Hello, AD9789 output impedance is 70 ohms and the recomended output circuit includes resistors that reduce the output impedance looking to the DAC to 50 ohms, after that it is recommended to route it to the secondary of a 2:1 transformer so the nominal impedance from the circuit output looking to the DAC will be around 25 ohms (at least in the lower ferquency range where the transformer is near ideal), a) is this system recomended in this manner for systems needing 50ohm, 75 ohm or both in spite of the impedance mismatch? b) we need to insert after the transformer or balun a sharp 50 ohm bandpass filter and I am unsure what is the real impedance the filter will see from the generator side, as well as if the DAC will "like" the transformed impedances that it will see from the filter input at all frequencies.

I wonder also, as all the output circuit is passive, if it would be convenient to use a differental amplifier instead the transformer, or the transformer first and the diffamp later because it would be the best way to isolate the impedances from the DAC output to the filter input. If the reply is yes please can your ecommend an ADI part that has been tested with this part.

Thanks in advance

Jaime MArtin

The recommended circuit you mention can be used to drive your 50 ohm bandpass filter. The circuit is tested with 50 ohm single-ended loads connected to one side of the primary coil with the other side grounded. We don't have an application circuit designed with a differential amplifier, but it certainly could be possible to make such a design.