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AD9838 - Jittering at Some Frequencies

Question asked by flyerrap18 on Nov 21, 2013
Latest reply on Dec 5, 2013 by lfnickel

     Good morning.  My customer wants to use the AD9838 DDS and is currently evaluating the part via the evaluation board.  His frequency range of operation is 40KHz up to 1MHz in 256 steps of 3.750KHz.  He has the IOUT pin going into the VIN pin as it is on the demo board using an RC filter composed by 300W resistor and a 0.1mF capacitor.  The calculated cut frequency for this filter is FC=5.3KHz.  The circuit is working and I get the square wave desired.  The only problem is the jittering in some frequencies on my scale.

 

     Question:  I'm planning to implement a 3-pole RC filter coming from IOUT to Vin to see if I can reduce jittering.  Which cut frequency do I need to have on my filter?  I have read the AN-837 talking about the filter, but a 7th order elliptical filter I feel is unnecessary.

 

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