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The left side impedance is 75+75 = 150 ohms.
I assume IOUT/IOUTB is a differential current sink, so it ideally won't affect the impedance.
The center-tap also has no effect on impedance.
The transformer is connected backwards, so it decreases the impedance by 1.5.
The output impedance is 150 ohms / 1.5 = 100 ohms.
The data sheet of the AD9858 states DAC output impedance as 100K. The shunt capacitance of the DAC output however, is not stated but must be measured. If you are operating up around 70MHz, as suggested by the flux type transformer, the imaginary part will have a major effect.
Well, speculations of echo47 are right till the statement that transformer "decreases the impedance by 1.5". Really transfromation factor of submitted configuration is R/n^2. So, the correct answer is 2*75/1.5^2=66.7Ohm.
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