On the appearance, both DDR3 and DDR2 use 240 pins and you can hardly see any differences between them. The only difference is the position of the key notch that prevents users from inserting the wrong type of RAM into their motherboards. The shift in the position of the key notch also means that DDR3 is not compatible with DDR2 and a separate DIMM slot has to be used.



The difference between DDR2 and DDR3


Other than the difference in key notch position, DDR3 RAM is also due to run at a higher frequency than DDR2 RAM. The specification provided by JEDEC provides more insight to its performance.

Official JEDEC Specifications
DDR 2 DDR3
Transfer Speed 400 - 800 Mbps 800 - 1600 Mbps
Voltage 1.8V +/- 0.1V 1.5V +/- 0.075V
Internal Banks 4 8
Prefetch 4bits 8 bits
Termination Limited All DQ signals
Topology Conventional T Fly-by
CAS Latency 3 - 5 6 - 10
Thermal Sensor No Yes (optional)
When JEDEC set the standard for DDR2 RAM, it never intends to make it run beyond 800MHz. The official specification states that DDR speeds ran to DDR-400, DDR2 from 400 to 800, and DDR3 800 to 1600. Due to the fact that DDR2 800 and 1066 (an overclock version of DDR2 800) exist, DDR3 800 and 1066 will become an overlap speed with that of DDR2. As such, it will only be 1333 that is the first DDR3 speed to have better frequency/speed performance over the DDR2 RAM.

While the difference between DDR and DDR2 RAM lies in the doubling of the frequency bus, the DDR3 works by doubling the prefetch data. This is achieved by doubling the internal bank to 8. Each internal bank can process 1 bit of data. DDR2 SDRAM have only 4 internal banks and process 4-bit samples, DDR3 SDRAM uses 8 internal banks to carry out 8-bit prefetch. The impact of the additional internal banks means that the DDR3 SDRAM could double the data transfer rate without having to make modifications to the original design.





Another advantage of DDR3 is the reduced voltage requirement. The required voltage for DDR3 is 1.5V, a 16% reduction of the DDR2 1.8V. This could translate to a 30% reduction in power consumption compared with DDR2 RAM operating at the same speed. Additional built-in power conservation features with DDR3 include partial refresh and thermal sensor that could allow the system to save further power by providing minimum refresh cycles when the system is not in high performance mode.

A point that requires attention and that brings DDR2 RAM back to the game is the price of the DDR3 RAM. A 1GB DDR3 1066MHz RAM is currently selling at over $200, which is double the price of a 1GB DDR2 1066MHz RAM. While there is no significant improvement of the DDR3-1066 RAM over DDR2-1066 RAM, there really isn’t any compelling reason to upgrade your current DDR2 RAM.

Conclusion

In the near future, it is inevitable that DDR3 is going to replace DDR2 as the standard for memory RAM. At this current moment, the hefty price hardly make it a ‘must-upgrade’ component, considering that you can get a DDR2 RAM operating at the same speed for under $150. Until the time when higher speed DDR3 RAM are more readily available and the price fall to a reasonable range that is comparable to DDR2, it is better to remain with your DDR2 RAM.


Difference Between DDR and DDR2 Random Access Memory (RAM)
DDR DDR2
Data Bus 64 bits 64bits
Data Rate 200/266/333/400 Mbps 400/533/667 Mbps
Bus Frequency 100/133/166/200 MHz 200/266/333 MHz
DRAM Frequency 100/133/166/200 MHz 100/133/166 MHz
Package Type TSOP-II FBGA
Densities 256MB 512MB 1GB 256MB 512MB 1GB
Voltage 2.5V 1.8V
Prefetch Size 2 bits 4 bits
Burst Length 2/4/8 4/8
CAS Latency 1.5, 2, 2.5 3+, 4, 5
Write Latency 1T Read Latency -1