A 1Gsample/s 6-bit flash A/D converter with a combined chopping and averaging technique for reduced distortion in 0.18(mu)m CMOS

摘要

Hard disk drive applications require a high Spurious Free Dynamic Range (SFDR),6-bit Analog-to-Digital Converter (ADC) at conversion rates of 1GHz and beyond.This work proposes a robust, fault-tolerant scheme to achieve high SFDR in an av-eraging flash A/D converter using comparator chopping. Chopping of comparatorsin a flash A/D converter was never previously implemented due to lack of feasibilityin implementing multiple, uncorrelated, high speed random number generators. Thiswork proposes a novel array of uncorrelated truly binary random number generatorsworking at 1GHz to chop all comparators.Chopping randomizes the residual offset left after averaging, further pushingthe dynamic range of the converter. This enables higher accuracy and lower bit-errorrate for high speed disk-drive read channels. Power consumption and area are reducedbecause of the relaxed design requirements for the same linearity.The technique has been verified in Matlab simulations for a 6-bit 1Gsamples/sflash ADC under case of process gradients with non-zero mean offsets as high as 60mVand potentially serious spot offset errors as high as 1V for a 2V peak to peak inputsignal. The proposed technique exhibits an improvement of over 15dB compared topure averaging flash converters for all cases.The circuit-level simulation results, for a 1V peak to peak input signal, demon-strate superior performance. The reported ADC was fabricated in TSMC 0.18 ??mCMOS process. It occupies 8.79mm2 and consumes about 400mW from 1.8V powersupply at 1GHz. The targeted SFDR performance for the fabricated chip is at least45dB for a 256MHz input sine wave, sampled at 1GHz, about 10dB improvement onthe 6-bit flash ADCs in the literature.