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Input matching network for AD9694

Question asked by dtaylor on Jun 20, 2018

Hi,

 

the input buffer of AD9694 provides a 200 ohm differential termination impedance according to the data sheet (left figure below). Both the suggested input matching networks (p. 24 in the datasheet, right figure below) thus present an approximate

220 || 100 + 20 = 89 ohm

impedance to the balun.

 

In the default configuration of the corresponding evaluation board, this impedance is transformed 1:1 by a double balun and presented to the (single-ended) analog input SMA connector which should be 50 ohm (figure below).

 

Am I missing something here? Why are the resistor values / baluns not chosen in a way such that the input impedance is 50 ohm single-ended / 100 ohm differential?

 

Thank you and best regards,

David

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