A Capacitor-Coupled Offset-Canceled Sense Amplifier for DRAMs With Reduced Variation of Decision Threshold Voltage

This article reports an offset-canceled DRAM sense amplifier with coupling capacitors to store and cancel the offset arising from random variations of the threshold voltages of the amplifying transistors. Analytical calculations of the average and standard deviation of the decision threshold voltages, defined as the voltage in the cell capacitor that bifurcates into binary levels when activated, are performed on various DRAM sensing schemes and their comparison results are presented. Based on the analysis, the proposed sense amplifier scheme using coupling capacitors is shown to offer the least amount of variation in the decision threshold, thereby increasing the sensing margin of the overall DRAM design. The coupling capacitors not only compensate for the random offset of the sense amplifiers but also mitigate the effect of the mismatch of the bitline capacitances in the open bitline scheme. Measurement on the experimental chip fabricated in a 65-nm CMOS process validates the analysis and confirms the superior performance of the proposed DRAM sensing scheme.