>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.

I agree with Dave, there is no need for fancy filtering here. Just a single RC would work. You may need to buffer the output AFTER the RC filter depending on what this needs to drive. Just use an R in series, then a C to ground. Cutoff frequency in Hz is 1/(2*pi*R*C) where R is in ohms and C is in Farads. Remember that you'll need a non-polarized cap since your signal goes + and -. A ceramic or polyester type should work fine.

The problem you were seeing was because the output of the op-amp is limited in how fast it can change (in terms of Volts per second). When you start to exceed that rate, you get a linear ramp output, which is why it was changing all but the slowly-changing parts of the sine wave into straight lines.

All linear systems pass sine waves undistorted because the most a linear system can do is add delayed,scaled copies of the input together to get the output. It cannot alter the frequency of the input. Any number of sine waves of different amplitudes and phases, as long as they are all at the same frequency, when added together will still give you a sine wave.

Sean


Sean




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