Question about the balun of the AD9250-250EBZ input circuit

Question

The circuit below has T103 and T100. 
 The T103 is a 50 ohm balun, and the T100 is a 75 ohm balun.

I wonder why they used different baluns. Also, why is the actually inserted balun a 75 ohm balun? 
 Thank you

jxav · Answer

Hi Justin, 
 Thanks for your interest in AD9250. 
 The use of different baluns are application-specific. For MABA-007159, it can cater a higher bandwidth compared to ADT1-1WT+. As for why the 75-ohm balun, the schematic is configured to match the impedance of the primary and secondary sides of the transformer for maximum power transfer. From the figure below, the aim is to have a Zsource = Zload = 50ohm most of the time, and the value of the characteristic impedance of the transformer is critical in that goal of 50ohm. 
 You can refer to the links provided below on the selection of characteristic impedance of the transformer and designs of transformer-coupled AFEs. Thank you. 
 
 MT-228 (analog.com) 
 AN-935 Designing and ADC Transformer-Coupled Front End Application Note (Rev. 0) (analog.com) 
 Transformer-Coupled Front-End for Wideband A/D Converters (analog.com) 
 Ask The Application Engineer&mdash;36 (analog.com) 
 Regards, 
 Xavier

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Question about the balun of the AD9250-250EBZ input circuit

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