[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 11:21:30 -0600
Thanks!
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 11:18 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: DDR Vref Bypassing
Charles,
The resistive divider alone would work fine if there were no parasitic
capacitance on Vref. But consider that Vref is distributed to about 100
receivers, and if each receiver has perhaps 10pF of parasitic
capacitance to ground, the bandwidth of the divider will be limited by
the RC time constant. The concept here is very similar to the
compensated attenuator in a scope probe, where R1C1 = R2C2 for a flat
frequency response.
-Bill
/************************************
/ billw@xxxxxxxxxxx /
/ /
/ Advanced Electronic Concepts, LLC /
/ www.aec-lab.com /
************************************
=======================================
Grasso, Charles wrote:
> 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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