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How to caculate the output impedence?

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John_li

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Dear all,
Can you tell me how to caculate the following schematics output impedence?
Thank you,
 

echo47

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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.
 

    John_li

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frannk

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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.
 

    John_li

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John_li

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Thank you,echo47 and Frannk,your reply is helpful to me.
 

sanim

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Hi,

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.

Rgds
 

    John_li

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echo47

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sanim, the schematic says "impedance ratio" is 1.5, not "turns ratio". So I think my calculation is correct.
 

    John_li

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