[time-nuts] Power Supply for AD9852 / AD9854

[Charles Steinmetz]

>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