[SI-LIST] Re: DDR Vref Bypassing
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 19 Jul 2005 16:07:52 -0400
Chris,
Yes, but keep in mind that with Vtt you've got some serious di/dt to
worry about. It's for this reason that I find the split termination
more practical. The current DDR2 design I'm working on uses buried-R to
terminate the address/control lines at the DIMM sticks (the split
termination for the data lines is built into the memory chips). When
you consider the power dissipated by the Vtt regulator, there really is
no power penalty.
With the x2y matching that good, it sounds like that is the best way to go.
-Bill
/************************************
/ billw@xxxxxxxxxxx /
/ /
/ Advanced Electronic Concepts, LLC /
/ www.aec-lab.com /
************************************
===============================================
Christopher R. Johnson wrote:
> 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
> ------------------------------------------------------------------
> ------------------------------------------------------------------ To
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