibration measurements to accurately determine LSB weight. The n+1 calibration measurements provide feedback information about actual capacitor weights, which is then used to correct for variations and achieve precise LSB weight estimation despite manufacturing tolerances.
Solution Approach 2:
The patent changes parameters by performing multiple calibration measurements (n+1 measurements) to statistically determine capacitor weights. This approach uses parameter variation through repeated measurements to achieve precise weight estimation that overcomes single-measurement inaccuracies in conventional designs.
3Measurement precision
If calibration scheme with n+1 measurements is implemented, then LSB weight estimation accuracy improves, but calibration time increases
Solution Approach 1:
The patent performs the time-consuming n+1 calibration measurements as a preliminary one-time action before normal ADC operation. Although this takes time, the calibration weights are stored and reused for all subsequent conversions, so the time cost is amortized over many operations, making the overall system more efficient.
Solution Approach 2:
The calibration system is self-service in that it automatically performs the n+1 measurements and weight determination without requiring external intervention. Once calibrated, the system uses its own stored weight information for subsequent operations, eliminating the need for repeated external calibration procedures.
Data Source
AI summary
A n-bit Successive Approximation Register Analog-to-Digital Converter, SAR ADC, is provided. The SAR ADC comprises a respective plurality of sampling cells for each bit of the n-bit of the SAR ADC. Each sampling cell comprises a capacitive element coupled to a cell output of the sampling cell in order to provide a cell output signal. Further, each sampling cell comprises a first cell input for receiving a first signal, and a first switch circuit capable of selectively coupling the first cell input to the capacitive element. Each cell additionally comprises a second cell input for receiving a second signal, and a third cell input for receiving a third signal. The third signal exhibits opposite polarity compared to the second signal. Each sampling cell comprises a second switch circuit capable of selectively coupling one of the second cell input and the third cell input to the capacitive element. The SAR ADC further comprises at least one comparator circuit coupled to the sampling cells. The at least one comparator circuit is configured to output a comparison signal based on the cell output signals of the sampling cells. Additionally, the SAR ADC comprises a calibration circuit configured to supply at least one respective control signal to the respective second switch circuit of the sampling cells for controlling the second switch circuits.