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TRF1305B2: Differential output impedance when disabled

Shunsuke Yamamoto
Expert 7301 points
Part Number: TRF1305B2
Other Parts Discussed in Thread: TRF1305

Tool/software:

Hi team,

What is the differential output impedance of TRF1305B2 when TRF1305B2 is disabled? Some RFSoCs has restrictions of differential output impedance of fully differential amplifier.

Best regards,

Shunsuke Yamamoto

  • 0
    TI__Prodigy 590 points

    Hi Yamamoto,

    I will measure today and get back to you.

    Thanks for the inquiry

  • 0
    TI__Prodigy 590 points

    Hi Yamamoto,

    Attached is the differential output impedance from the TRF1305B2, with channel 1 being in red and channel 2 being blue. The real portion of the impedance is on the left plot, and the imaginary portion is on the right.

    As you can see from the plot, the device is high output impedance (250 real ohms differential) at low frequencies, below 1 GHz. After 1 GHz, the differential output impedance can be approximated by a series 50 ohm resistor with a 5 pF capacitor.

    The S parameters for my measurement are attached so you can play with them.


    TRF1305B2_PD_Zout.s4p

    Port 1 and 3 are OUTP and OUTM respectively for channel 1, Port 2 and 4 are OUTP and OUTM for channel 2

    Gideon

  • 0 in reply to Gideon Townsend
    TI__Expert 7301 points

    Hi Gideon-san,

    Thank you so much for the reply. It is good to know you support my case. My customer has an additional question regarding the connection of S2D amp and ADC. My customer doesn' plan connect filters between S2D amp and ADC. If so, Is the figure below the correct connection of S2D amp and ADC?

    Best regards,

    Shunsuke Yamamoto

  • +1 in reply to Shunsuke Yamamoto
    TI__Prodigy 590 points

    It depends on the input impedance of the ADC. For a wideband approach like this one, a pi or tee attenuator is recommended to be placed directly at the output of the TRF1305, and then connected to the ADC by a transmission line if the ADC input impedance is good. The series 50 ohm as pictured will work as an attenuator, but have more loss than a pi or tee attenuator doing the same job.

    If the ADC has some mismatch, then another attenuator will be needed at the ADC depending on how far away the 1305. If you are able to put the 1305 close to the ADC, then the attenuator values needed can be quite small, to limit loss.

    Thanks,
    Gideon