[SI-LIST] Re: DDR Vref Bypassing
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 19 Jul 2005 21:01:39 -0400
Steve,
I'll try to answer your comments/questions as I understand them (see
response below).
Regards,
-Bill
======================================
steve weir wrote:
> Bill, I really question where this assumption of average between Vdd and
> Gnd comes from. The noise level on Vdd at the transmitter is not going to
> be the same as the instantaneous noise at the receiver, so I don't buy an
> argument based on the launch. That leaves us with the transmission
> channel. What percentage of what couples onto the transmitted signal
> depends on how the board is constructed.
If the noise on Vdd and Vss at the transmitter propagate to the receiver
with the same velocity as the signal, then the noise at both the
transmitter and receiver will be the same but delayed by the flight
time. Granted, the construction of the board can and will impact this
assumption. The premise is to make the noise common to both sides of
the differential receiver, so that the receiver will reject the noise by
virtue of its CMRR. At launch, a logic '1' will replicate Vdd noise
which will be attenuated by terminations at both ends and also
influenced by any noise on the reference planes as it travels to the
receiver. Similarly, a logic '0' will replicate Vss noise. Trying to
make Vref equal to 50% of the difference between the two rails is
admittedly a compromise and, as you point out, other factors will reduce
the cancellation further. Yet I fail to see a better alternative.
>
> A split filter is in essence a 2Y common mode filter turned inside
> out. Impedance mismatch gives rise to mode conversion.which throws off
> that 50% divider assumption for equal value ( data sheet )
> capacitors. This is why we see 2Y RFI filters with a much bigger X
> capacitor shunting the two lines together- to swamp out the mode
> conversion. In the Vref application, the X capacitor is represented by the
> bypass network from Vcc to Vss. That is really ugly, because it basically
> says that we need to bypass the heck out of Vdd to get around mode
> conversion in the Vref bypass caps.
I agree. This is essentially a 2Y CM filter, and Vdd must be bypassed
to the greatest extent practical.
>
> The two capacitors in an X2Y match so well that even for analog
> instrumentation they do not need an X capacitor. I have an application
> note on this in ADI's instrumentation amplifier designer's guide, based on
> real circuit measurements. An X2Y configured as: Terminal A => Vdd,
> Terminal B => Vss, Terminals G1, and G2 => Vref matches to better than
> 1%. So, if one is bent on implementing the divider, X2Y capacitors do the
> job in a way that is basically impossible using separate capacitors to each
> rail.
Again, I agree. I hedged in my response because I was unsure of the
matching that could be achieved with x2y capacitors.
>
> If someone is really bent on this divider approach, then X2Y is definitely
> the way to go. But given that people have been building with it, and
> apparently it has "worked" despite the mode conversion with regular caps, I
> really question the validity of the approach in the first place. Do you
> know what the physical basis for the rationale of the divider is supposed
> to be?
Hopefully, unless I've missed something, I've answered this in my
response to the first paragraph. Please let me know if I haven't.
>
> Regards,
>
>
> Steve.
>
> At 12:48 PM 7/19/2005 -0400, 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 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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