AD9951: Clock signal

Document created by analog-archivist Employee on Feb 23, 2016
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In AD9951 datasheet, the description of reference clock signal is limited. It
is specified only the max applicable power. Can you confirm that using
sinusoidal signal is equivalent than squared (ECL?) signal? What is the impact
on DDS phase noise? Using differential input can lead a better phase noise than
single ended input?


The AD9951 reference input is normally driven with a sine wave.
It is AC  coupled (with a capacitor) and if a single-ended drive is used the
other  clock input is decoupled with another capacitor.
The drive level must  not be over 1.8 V pk-pk (0.64 V r.m.s.) with a 1.8V
supply, but may be  as low as 100 mV r.m.s.
However it is better to use as large a drive as  possible to minimise phase

If you drive the reference input with a square wave it must have a duty  cycle
of close to 50% and logic levels (for 1.8V supply) of logic 0<.4V  and logic
If you use the PLL clock multiplier with logic inputs  the duty cycle may be
slightly worse - between 35% and 65%.

Driving differentially is more recommended. Better common mode rejection is
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