series termination in DDR's signal is for EMI issues?

[mayank.bhaintwal]

is series termination in DDR's signal is for EMI issues? y not we can terminate them directly?

 

[FvM]

Series termination is an optional means to improve the signal quality, parallel termination is basically required. Both are not particularly EMI related. You can refer to the applications notes of RAM vendors and processor reference designs.

 

[nelsonys]

Transmission line that is not properly terminated will experience reflection, which will attenuate and/or distort the transmitted signal to the receiver side, leading to read error. Controlled impedance of trace and terminating resistors are needed for this case.

Correct me if I'm wrong.

 

[ea.arun]

keeping a series resistor alone is not sufficient in terms of termination/for reducing the reflection.....a parallel termination also required to achieve trhe optimum performance........
but a series resistor can damp the ringing and reduce the EMI effectively.....but there is a price to pay with noise margin.....it basically slow down the rise time.....

 

[FvM]

Transmission line that is not properly terminated will experience reflection, which will attenuate and/or distort the transmitted signal to the receiver side, leading to read error. Controlled impedance of trace and terminating resistors are needed for this case.

You'll face some difficulties to explain a DDR RAM design in terms impedance matched transmission lines, although it's based on the principle, of course. You'll find e.g. an application note suggestion (from Micron TN4607:

Location: The series resistors should be located near the first DIMM for ease of routing.

The statement may be read as other words for "they should be place elsewhere, but it's not practical". But even a misplaced series resistor can help to reduce reflections in a mismatched transmission line. DDR RAM design is mostly a compromise.