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AD9789 output interface for 50 ohms systems

Question asked by Jaime on Mar 19, 2013
Latest reply on Mar 19, 2013 by danf

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