[SI-LIST] Re: Ground, the preferred reference plane

From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
To: chris.cheng@xxxxxxxxxxxx, scott@xxxxxxxxxxxxx
Date: Thu, 12 Dec 2002 19:01:02 -0800
Chris,
In some previous platforms, we chose to generate VTT and VDDQ separately =
but with tight tolerances on both of 1-2%.  In most of our reference =
boards and in customer platforms, VTT is always generated from a divider =
off of VDDQ. =20

The intent of this is to ensure DC tracking only, not rejection of =
common mode noise.  There is no need to make the VTT track AC noise in =
any case, our only goal is to make it act like a voltage source across =
the frequency range we need while coupling the minimum amount of noise =
between signals. =20

It might be possible to get some common-mode reduction by generating =
VREF at the driver and piping it to the receiver as a signal (as in =
AGP4x) but the existing DIMM layouts don't support this, and I doubt the =
benefits would be worth the extra pin(s) and the routing hassle.

regards,
Jeremy



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
Sent: Thursday, December 12, 2002 4:29 PM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground, the preferred reference plane



Jeremy,
You know what, I stand corrected. You own company's reference schematics
shows the VDDQ and VTT are generated from two seperated resistor =
dividers
and going to two seperate feedbacks. Hard for me to imagine they even =
track
each other.
Chris


-----Original Message-----
From: Chris Cheng [mailto:chris.cheng@xxxxxxxxxxxx]
Sent: Thursday, December 12, 2002 4:09 PM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground, the preferred reference plane



No there is no confusion. Even though the VTT is generated by the using
VDDQ/2 as reference to the voltage regulator any high speed changes on =
VTT
will not be propagate back to VDDQ and vice versa. They are just DC =
tracking
each other but AC isolated.

-----Original Message-----
From: Jeremy Plunkett [mailto:jeremy@xxxxxxxxxxxxxxx]
Sent: Thursday, December 12, 2002 3:47 PM
To: chris.cheng@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Ground, the preferred reference plane


Hi Chris,
I think you are mixing up Vref and Vtt...Vref is the threshold reference
provided to each DDR DIMM and controller ASIC, the only current drawn on =
it
is leakage from the input comparators.  Therefore Vref is typically
generated by a voltage divider off of VDDQ as close as possible to the =
chip
it is supplying, and the number of resistors required is quite =
reasonable.

Vtt on the other hand is the termination supply for all the DDR signals =
and
must source and sink substantial amounts of current.  This requires a =
DC/DC
converter, but still the VTT should be set up to track the VDDQ voltage =
to a
tight tolerance by using a voltage divider from VDDQ(suitably filtered) =
as
the reference voltage for the regulator.

regards,
Jeremy

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References:
- [SI-LIST] Re: Ground, the preferred reference plane
  - From: Chris Cheng
[SI-LIST] Re: Ground, the preferred reference plane

Other related posts:
