[SI-LIST] Re: DDR Vref Bypassing

  • From: "Christopher R. Johnson" <crjohnson11@xxxxxxxxxxx>
  • To: billw@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Tue, 19 Jul 2005 14:03:08 -0400

 Bill,
Would the same argument hold for Vtt bypassing (i.e. are capacitors to VDD
and GND are better)?  The termination supply (assuming you are not using
Thevenin resistors) generates VDD/2, but its frequency response is such that
the bypass capacitors really determine what happens at the scale of
switchingnoise.  Is the goal to hold Vtt at VDD/2 or at (VDD-GND)/2?  Since
you have a differential receiver, couldn't you just bypass BOTH Vtt and Vref
just with respect to GND?  Do you somehow get more noise margin bypassing to
both rails?

As for X2Y capacitors, since both capacitors are on the same monolithic
substrate on an X2Y cap,  the matching is supposed to be very good (~1%-2%),
and thus it should provide an accurate divider.

Regards

Chris


Bill Wurst wrote: Chris, The answer to your question lies in understanding
the function of the Vref line. DDR, as well as DDR2, utilize differential
receivers to process single-ended inputs that have been generated by drivers
which swing in a balanced fashion around the mid-point of the VDD/GND
system.To properly process these single-ended inputs, the inverting input of
each differential receiver is connected to Vref. The receivers will work
bestwhen Vref equals exactly 0.5*(VDD-GND), including any noise that is
present on the VDD/GND system. The purpose of placing an equal amount of
capacitance from Vref to VDD and from Vref to GND is to form an ac divider
that keeps Vref equal to 0.5*(VDD-GND) over all frequencies. The capacitance
should be large enough to swamp out any parasitic capacitance that exists
which could imbalance Vref. I'll have to hedge on the second question which
was "whether an x2y capacitor is better than two discrete capacitors" since
Idon't know enough about x2y devices. Properly configured, an x2y capacitor
could perform better, but the bottom line comes down to the accuracy of the
ac divider. Regards, -Bill /************************************ /
billw@xxxxxxxxxxx[1] / / / / Advanced Electronic Concepts, LLC / /
www.aec-lab.com[2] / ************************************
=================================================================
ChristopherR. Johnson wrote: I have seen references that have Vref bypass
capacitors to both VDD and GND. Other references have capacitors only to
GND.Is it really necessary to have "balanced" capacitors on the Vref lines?
Why? If the "balanced" design is desirable, would an X2Y capacitor be a good
choice, since it is "2 capacitors in one"? Regards, Chris Johnson
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