AM3352 DDR3 - Nominal Impedance

Byron Loader

Hi Guys,

I'm have routed a DDR3 interface with 2 DDR3L (x8) chips. I have selected a single single ended impedance of 60R and a differential impedance of 120R since the On Device Termination of the ram chip supports 60R and not 50R

My PCB manufacturer has come back and stated that he can only manufacture to a +-10% nominal tollerance which is outside the Zo+5 Zo-5 tollerance.

Would I get a result if I changed the nominal impedance to 50 ohms or will the design tollerate a Z0 +-6R. Please note my design includes a VTT termination regulator.

over 13 years ago

0 Biser Gatchev-XID over 13 years ago

TI__Guru**** 393215 points

Hi Byron,

The Zo+5 Zo-5 tollerance is in Ohms, it's +/-5Ohms not +/-5%. Zo=50Ohms is normally used in DDR3 designs and 100Ohms for the differential lines.

0 Byron Loader over 13 years ago in reply to Biser Gatchev-XID

Prodigy 140 points

Hi Biser,

That being said, the Zo is specified from 50R - 75R and is controlled by Rext. Since DDR3 ODT no longer supports 50R as an Impedance is 60R not a better impedance for the DDR3 bus?

Kind regards

0 Biser Gatchev-XID over 13 years ago in reply to Byron Loader

TI__Guru**** 393215 points

There are a lot of application notes for DDR3 layout. From what I've read, lower track impedances give better margins. 50Ohm is widely used.

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AM3352 DDR3 - Nominal Impedance