[SI-LIST] Re: DDR Vref Bypassing
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: "'billw@xxxxxxxxxxx'" <billw@xxxxxxxxxxx>, "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Jul 2005 10:57:52 -0600
Hi Bill,
Why should the caps work any better than the resistor
divider that typically is used to derive Vref from Vdd?
Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Pager/Short Message: 3032042974@xxxxxxxx
Email: charles.grasso@xxxxxxxxxxxx;
Email Alternate: chasgrasso@xxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Bill Wurst
Sent: Tuesday, July 19, 2005 10:48 AM
To: si-list@xxxxxxxxxxxxx
Subject: [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 /
************************************
=================================================================
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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