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Old 10-31-2007, 11:28 PM
Jim Granville
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Default Re: Ping Jim: The PFD is dead!

John Larkin wrote:

> On Wed, 31 Oct 2007 11:22:20 -0700, Peter Alfke <[email protected]>
> wrote:
>
>
>>Hi, John
>>I suppose you know about the old Xilinx app note:
>>http://direct.xilinx.com/bvdocs/appnotes/xapp028.pdf
>>which would benefit from your diode trick.
>>Cheers
>>Peter Alfke, Xilinx Applications

>
>
> The diode thing allows true overlap (ie, zero deadband operation)
> without worrying about relative pfet/nfet drive strengths or tristate
> enable times. It also eliminates a more subtle problem related to pin
> capacitances, which would add yet another nonlinearity to the already
> nonlinear xapp circuit.
>
> Skyworks makes some 0.22 pF, SC79 schottkies that would be ideal here.
>
> Loop gain doubles in the overlap region, but that's easy to deal with.
> It's sure better than a flat spot.


Yes, better systems take effort to avoid any dead-band

>
> The opamp situation can be interesting.


With the single-resistor drawn in the Xilinx app note, it gets worse
- there is buffer contention during that (short) time = more unknowns

-jg

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