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AD9910 eval bd, R term differ from AN-912 and DAC_Xfmr_Coupled_Deluxe.xls

Question asked by hiro.sp on May 23, 2013
Latest reply on Jul 25, 2013 by hiro.sp

The schematic of AD9910 EVAL has 49.9ohm(=50ohm) each with IOUT, #IOUT. (R7,R9)

But, I calcurated the R value with DAC_Xfmr_Coupled_Deluxe.xls it says 25ohm should be used with 50ohm impedance circuit.

And I simulated following 400MHZ LPF circuit assumed with 50ohm, it works well. the circuit seems designed with 50ohm whole.

 

Here, I have some questions.

(1) Why 50ohm R used at primary of transformer 1:1 ratio .

(2) Why 75ohm specified transformer used (ADT1-1WT, it specified with 75ohm)

 

The circuit seems it has 100 ohm output impedance at transformer output.

If it assumed 100ohm input impedance, 400MHz LPF pass band characterestic waves(degraded) more than assumed 50 ohm.

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