The Importance of Impedance Calibration and the Modeling Practice for Simulation in DDR Memory

As more and more industrial or mobile communication and portable electronic systems use Double Data Rate (DDR) memory for faster data transfer rate and overall higher system performance, the essential need of cost reduction can be obtained by using conventional passive bus termination resistors. Termination resistors are used to match the DDR termination line impedance to the source impedance in order to reduce the amplitude of the reflection to zero. Resistor mismatching having a resistor larger or smaller than the targeted impedance can results in data accuracy error during data write, read or high-low transition. In this regard, the V
_TT
tolerance voltage should not exceed ± 3% of (VDDQ/2), where VDDQ is the memory supply rail.