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

Peter Alfke 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 tristate drive is seen in many PFD's.
Tristate works well, but does float the FPGA pin
at the Opmap Bias point, and is also a noise-injection
point.
Also either the Diode, or Tristate directly couple
the FPGA Vcc and GND noise into the integrator (when ON)

So the "purists PFD", would use a TinyLogic analog Switch,
[example of 2 in one package : 74LVC2G66 ]
and keep the Hi-Z integrator node tiny, and shielded from
digital charge injection, and power supply noise.

Also, some designs have deliberate overlap in the PFD
impulses, as that avoids a dead-band, which can give
higher spurious noise spurs.

-jg

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