By design, DDS "stones" like AD9852 from Analog Devices, required
separated power lines for AVDD, DVDD and VCC. What will is simple
solution for that ? I am planing to use following approach: +5V
from linear PS, then three LC filters, then three 3.3V voltage
regulators (Ex.: MC33269T) connected to each filter. Is it good
enough ? May be its better solution for this ? Or may be that could
be simplified to join AVDD and VCC (AVDD will be connected to VCC via 100 Ohm).
Do you have the AD evaluation board, or are you starting with the bare chip?
If you really want to know how simple you can make it, why not try it
yourself, and see what you need? You will learn a lot more that way
than by asking first every time a question occurs to you.
Follow the evaluation board plan and put a 0.1uF (100nF) monolithic
ceramic capacitor right at each power input pin of the IC itself
(something like 10 capacitors per supply).
First, use one 3.3v regulator and feed its output straight to all
three circuits, with simply a local bypass cap for each one (plus the
per-pin capacitors as noted above). Run the DDS and see how it performs.
Then, see how three separate LC filters perform (each LC fed by the
regulated 3.3v supply).
Finally, feed the unregulated supply to the "upstream" side of each
of the three LC filters, and use a separate 3.3v regulator on the
"downstream" side for each supply.
In each case, note carefully (at a lot of different output
frequencies) the general output noise level and the presence of any
spurs and birdies in the output, as well as any logic faults you find
(wrong frequency, system hangs up, bus errors, etc.).
It might be more instructive to run those steps backwards -- first,
see how it works with the most complex (and presumably best) supply,
then try the simpler circuits and see what problems crop up.
Of course, with either test protocol it is difficult to know whether
you have tried every operating state that could cause a problem, so
play with it quite a while with each setup and try to use every
function and combination.
As Chris said, you need to be very careful with your grounds. These
chips are intended to be put on boards with four or more layers. The
AD evaluation board has four layers with a common ground plane for
the analog and digital circuitry -- it is possible you could do
better with more careful attention to grounding.
Best regards,
Charles
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