01-11-2010, 12:08 PM
LOW POWER HIGH SPEED SWITCHED CURRENT COMPARATOR
Y. SUN, Y.SWANG, F.C. LAI
HARBIN INSTITUTE OF TECHNOLOGY, CHINA
ABSTRACT:
Current mode implementation provides an alternative to high speed data conversion systems for low voltage applications. The pursuing of speed and accuracy of data conversion makes comparator critical. This paper presents a novel switched current (SI) comparator which achieves high speed without sacrificing either accuracy or power dissipation. Employing a class AB current mirror as the input stage, the desired accuracy is attained and a dynamic class AB latched comparator is used to achieve high operation speed. Both of the input stage and the latched comparator are low power blocks and the average supply current is only 85μA during comparison. The proposed comparator is designed and simulated in TSMC 0.25μm CMOS process with 1.8V supply voltage. The proposed SI comparator achieves a current sensitivity up to 0.2μA and a sampling frequency up to 100MHz, with only 153μW of total power consumption.
INTRODUCTION
Current mode signal processing implemented in CMOS technology has received increasing interest in the past decades [1-3]. The circuit implemented in current mode technique occupies small area, consumes less power dissipation and achieves high operation speed. Moreover, many sensors in SoC such as temperature sensors, photo sensors provide current signal. In these applications and high speed data converters such as RF A/D D/A converters, oscillators, et al., where the function of comparison is a limiting component for accuracy, noise and power consumption reasons, the introduction of current mode solutions is highly desirable. The current comparison process is injecting one or two current flowing into the comparator and distinguishing the current (or the difference of two current) is positive or negative. The output nodal voltage generated by the output current is used conveniently to indicate the result of the comparison. The comparison process is relatively simple, but the implementation of the current comparator is not piddling. Low input impedance, which is required by current mode circuits, should be considered first. Secondly, a quick time response is demanded by the current comparator. The main limitation to the time response usually comes from the initial balance of the output branches that often leads to the triode region some output transistors. Finally, accuracy is a crucial parameter for comparator, and it depends on the offset caused by the mismatch of transistors. Many good implementations have been reported in the past decades [4-7], many of them emphasis on one or several aspects at the cost of deterioration in other characteristics.
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