Impedance requirement for SDRAM signals

[engr_joni_ee]

Hi, I am looking for the impedance requirement in PCB layout of SDRAM MT41K256M16 by Micron. I have looked at the datasheet but I am not sure if I am looking at the right sections or not. On page 62 it says output driver impedance 40 Ohm. I am wondering if it applies to all DQ[0:15], ADR[0:13], BA[0:2], and other signals or maybe not.

 

[FvM]

Micron as number of technical notes regarding SDRAM system design. A typical PCB impedance suggestion is 50 ohm.

 

[engr_joni_ee]

Hi, I have now looked at this document "TN-46-14: Hardware Tips for Point-to-Point System Design"

https://www.micron.com/support/~/media/f9b2fb1d2b8c4efc8078247232e25b56.ashx

On page 3 "Standard characteristic impedance (ZO) of 50–60Ω is recommended for all traces. The 60Ω level also provides a good match to the sum of the output impedance of the controller/FPGA driver and any series resistors used; 60Ω ±6Ω (10 percent tolerance) is Micron’s module and reference design target. Designers are advised to specify ZO, enabling board manufacturers to adjust dielectric thickness and line width to achieve the specification."

Then I guess 50 Ohm impedance on all routing layers should be fine but I don't know what is "output driver impedance" on page 67 of the main datasheet which is 34 Ohm in the following link.

"34 Ohm Output Driver Impedance"

https://media-www.micron.com/-/media/client/global/documents/products/data-sheet/dram/ddr3/4gb_ddr3l.pdf?rev=8d4b345161424b60bbe4886434cbccf4

 

[d123]

Hi,

The output driver appears to have an impedance of 34 to 40 ohms, not sure how calculated here, same as a source or sink, or a battery, have impedance, it has nothing to do with trace impedances, I think. 34 ohms is pretty low-current-capability. Didn't read pdf enough to guess answer to first post question(s).

 

[engr_joni_ee]

Then I think it should be fine to have characteristic trace impedance 50-60 Ohm for all single ended signals on routing layers in PCB layout. This include DQ[0:15], ADR[0:13], BA[0:2], CS, CKE, RAS, CSA, WE, ODT, LDM, UDM.

How about maintaining 100 Ohm characteristic impedance for differential signals CLK and DQS.