An Adaptive DAC Settling Waiting Time Optimized Ultra Low Voltage Asynchronous SAR ADC in 40 nm CMOS


IEICE TRANSACTIONS on Electronics   Vol.E96-C   No.6   pp.820-827
Publication Date: 2013/06/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E96.C.820
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Analog Circuits and Related SoC Integration Technologies)
ADC,  SAR,  ultra low power,  asynchronous,  

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An ultra low power and low voltage successive-approximation-register (SAR) analog-to-digital converter (ADC) with timing optimized asynchronous clock generator is presented. By calibrating the delay amount of the clock generator, the DAC settling waiting time is adaptively optimized to counter the device mismatch. This technique improved the maximum sampling frequency by 40% keeping ENOB around 7-bit at 0.4 V analog and 0.7 V digital power supply voltage. The delay time dependency on power supply has small effect to the accuracy of conversion. Decreasing of supply voltage by 9% degrades ENOB only by 0.1-bit, and the proposed calibration can give delay margins for high voltage swing. The prototype ADC fabricated in 40 nm CMOS process achieved figure of merit (FoM) of 8.75-fJ/conversion-step with 2.048 MS/s at 0.6 V analog and 0.7 V digital power supply voltage. The ADC can operates from 50 S/s to 8 MS/s keeping ENOB over 7.5-bit.