Quiz Answer: PCB Zo impedance for DDR memory routing


Quiz answer is: Characteristic impedance for routing DDR3 memories should be 50 ohm for single ended traces and 100 ohm for differential traces.

Where these values come from? It’s will be subject to a bit longer post which I will do this week 🙂

I’ll try to answer these questions: What is characteristic impedance? Were these numbers 50  / 100 ohms come from? How we decide how to stack-up PCB layers before start design with DDR memory?

2 Comments (+add yours?)

  1. LIME-fan
    Nov 17, 2015 @ 01:40:03

    Seems like i read the wrong datasheets (micron) 🙂
    I read a lot about how the driver influences the needed impedance and yes, i also read about the differential impedance but i didnt make the connection to your question as it was obiously always the doubled impedance and seemed too easy 😉

    So i guess now, the H3-memory-driver is a 50Ohms type – which i could had found in the H3-Datasheets?

    I am really looking forward to your PCB-layer blog entry!


  2. miLinux
    Mar 31, 2016 @ 12:27:18

    Hello, you promised new post about impedance and layer stack..:)
    Can we get some more details about impedance?
    I am trying to make board with A20 and K4B4G1646D ddr3 memory.
    Can we get some hints with stack-up and characteristic impedance?
    Does 50 and 100 ohm matching my memory and does it depend only from memory type or also from cpu?
    For now i have 5 layers:
    1Layer – TOP copper signal layer
    – prepreg
    2Layer – internal copper GND plane
    – Core
    3Layer – MIDDLE copper signal layer
    – Core
    4Layer – internal copper GND plane
    – prepreg
    5Layer – BOTTOM copper signal layer

    Can i use internal copper layer for some signal routing on the other side of PCB where i do not have high speed signals?

    Length matching for DDR:
    I suppose that signals can be split in few banks.
    For example:
    Bank 0: address lines -> length 15 mm +- 10 mils
    Bank 1: data lines -> length 25 mm +- 10 mils
    Bank 3: clock and other diff lines -> length 18 mm +- 10 mils
    Is this ok? How to split lines to bans in my case? Can clock lines be longer then address or data lines and vice versa?


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