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 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng Sent: Thursday, December 12, 2002 1:48 PM Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Ground, the preferred reference plane 100% agree but in reality, a lot of PC board doesn't have enough space = to fit so many rpack's. Take a look at most of the reference designs for PC chipsets, most of them call out generating vref with a DC/DC converter = and terminate with a single resistor. -----Original Message----- From: Vinu Arumugham [mailto:vinu@xxxxxxxxx] Sent: Wednesday, December 11, 2002 11:55 PM To: chris.cheng@xxxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Ground, the preferred reference plane Chris Cheng wrote: >Take an example of DDR in most PC design. The vref on receivers is basically >the VTT which usually is generated from an external DC/DC converter = that >generates VDDQ/2 and is completely isolated from the driver VDDQ.=20 > That does not sound like a good thing... The receiver Vref should be driver Vddq/2. We usually generate it with a = resistor divider from driver Vddq. This way, Vref will track Vddq better... Thanks, Vinu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages=20 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu