[SI-LIST] Re: Vref_DQ Vref_CA VTT decoupling in DDR3 DIMM
- From: "Zhenwei Wang (zhenwwan)" <zhenwwan@xxxxxxxxx>
- To: <hermann.ruckerbauer@xxxxxxxxxxxxx>, "Vinu Arumugham (vinu)" <vinu@xxxxxxxxx>
- Date: Tue, 12 Jun 2012 09:58:21 +0800
My opinion: there should be a very low impedence between the power plane and
gnd plane in a good design. Two plane adjacent in core board, adequate
decoupling capacitors(low esr and low body inductance), integrite return
path... will be the same thing. So the noise on power plane will return back to
gnd plane. So power plane is the same as gnd plane in AC.
BR
Zhenwei(Jason) Wang
Cisco Systems (Shanghai) Video Technology Co., Ltd.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Hermann Ruckerbauer
Sent: 2012年6月9日 2:56
To: Vinu Arumugham (vinu)
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Vref_DQ Vref_CA VTT decoupling in DDR3 DIMM
Hello Vinu,
not exactly, only as long as you think about VSS as fixed potential. As I
mentioned I think both are equal designed ... so you can get noise on both VDD
or VSS. And both will be transferred equally to VTT ...
But as I mentioned .. I think this is more a question of design philosophy ...
Hermann
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schrieb Vinu Arumugham:
> Hermann,
>
>
> "Nevertheless I prefer to do a decoupling to both VDD and VSS."
>
> If you decouple VTT to both VDD and VSS, you create a AC voltage divider
> between VDD/VSS and connect it to VTT. So any noise on VDD w.r.t VSS will be
> divided in half and be injected on to VTT. Decoupling to VSS or VDD alone
> would avoid this noise injection. Do you agree?
>
> Thanks,
> Vinu
>
>
>
> On 06/08/2012 10:14 AM, Hermann Ruckerbauer wrote:
>> Hello,
>>
>> yes, Vinu's explanation is very good!
>> Just one comment: VTT is just between VDD and GND, so this would be
>> no reason to decouple to VDD. The real issue is the referencing on
>> the DIMM (which comes from the limited number of supply pins on the
>> DIMM connector)
>>
>> Nevertheless I prefer to do a decoupling to both VDD and VSS.
>> This is also some kind of philosophy. Some people think that VSS is a
>> stable potential, while VDD is something unstable .. I think both
>> should be handled with the same care. But this means, that both might
>> have noise (seen in reference to some imaginary solid and fixed potential).
>> Based on this "philosophy" i prefer to have e. g. two same size
>> capacitors to VDD and GND compared to two different sized capacitors
>> to GND (or VDD for CA).
>>
>> But thats just my way of looking into any "stable" potential ...
>>
>> Hermann
>>
>> Our next Events:
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>> "Open the Black Box of Memory"
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>> www.EyeKnowHow.de
>> Hermann.Ruckerbauer@xxxxxxxxxxxxx
>> Veilchenstrasse 1
>> 94554 Moos
>> Tel.: +49 (0)9938 / 902 083
>> Mobile: +49 (0)176 / 787 787 77
>> Fax: +49 (0)3212 / 121 9008
>>
>>
>> schrieb Vinu Arumugham:
>>> My understanding is that on the DIMM, DQ routing is referenced to a
>>> VSS plane and CA routing is referenced to a VDD plane. Therefore
>>> VrefCA is bypassed to VDD and VrefDQ is bypassed to VSS. Since VTT
>>> is used to terminate CA, it is also bypassed to VDD.
>>>
>>> Thanks,
>>> Vinu
>>>
>>> On 06/07/2012 11:29 PM, Tesla wrote:
>>>> Hi, experts
>>>> I read the standard "DDR3 SDRAM Unbuffered DIMM Design Specification"
>>>> On the page 32, it states three kinds of decoupling for
>>>> VTT,Vref_DQ,Vref_CApower rail:
>>>> 1 VTT, Minimum of one decoupling capacitor to VDD per every two
>>>> termination resistors
>>>> 2 Vref_CA,Minimum of one decoupling capacitor to VDD per DRAM
>>>> 3 Vref_DQ,Minimum of one decoupling capacitor to VSS per DRAM.
>>>>
>>>> Confusion:
>>>> Why VTT decoupling capacitor to VDD not VSS(usually GND)? will this
>>>> increase noise coupling between VTT and VDD?
>>>> Vref_CA and Vref_DQ has same function as the reference voltage for SSTL-15.
>>>> Vref_CA decoupling capacitor connected to VDD, but the Vref_DQ connected
>>>> to VSS.
>>>>
>>>> I think decoupling capacitors of all above power rails should be connected
>>>> to VSS and corresponding power rail net, this will be better decoupled.
>>>>
>>>> Thanks.
>>>>
>>>> Tesla
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