[SI-LIST] Re: DDR Vref Bypassing

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 best when 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 I don'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         /
     /                                   /
    / Advanced Electronic Concepts, LLC /
   /           www.aec-lab.com         /
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Christopher R. 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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