AD9957 PLL_Loop_Filter Connection


Please advise on AD9957 PLL_LOOP_FILTER (PIN2) connection. if I am using an external differential REF_CLK source, do I leave the PLL_LOOP_FILTER pin open/NC? How about the PLL_LOCK pin?

Also, I intend to use the ADF4360-7 as the REF_CLK synthesizer and interface directly to the AD9957 by following the suggested interface matching from their perspective data sheet.


  • 0
    •  Analog Employees 
    on Jul 10, 2012 5:50 AM


    PLL_LOOP_FILTER pin provides a connection interface to attach the external loop filter components.

    You may refer to the evaluation board schematic here:

    The connection depends on what is your available external differential REF_CLK source. If you have a clock source value that is within the range (please refer to AD9957 datasheet) when PLL is disabled, then you can leave it not connected. That is, you can leave the 0Ω resistor not connected.

    You can also have an option of connecting it (for future flexibility). When in cases you don't have higher REF_CLK source, then you would be able to use the internal PLL. In this case, just put a 0Ω resistor in between the PLL_LOOP_FILTER pin and C13. Just refer to the datasheet on how to calculate the loop filter component values.

    PLL_LOCK pin provides an active high indication that the PLL has locked to the REFCLK input signal. So it is better to connect it to a LED for this purpose.