On DDR memories, the necessary resistive termination is located on the motherboard, while on DDR2 and DDR3 memories this termination is located inside the memory chips – a technique called ODT (On-Die Termination).
This is done in order to make the signals “cleaner.” In Figure 5, you can see the signal that reaches the memory chip. On the left hand side you see the signals on a system that uses motherboard termination (DDR memories), while on the right hand side you see the signals on a system that uses on-die termination (DDR2 and DDR3 memories). Even a layman can easily see that the signals on the right-hand side are cleaner and are more stable than the signals on the left-hand side. On the yellow square you can compare the time frame difference; this time frame is the time the memory has to read or write a piece of data. With the use of on-die termination, this time frame got wider, allowing higher clocks to be achieved since the memory has more time to read or write a data chunk.