A SAR ADC Capacitance Mismatch Calibration Technique Based On Metastability Detection

Analog to Digital Converter(ADC)is a type of device used to convert analog continuous signals into digital discrete signals.ADCs are widely used in communications equipment,medical electronics,digital oscilloscopes and other fields.Many modules on the Successive Approximation Register(SAR)ADC structure are replaced by digital circuits,so that the size can be closely followed by the process selection.SAR ADC can achieve medium-high-level resolution and medium-high-speed sampling rate with its low power consumption and area.SAR ADC have become more and more popular in academia and industry in recent years.At the same time,the capacitance mismatch in SAR ADC is a very important error parameter.For high performance SAR ADCs,capacitive mismatch calibration techniques are increasingly being applied to SAR ADCs.This article explains in detail the working principle of SAR ADC,and introduces some sources of error in SAR ADC,including charge injection and clock feedthrough of sampling switch,capacitor mismatch and comparator offset.Some calibration methods for comparator offset calibration and capacitance mismatch are introduced.In order to calibrate the SAR ADC capacitance mismatch,this paper proposes a capacitance mismatch calibration algorithm based on comparator meta-stability detection.Firstly,the meta-stability of the comparator and its influence are introduced.A pseudo-random noise(PN)injection is introduced into the algorithm.The PN code assigns a value to the comparator output when the comparator is meta-stability,thereby obtaining the difference between the actual output and the ideal output,and using the difference to calculate the calibration code.The introduction of the algorithm leads to the distinction between the ADC calibration mode and the normal operating mode.Secondly,based on the MATLAB platform,the simulink simulation model is built,and the basic workflow of the simulation module is introduced.Some parameters in the algorithm model are analyzed,such as the mismatch tolerance of the unit capacitance,the determination of the meta-stability detection range,and the average number of points of the capacitance mismatch error code.Finally,the algorithm is applied to the 12 bit 150Msps SAR ADC.The role of redundant bit capacitance for digital calibration,the pseudo meta-stability phenomenon in the metastable detection circuit and the transcoding of the output codeword are introduced.In addition,the decimal point of the capacitance mismatch error code is simply analyzed and the effects of different ideal output codes on the calibration are introduced.The simulation results show that the SNDR is improved from 63.12 dB to 71.06 dB and the SFDR is increased from 66.85 dB to 82.37 dB after calibration.