Future-Ready Ultrafast 8bit CMOS ADC for System-on-Chip Applications Jincheol Yoo , Daegyu Lee , KyusunChoi , and Ali Tangel Department of Computer Science & Department of Computer & Engineering Communications Engineering The Pennsylvania State University University of Kocaeli University Park, PA 16802 41040 ´ Yzmit, Turkey jyoo,dalee,kyusun @cse.psu.edu atangel@kou.edu.tr Abstract Design and performance of an ultrafast 8bit 0.25 CMOS flash ADC based on the thresholding inverter com- parator are presented. 1. Introduction The semiconductor technology is now approaching 100 nanometer feature size and it is going toward below 100 nanometer. This technology trend presents new challenges in analog-digital mixed signal circuit design. A mixed sig- nal circuit must be integrated on a single chip along with logic and memory circuits to form a system-on-chip. The mixed signal circuit must operate at fast speed along with digital logic and memory circuits; otherwise, it becomes the bottleneck of the system. Thus the authors present in this paper a noble approach to high-speed Analog-to-Digital Converter (ADC) design suitable for the technologies below 100 nanometer and suitable for system-on-chip integration. Flash ADC architecture is known for its high speed oper- ation. An analog input voltage is simultaneously compared by voltage comparators in an -bit flash ADC. The comparators are, perhaps, the most critical components in a flash ADC. The high-speed comparators are realized with differential amplifiers using bipolar transistors. The com- parator realization in the CMOS flash converters consists of auto-zeroed inverter with switched capacitor input. Yet, another CMOS voltage comparator design is considered. This work was supported in part by Pittsburgh Digital Greenhouse (EDTD00-2) through a grant from the Commonwealth of Pennsylvania, Department of Community and Economic Development. 2. Threshold Inverter comparator The authors propose an ultrafast 8bit CMOS flash ADC, shown in Figure 1, featuring the Threshold Inverter (TI) comparators. Circuit wise, the TI comparator is an inverter. Figure 2 shows the TI comparator and the differential am- plifier type voltage comparator. The inverter circuit is inher- ently simpler and faster than the differential amplifier type voltage comparator. D2 D3 D4 D5 D6 D7 D8 D1 Vin V1 V1 gain booster gain booster gain gain booster booster thermometer to binary encoder TI comparator V255 V254 V253 V255 V254 V253 Figure 1. Proposed 8-bit CMOS flash ADC Highlights of the TI comparators are as follows: The comparison voltage of the TI comparator is internal to the inverter, fixed by the transistor sizes. On the other hand, the comparison voltage of the differential comparator is external to the comparator, typically generated by a resistor ladder circuit.