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ADF4351 Differential to single ended output


I want to use the ADF4351 as single ended out put. how should i do this in the eval board schematics?


  • Hi,

    You should terminate the unused output with a 50 ohm termination to ground. If you are making your own board, you can put a 50 ohm resistor to ground after L1/L4 inductors. If you are using evaluation board, you can put a 50 ohm load to SMA connected at the output of unused channel. 


  • Thanks Kudret.

    Can I not terminate directly at RFOUTA-  PIN with 50 ohm by discarding R27,L3,C35 and L1 ?

    ADP150 used in eval board is 3.3V or 3.0v?

    Is it required to use two  separate LDOs  for VDD and VOUT as used in Eval board?


  • Hi,

    You need to provide similar output structure at both pins to decrease the mismatch between the pins. 

    ADF4351 requires a pull-up path to Vdd at the RF outputs as a current path. This pull-up can be provided with a 50 ohm resistor which provides wider matching bandwidth but less output power or a pull-up inductor which providers narrower matching bandwidth but higher output power. The value of pull-up inductor depends on the output frequency that you use. You can review the Output Matching section of the datasheet. 

    3.3V ADP150 is used in the evaluation board. The reason of using separate LDO for VOUT is to increase the isolation at the outputs. 


  • Hi Kurdet,

    How to select the inductor for a particular (single) frequency?

    Isolation at outputs. can you please elaborate little more to understand?


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  • Hi ,

    I attached the s-parameters of ADF4351 outputs. You can match the output for your frequency interval.

    By saying isolation, I meant the spurs that seen on the output. Using same LDO for all supplies increases the cross-talk between circuit blocks and you can see unwanted spurious signals on your output clock. Separating output buffer supply from the rest of the circuit improves the spurious performance.