[SI-LIST] Re: DDR3 VTT Bypassing
- From: "Stefan Milnor" <stefan.milnor@xxxxxxxxxxxxxx>
- To: <fbhatti@xxxxxxxxxxxxxxxx>
- Date: Sun, 9 Sep 2012 15:36:39 -0700
I guess that is one way to hedge your bets.
On Sep 9, 2012, at 12:35 PM, "Farooq" <fbhatti@xxxxxxxxxxxxxxxx> wrote:
> Hi OP
> I want to second Herman. I agree GND & VDDQ should treated same
> I ALWAYS bypass VTT to GND & VDDQ
>
> Regards
> Farooq Bhatti
>
> Sent from my iPhone
>
> On Sep 9, 2012, at 10:47 AM, Hermann Ruckerbauer
> <Hermann.Ruckerbauer@xxxxxxxxxxxxx> wrote:
>
>> Hello all,
>>
>> I'm a bit late to jump into the discussion, and I can anyhow only repeat
>> what I mentioned last time:
>>
>> I do not like the idea of a solid ground plane and a weak supply plane.
>> The only thing that matters from a voltage point of view is the
>> difference between them. So I try to treat them equally.
>> A rock solid GND does not help if the VDD is having the noise.
>>
>> Additionally one often does not know the used referencing of all
>> contributors (which seems the case in this discussion). Especially for
>> DRAMs with mid level termination both current return rails matters. So I
>> still stay with the target to have GND and VDD treated with the same
>> care, and if I don't know the whole current return I implement a
>> symmetrical decoupling.
>>
>> Hermann
>>
>>
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>> schrieb Gregory R Edlund:
>>> The signal arrives at the termination resistors (or the DRAM) in the form
>>> of an electromagnetic wave. Do the E and B fields really store any
>>> information about which MOSFET generated them and which power supply it was
>>> connected to? I don't think so. In this argument, I don't think the
>>> driving MOSFETs matter at all so long as the IO circuits are sufficiently
>>> decoupled on chip.
>>>
>>> If it were me, I'd decouple VTT to GND because that's the reference plane,
>>> but I don't have any lab data to support that. In fact, I haven't seen
>>> many papers that show lab data for a real-life return current discontinuity
>>> (I hate the phrase). Maybe it's better just to call it what it is: a
>>> transmission line reference discontinuity.
>>>
>>>
>>> "The reason for bypassing to 1.5V instead of GND is that VTT is both
>>> sinking and sourcing current. When high side mosfet of the buffer is
>>> turned ON, for logic HI, the current flow is from 1.5V to MOSFET to
>>> termination resistor to VTT power supply. Signal is switched referenced
>>> to VTT, so we need to decouple the buffer's 1.5V pin to VTT, not to GND.
>>>
>>> No need for the decap between buffer's GND to VTT, because GND is much
>>> more robust than 1.5V rail (usually a massive plain) and can supply
>>> necessary instantaneous current for logic LOW."
>>>
>>> Greg Edlund
>>> Senior Engineer
>>> Signal Integrity and System Timing
>>> IBM Systems & Technology Group
>>> 3605 Hwy. 52 N Bldg 050-3
>>> Rochester, MN 55901
>>>
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