In the datasheet for the AD9744, Figure 24 shows a typical test set up, which connects the differential DAC output to a single ended 50 ohm input into a spectrum analyzer. Assuming there is a 50 ohm resistor terminating each side of the primary side of the transformer, that would provide 100 ohms on the primary side and 100 ohms on the secondary side (because the transformer T1-1T is a 1:1 transformer. Is this the correct reference circuit for this application? (I correct what seems to be a mistake in the datasheet - with two 50 ohm resistors on IOUTB).
The datasheet says the the 9744 equivalent output circuit is a current source with parallel 100k ohm and 5 pF impedance, so it seems like the external 50 ohm resistors would dominate the impedance.
The evaluation board for this device shows a similar situation. (The ADTT1-1+ is also a 1:1 transformer).
Why do both circuit examples transform to a singled ended 100 output impedance?
Is there an equivalent circuit you can recommend that I can use to simulate this DAC output in an ADS S-parameter simulation?