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DDR3 impedance matching

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dharithothi

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Hello,

As we are following 50Ohm impedance in single ended signals and 100Ohm impedance differential signals in routing.

My question is in routing differential like Clock+ and Clock- we follow 100Ohm differential impedance. Is it necessary to follow 50Ohm for individual signals? Means we need to follow 50Ohm for Clock+ and 50Ohm for Clock-???

55Ohm for Clock+ and 45Ohm for clock- can work???


Regards
Dharit
 

Although you can use two uncoupled single ended transmission lines for differential signals, that's neither the usual nor a practical way.

Consider that the effective differential impedance is formed by the parallel circuit of differential (wire-to-wire) and single ended (wire-to-ground) components. In PCB design, space saving demands to minimize trace separation, the single ended component will be respectively higher than 50 ohms, e.g. 70 ohms. Check by comparing typical single ended and differential trace calculation results of a PCB tool like Saturn toolkit.

Apart from the discussed point, 45 + 55 ohms S.E. involves an unwanted imbalance, but is probably roughly within design margins.
 

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