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EVAL-ADF4355 input SMA bypassing

Category: Hardware
Product Number: ADF4355

Hi. I have a quick question regarding the ADF4355-2. I own the official EVB by Analog that allows to provide the input reference though the SMA connector, thus with a nominal 50ohm single-ended input impedance.

I was wondering if there is the possibility of "bypassing" the SMA input and provide a high-impedence signal with 3.3Vpp amplitude directly to the ADF4355, in order to lock the PLL to this kind of reference.

If this is not feasible, how do you suggest me to proceed? I was thinking about a voltage divider, but it seems to me it's a too naive solution.

  • Hi,

    If your signal is strong enough to drive some capacitance due to cables, SMA connector and Ref path, you can connect that signal directly to REFinA through a series capacitor.

    Just ensure that you are setting the Single-ended reference input mode (DB9 in register 4). This will disable the input bias generator and the common mode level should settle to mid-point. It is better to insert a small resistor (say 47ohm) in series to prevent any mismatch without impacting the amplitude and slew rate. You can confirm that the levels on REFinA and REFinB are not exceeding the Absolute ratings and tune the resistor and capacitor value if needed.

    Regards, Kazim 

  • Thanks for your kind reply Kazim. I just want to check if I understood properly. I want to specify the input signal will have a frequency around 10 MHz.

    Is the REFIN A/B input (on the EVB, so through the SMA) exposing an actual 50 ohm impedence or am I wrong?

    What you are suggesting is to build a kind of matching network with the resistance near the 50 ohm value and a (high? given the frequency) value for the capacitor in order to block the DC.

    My only worry is right about the capacitor value, that given the frequency may be too high. What do you suggest?

    Another question is about the maximum ratings: on the ADF4355 datasheet I found a dependancy from the AVdd supply. However, on the EVAL-ADF4355 I did not found any value for this parameter. Any clue or am I missing something?


  • Hi, The reference input impedance is 2.5kohm as shown in Figure-22 in the datasheet.

    Basically, what I have suggested is not a matching circuit. Just a series resistor to prevent reflection and a cap to block DC. I do not think the capacitor value will be an issue.

    On the eval board, you need to remove the R27 (100ohm) and power-down the on-board oscillator by removing the R12 (0ohm). The oscillator is still on the same path, that should be OK but you can remove that or cut the trace to ensure no loading from that side. Also, it is better to AC-couple the REFinB to GND i.e. remove the 1000pF from C42 location and solder that to R10 location.

    Regards, Kazim 

  • Thanks Kazim for your precious and precise answers.

    I'm having other kind of problems with the ADF4355 but not related to this specific topic, so I'm going to close this one and opening a new one.