Q
I would like to know what reference clock is the most adapt for the AD9951.
I want to used a cristal oscillator for a high frequency stablility but I
don't find in the AD9951 datasheet what kind of level (CMOS, clipped sine,
sinus, etc) I have to use. And the input impÃ
dance is 1.5KOhms, isn't it ?
because the input power expressed in dBm is usualy for 50ohms.
do a small clock amplitude reduce a lots the phase noise performance ?
A
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
noise.
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
1>1.25V.
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
achieved.
However, two things to be careful about
- Common mode level of the AD9951 input.
The differential inputs are biased around 1.35V which is different from the
common mode level of LVPECL outputs. Probably best therefore to ac-couple the
clock (using series capacitors) if using ICS8432-101 3.3V LVPECL Frequency
Synthesizer or similar.
- Jitter of the output. The stated output jitter is 5ps rms. While this might
seem good performance, it is not really adequate to get 14 bit SNR performance
at more than 1 or 2 MHz.
