Why does the AD9540 require special filtering on its analog output. What are the requirements of this filter?

To use a DDS-based device as a clock driver, the output sinewave must be converted to a rectangular wave with fast edge transitions (high slew rate). In order to do this, it is necessary to pass the AC output signal through a high- speed comparator, with an output of the correct voltage level.

An important factor in creating a clean, low jitter clock signal is the reconstruction filter, which is required between the DAC output and the comparator. The sampled sinewave output of the DDS must be low pass (or bandpass) filtered in order to remove the sampling artifacts (images) from the AC signal. Another way to look at this is that the filter acts to smooth the stairsteps in the DAC output. This smoothed sine wave is then applied to the input of a high-speed comparator. The duty cycle of the resulting clock signal can be varied by adjusting the reference voltage on other input of the comparator, if desired.

For applications requiring the lowest jitter, a bandpass filter may provide better performance than a low pass filter. Also, differential filters offer performance advantages over single-ended ones.

Several of the ADI DDS' have high-speed comparators integrated with the DDS in order to make clock signal generation simple. It is also always possible to use an external, high-performance, high-speed comparator from ADI. But, in all cases – don't forget the low pass (band pass) filter.