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Very low frequency oscillation in AD9958 DDS output

Question asked by rdgraham on Sep 17, 2014
Latest reply on Sep 22, 2014 by rdgraham

Hello,

 

I am using an AD9958 DDS and am having a great deal of difficulty getting a good signal out. I am looking for any more information beyond the data sheet as to how to couple the differential signal out to a small pre-amplifier.

 

The DDS itself seems to be working fine but I am seeing very slow oscillations at the output, around 10 - 50 Hz, with amplitude several times larger than the RF output (100 - 200 MHz). I have tried several different output coupling schemes but always see this behaviour. Adding some components to high-pass filter it works to so some extent (factor ~2) but I can't remove it entirely even though it should be massively attenuated at these frequencies. The oscillations are always proportional in frequency to the RF frequency that I set so I don't think they are coming from somewhere else.

 

Specifically, I have tried terminating the output via a pair of 50 ohm resistors into Vdd, which then in turn drives a AD8009AD op-amp with gain of 5 to 10. I have also tried the centre tapped transformer arrangement suggested in the data sheet by itself and driving a OPA846 op-amp. The output termination and amplifier are connected to the DDS via about 5cm of differential pair traces on the PCB.

 

I'm guessing there is something here I really don't understand. Any suggestions appreciated.

 

Thanks,

-- Richard

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