15 ohm resistor in DDR3 DIMMs

Hi,Could anyone please specify the reason why DDR3 DIMMs use a 15 ohm resistor right after the connector finger?The resistor value used to be 22 ohm for DDR2 but has been changed to 15 ohms for DDR3. It seems to be too far from either memory or controller to serve as a series termination/damping resistor.What purpose does this resistor serve ( for DDR2 or DDR3) since it is right in middle of the trace between the controller and memory which are 40/50 ohms impedance?Regards,Sumant
Sumant.Srikant 7 years 3 months 5 days

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Answered byvinu 7 years 2 months 4 days
One can also view it as a way of somewhat reducing the effect the stub will have on the signal traversing the main line.This concept is the basis for Stub Series Terminated Logic (SSTL).Thanks,VinuOn 09/02/2013 03:48 AM, Hermann Ruckerbauer wrote:> Hello,>> memory design with mutiple stubs and bidirectional signaling will never> be a perfect matched system. This is always a trade of between voltage> and timing margin.>> The resistor is used to give a bit of damping of reflections between> different slots.>   > So if you have a point to point solder down connection it is not> required to be implemented.> If your rely on the engineering that was done during development of the> interface spec and your are re-build the DIMM case you can leave it in.> If you want to avoid it (e. g. for cost and space daving) you should> take care that the changes you make  are still fulfilling the signal> quality requirements for DRAM and Controller.>> Hermann>> EKH - EyeKnowHow> Hermann Ruckerbauer> www.EyeKnowHow.de> Hermann.Ruckerbauer@xxxxxxxxxxxxx> Itzlinger Strasse 21a> 94469 Deggendorf> Tel.: +49 (0)991 / 29 69 29 05> Mobile:       +49 (0)176  / 787 787 77> Fax:  +49 (0)3212 / 121 9008>> schrieb Srikant, Sumant:>> Hi,>> Could anyone please specify the reason why DDR3 DIMMs use a 15 ohm resistor >> right after the connector finger?>> The resistor value used to be 22 ohm for DDR2 but has been changed to 15 >> ohms for DDR3. It seems to be too far from either memory or controller to >> serve as a series termination/damping resistor.>>>> What purpose does this resistor serve ( for DDR2 or DDR3) since it is right >> in middle of the trace between the controller and memory which are 40/50 >> ohms impedance?>>>> Regards,>> Sumant>>>>>> 
Answered byHermann.Ruckerbauer 7 years 3 months 5 days
Hello,memory design with mutiple stubs and bidirectional signaling will neverbe a perfect matched system. This is always a trade of between voltageand timing margin.The resistor is used to give a bit of damping of reflections betweendifferent slots.So if you have a point to point solder down connection it is notrequired to be implemented.If your rely on the engineering that was done during development of theinterface spec and your are re-build the DIMM case you can leave it in.If you want to avoid it (e. g. for cost and space daving) you shouldtake care that the changes you make  are still fulfilling the signalquality requirements for DRAM and Controller.HermannEKH - EyeKnowHowHermann Ruckerbauerwww.EyeKnowHow.deHermann.Ruckerbauer@xxxxxxxxxxxxxItzlinger Strasse 21a94469 DeggendorfTel.:   +49 (0)991 / 29 69 29 05Mobile: +49 (0)176  / 787 787 77Fax:    +49 (0)3212 / 121 9008schrieb Srikant, Sumant:> Hi,> Could anyone please specify the reason why DDR3 DIMMs use a 15 ohm resistor > right after the connector finger?> The resistor value used to be 22 ohm for DDR2 but has been changed to 15 ohms > for DDR3. It seems to be too far from either memory or controller to serve as > a series termination/damping resistor.>> What purpose does this resistor serve ( for DDR2 or DDR3) since it is right > in middle of the trace between the controller and memory which are 40/50 ohms > impedance?>> Regards,> Sumant>>> 
Answered byweirsi 7 years 3 months 5 days
The resistor provides a limited amount of damping.Steve.On 9/2/2013 2:39 AM, Srikant, Sumant wrote:> Hi,> Could anyone please specify the reason why DDR3 DIMMs use a 15 ohm resistor > right after the connector finger?> The resistor value used to be 22 ohm for DDR2 but has been changed to 15 ohms > for DDR3. It seems to be too far from either memory or controller to serve as > a series termination/damping resistor.>> What purpose does this resistor serve ( for DDR2 or DDR3) since it is right > in middle of the trace between the controller and memory which are 40/50 ohms > impedance?>> Regards,> Sumant>>>