A Current-Mode Bit-Block Circuit Applicable to Low-Voltage, Low-Power Pipeline Video-Speed A/D Converters

Yasuhiro SUGIMOTO  Shunsaku TOKITO  Hisao KAKITANI  Eitaro SETA  

IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E79-A   No.2   pp.199-209
Publication Date: 1996/02/25
Online ISSN: 
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Analog Technologies in Submicron Era)
ADC bit-block circuit,  current-mode circuit,  pipeline-type ADC,  video-speed ADC,  low voltage,  low power,  

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This paper describes a study to determine if a current-mode circuit is useful as an analog circuit technique for realizing submicron mixed analog-and-digital MOS LSIs. To examine this, we designed and circuit simulated a new current-mode ADC bit-block for a 3 V, 10-bit level, 20 MHz ADC with a pipeline architecture and with full current-mode approach. A new precision current-mode sample-and-hold circuit which enables operation of a bit block at a clock speed of 20 MHz was developed. Current mismatches caused by the poor output impedance of a device were also decreased by adopting a cascode configuration throughout the design. Operation with a 3 V power supply and a 20 MHz clock speed in a 3-bit A/D configuration was verified through circuit simulation using standard CMOS 0.6 µm device parameters. Gain error, mismatch of current, and linearity of the bit block with changing threshold voltage of a device were carefully examined. The bit block has a gain error of 0.2% (10-bit level), a linearity error of less than 0.1% (more than 10-bit level), and a current mismatch of DAC current sources in a bit cell of 0.2 to 0.4% (more than 8-bit level) with a 3 V power supply and 20 MHz clock speed. An 8-to 9-bit video-speed pipeline ADC can be realized without calibration. This confirms that the current-mode approach is effective.