A monolithic 8b two-step parallel ADC without DAC and subtractor circuits

THE DEVELOPMENT of high-speed monolithic 8b A/D converters for digital processing of video signals has been receiving a high priority. Most designs have incorporated a flash-type ADC with 2"-1 comparators, involving complex circuitry. It has been found that this complexityc an be reduced, in general, through the use of two-step parallel A/D conversion, where local D/A converter (DAC) and subtractor circuits are required. But accuracy has been affected. This paper will describe a two-step parallel 8b ADC capable of sampling at rates up to 30 megasamples-per-second (Msls), by using monolithic IC technology. A single resistor-string is utilized to generate dc reference voltages for all of the comparators, and it is unnecessary to use the local DAC and subtractor circuits. The device has been fabricated with 5pm-rule standard bipolar process technology, using double-layer aluminum nterconnection; about 2300 elements are on a 3.95mm x 5.35mm chip. The basic processing sequence requires nine masks including overpassivation.