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I am referring to Micron MT46V16M16CY-5B IT:M DDR SDRAM. In its datasheet, I see two different set of pins, VDD-VSS and VDDQ-VSSQ.

What is the difference between them?

The datasheet suggests that the VDDQ and VSSQ have been isolated on the die for improved noise immunity. Does that the VDDQ-VSSQ pair doesn't need a decoupling capacitor between them?

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Vdd/Vss are generally for the internal logic supply, while VddQ/VssQ are specifically for the data bus I/O pin drivers. The latter are kept separate on the die in order to prevent the large transient currents from affecting internal operations.

Both sets of pins should have their own bypass capacitors.

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  • \$\begingroup\$ and the bypass cap values can be maintained same? \$\endgroup\$ – Harshad D Jul 22 '16 at 23:04
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    \$\begingroup\$ Probably not. You will probably want beefier caps on the VddQ pins in order to better supply their higher transient current needs. But if you use that same size on the Vdd pins, it doesn't hurt anything. \$\endgroup\$ – Dave Tweed Jul 22 '16 at 23:15
  • \$\begingroup\$ So considering a SDRAM application, would you recommend 0.1uF for all pin pairs? \$\endgroup\$ – Harshad D Jul 22 '16 at 23:17
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    \$\begingroup\$ I would first thoroughly read the manufacturer's application notes to see what they recommend for bypass and layout. Failing that, I would do an analysis of the required power supply impedance at the operating frequency and pick the capacitor value and ESR accordingly. I'd also be watching out for any self-resonant effects. \$\endgroup\$ – Dave Tweed Jul 22 '16 at 23:22

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