[SI-LIST] Re: Arguments against Thevenin bias/termination for ddr2 Vtt
- From: agathon <hreidmarkailen@xxxxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Tue, 29 May 2007 13:39:07 -0700
Oh great. All I get is a Russian working for Cisco (for peanuts, most
likely, in some hinterland outpost) but not too busy to answer me ... in
RUSSIAN???? This is getting embarassing.... :-)
PS: Uh, in case yer wondering, I'm just kidding.
On 5/29/07, Michael Grobman (mgrobman) <mgrobman@xxxxxxxxx> wrote:
>
> éÒÁ, áÎÄÒÅÊ,
>
> ëÓÔÁÔÉ,÷Ù ÐÏÄÌÀÞÅÎÙ Ë ÔÅÌÅÆÏÎÕ Skype × ÉÎÔÅÒÎÅÔÅ.
> íÏÖÎÏ ÏÂÝÁÔÓÑ ÂÅÓÐÌÁÔÎÏ ÞÅÒÅÚ ËÏÍÐ.
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of agathon
> Sent: Tuesday, May 29, 2007 9:10 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Arguments against Thevenin bias/termination for ddr2
> Vtt
>
> 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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