[SI-LIST] Re: AW: Re: Reference layers for high speed diff pairs
- From: Michael Vrbanac <vrbanacm@xxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx
- Date: Wed, 16 Jan 2013 11:48:17 -0600
I agree, Scott.
Michael
On 01/16/2013 10:52 AM, Scott McMorrow wrote:
> On Wed, Jan 16, 2013 at 11:28 AM, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
>> In my experience in server designs, there is virtually zero impedance
>> between VDDQ and GND planes, even on a bare board. Shooting a TDR between
>> them, you see a dead short (unless you wait a very long time in which case
>> you'll see the classic charging of a capacitor). Thus, it doesn't matter
>> which plane you choose to reference your signal to -
>>
>>
> Jeff, this only holds true below the power system parallel resonant
> frequency (PRF). For single ended low speed signals, such as DDR, you may
> be correct (as long as the signal bandwidth is below PRF). However, it is
> pretty much impossible to design a power delivery system with a PRF high
> enough to support common SERDES switching rates. It is in these cases with
> a high Nyquist frequency that dual-referenced or power plane only
> referenced signals should be avoided.
>
> As a pretty decent rule of thumb, I would say that dual-referenced signals
> might be justified to a bandwidth of approximately 1 octave below power
> system PRF (PRF/2), but not without additional noise, since the power
> impedance is on a rising inductive slope at this point. Above that point, I
> would not dare run diff pairs with anything but ground.
>
>
> regards,
>
> Scott
>
>
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