Sean Schouten wrote...

>I have a designg for a digitally controlable sign wave generator, using one
>of Analog Devices AD9834 DDS's and am looking for a way to turn it's output
>signal into a sine-wave that looks near-perfect on an old analog scope.

Given that you intend to clock the AD9834 at 10MHz and are going
to be generating sine waves only up to 25kHz, that should be very
easy.

>I had the idea to use a Sallen-Key filter with a butterworth configuration
>(2nd order), to filter my DDS's output, but after numerous simulation it
>seems that the filter starts deforming the DDS's output because of it's
>fase-shift  characteristic of the filter. 

You don't say exactly how the sine wave is being "deformed", but
whatever the nature of this distortion it is unlikely that it has
anything to do with the filter's phase shift.  When passing a
sine wave, the phase shift of a filter will only affect the delay
between input and output-- not the shape of the wave.

I would guess that any distortion you are seeing in your
simulation is the result of some fluke in the simulation device
models, or a consequence of the way your particular circuit is
designed; without seeing the circuit, it's hard to tell.

In any case, filtering your DDS chip's output to get a
high-quality sine wave should be very easy, since the clock
frequency of your DDS chip is 400X the maximum frequency you want
to generate.  A single-pole passive RC filter with its pole
around 50kHz-100kHz should be more than adequate.

Hope this helps a bit...

Dave D.


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