Hello,

I need to generate frequencies with a range 100Hz - 2MHz using AD9959 eval board. The specs says the min frequency is 1MHz. Nevertheless in one of the previous discussions it was said that it was transformer who sets this limit and that DDS can actually generate all range of frequencies.

Question is: What modifications to the eval board shall I do to get out the lower frequencies? (Desoldering transformer and populating the resistor instead etc.?)

I am interested in making the least modifications possible to the board.

Many thanks in advance,

Reni

Your circuit seems satisfactory. It is, after all, a standard differential opamp circuit with a gain of 28. As long as the opamp has sufficient bandwidth, it should be a workable solution.

Be aware, however, that any DC mismatch in the actual DAC output currents will be amplified by a factor of 28. Of course, you can mitigate this DC offset altogether by placing a capacitor between each DAC output node and the associated 250 ohm resistor. Then the only DC offset present will be that introduced by the opamp.

By the way, modelling the current sources as 5mA peak with +5mA DC offset is spot on.