[SI-LIST] Re: Arguments against Thevenin bias/termination for ddr2 Vtt
- From: steve weir <weirsi@xxxxxxxxxx>
- To: hreidmarkailen@xxxxxxxxx
- Date: Wed, 30 May 2007 13:28:23 -0700
agathon it would be a lot more palatable to help you if you would stop
behaving like a jerk. Bill's positions are well founded in science.
Your original postulates and those you offer today are filled with
assumptions a number of which are very dubious.
Steve.
agathon wrote:
> My apologies to Bill for doubting him and being blinded by science... :-)
> I stand justly rebuked.
> Many thanks to all......
>
> Steve, sorry to query for detail... maybe there's a quick ref. you can point
> to?
>
> in #2 answer do you mean extend a very wide finger or shape instead of using
> the solid plane?
> I realize this wouldn't necessarily be a hard rule.
> #3: Thev. term. reduces cavity Q? Can you suggest a Spice setup to show
> this? I don't see how there's any added R series to the cavity. Or, it's
> just due to the added Thev. vias, or the loop with 100ohms - instead of Vtt
> regulator with the lower Z whole signal path?
>
> #4: "In the Thevenin case you get much less signal crosstalk for equivalent
> bypass impedance as with the end termination into Vtt."
> Due to less delta-i from Vddq I suppose, due to divider? We aren't using
> series R. I assume you're implying that each termination requires its own
> decap for best results (?).
>
> #5: -----
>
> #6: But here you mention "shared bypass" as a potential helper. I see, just
> happens to reduce delta-i through nearest cap (?). Ie: data dependent.
>
>
> Overall, I seem to get from the replies: Use separate large shape from vdd
> for vddq to Thevenin terminations. The shape branches off from a point
> close to vdd at ctlr, since it is tx for Addr/Cmd signals. Ie: As jedec
> specifies, Vtt & Vref must track Vddq of tx. Larger decap at branch.
> Added decap per termination, right at divider.
>
>
>
> On 5/29/07, steve weir <weirsi@xxxxxxxxxx> wrote:
>
>> Agathon I mostly agree with Bill here. Taken point by point:
>>
>> 2. The PDN (Vddq) is used dc-coupled, so its behavior statistically
>> influences Vtt. That is, Vtt is more subject to Vddq and other noise.
>>
>> Not necessarily so. It depends on how you do the board layout and bypass
>> for Vddq in the Vtt area. In the ideal case Vddq from the transmitter
>> forwards to the Vtt divider island. In that case Vtt tracking against the
>> actual switching signals which is what we want can be better than a fixed
>> Vtt, ie lower jitter.
>>
>> 3. Vtt is then subject to board PDN resonances.
>>
>> Only if you have one giant cavity AND that cavity has resonance
>> issues. As Bill infers if you have a significant resonance issue, that's a
>> problem you probably need to fix anyway. The Thevenin case has a tendency
>> to reduce cavity Q.
>>
>> 4. The Thevenin method makes Vin (rcvr) more sensitive to Vddq noise
>> merely
>> from the linear network analysis point of view, or to Vtt offset at worst
>> case pullup/down values, than when Vtt is regulated and terminated with 50
>> ohms (nom.). ... I'm verifying this now; may not be true. I assume 1%
>> resistors.
>>
>> At anything like the bit rate, whether you use a linear supply or not the
>> bypass scheme controls the noise. In the Thevenin case you get much less
>> signal crosstalk for equivalent bypass impedance as with the end termination
>> into Vtt. As mentioned before, whether or not other noise on the board
>> impacts the local Vddq is a design issue.
>>
>> 6. Vtt current switching noise is injected into the PDN. Not very nice if
>> layout or margins are poor.
>>
>> A bad design is still a bad design. Under the right circumstances a bank
>> of Thevenin terms could send one over margin, but it could for reasons
>> stated above just as likely pull a design that is out of margin back in due
>> to the increase in shared bypass and improved damping. Either way the
>> design is done properly, or life is bad. If the design is better off
>> isolating Vddq in the region of the terminations, that is an easy task.
>>
>> Steve
>>
>>
>> agathon wrote:
>>
>>> Bill,
>>> au contraire, mon frere....
>>>
>>> #2-4, 6 are also specific to the Thevenin bias/termination, as opposed
>>>
>> to
>>
>>> Vtt separate regulator.
>>>
>>> Come on, you frikkin experts. :-) This should be easy. I double
>>>
>> dare
>>
>>> ya.
>>>
>>>
>>>
>>> On 5/29/07, Bill Owsley <wdowsley@xxxxxxxxx> wrote:
>>>
>>>
>>>> Only one and five are accurate. The rest are symptoms of other
>>>>
>> problems
>>
>>>> that should have been addressed.
>>>>
>>>>
>>>> *agathon <hreidmarkailen@xxxxxxxxx>* wrote:
>>>>
>>>> Hello,
>>>> Assume a 50 ohm environment so, for example, a 100 ohm pullup to Vddq (
>>>> 1.8V)
>>>> and pulldown to gnd, for Address/Command signal group.
>>>> The typical method is to use a lin. regulator with outputs for Vref and
>>>> Vtt,
>>>> so they supposedly track each other better.
>>>>
>>>> The only arguments against the pullup/down I can come up with are:
>>>>
>>>> 1. Extra dc current (but the regulator has Vout-Vin losses, too). For
>>>> Addr./Cmd it's around 200mA. for a single port, and doesn't increase w/
>>>> memory size.
>>>> 2. The PDN (Vddq) is used dc-coupled, so its behavior statistically
>>>> influences Vtt. That is, Vtt is more subject to Vddq and other noise.
>>>> 3. Vtt is then subject to board PDN resonances.
>>>> 4. The Thevenin method makes Vin (rcvr) more sensitive to Vddq noise
>>>> merely
>>>> from the linear network analysis point of view, or to Vtt offset at
>>>>
>> worst
>>
>>>> case pullup/down values, than when Vtt is regulated and terminated with
>>>>
>> 50
>>
>>>> ohms (nom.). ... I'm verifying this now; may not be true. I assume 1%
>>>> resistors.
>>>> 5. Uses more pcb space and routing area.
>>>> 6. Vtt current switching noise is injected into the PDN. Not very nice
>>>>
>> if
>>
>>>> layout or margins are poor.
>>>>
>>>>
>>>>
>>>> Arguments in favor:
>>>> 1. Using Vddq actually may force Vtt to track it better. Regulators
>>>> providing Vtt and Vref (with Vddq sense) cannot track Vddq as quickly
>>>>
>> or
>>
>>>> accurately.
>>>> 2. The dc current penalty is small.
>>>> 3. ???
>>>>
>>>>
>>>>
>>>> Thanks very much.
>>>>
>>>> -----------
>>>>
>>>>
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