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About the termination resistor Zo of DDR3 VTT

Garth
Prodigy 130 points

The datasheet ask Zo value range is from 50 ohm to 75 ohm,but on the C8168 EVM，VTT termination resistor is 49.9 ohm，so my question is :whether the accuracy of the Zo value  is important.If I use 51+-5% termination resistor is ok?

  • 0
    TI__Intellectual 1610 points

    The spec requires Zo to be controlled to within 5 ohms.  Assuming a Zo of 50 ohms, a 51 +/- 5% ohm resistor should be fine as it will be 48.45 ohms to 53.55 ohms and the allowable range of impedance is 45 to 55 ohms.  The EVM is a Zo = 50 ohm board, thus the reason why the VTT termination resistors are 49.9 ohms.

    You want to keep the VTT resistor value to within the board impedance spec.  Ideally its nominal value equals Zo.   In the end, it's nominal value + tolerance must be within the board impedance spec of nominal Zo between 50 and 75 ohms and controlled to within 5 ohms.

    -Mike