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Old 10-31-2007, 10:39 PM
John Larkin
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Default Re: Ping Jim: The PFD is dead!

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.

The opamp situation can be interesting.

John


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