AD9958 loop_filter components

Hi all,

Our customer will design the custom board with AD9958.

They have the question about the Loop_Filer, 27pin.

@ the datasheet rev.c,
27 pin description ;
Connects to the external zero compensation network of the PLL loop filter.
Typically, the network consists of a 0 Ω resistor in series with a 680 pF capacitor tied to AVDD.

@ the AD9958 Evaluation Board Data Sheet rev.0,
Figure 48. AD9958/PCB Schematic, Page 1 ;
R9 698 ohm
C38 680pF

Which is correct the resistor values of network, 0 or 698 ohm ?

Best regards,
sss

  • 0
    •  Analog Employees 
    on Mar 4, 2021 6:07 AM

    Hi sss,

    The correct resistor value is 0 ohm as indicated on the AD9958 data sheet rev. C, and used in the evaluation board schematic rev. C.

    Best regards,

    Mark

  • Hi Mark - san,

    Thank you for quick reply.

    I'm sorry, I have another question.
    Do "CLK_VDD" and "AVDD" have the same meaning?
    Where should we connect this network ?

    Best regards,

    sss

  • +1
    •  Analog Employees 
    on Mar 9, 2021 7:37 AM in reply to sss@jpn

    Hi sss,

    CLK_VDD is also an AVDD based on the pin configuration of the AD9958. CLK_VDD, BG_VDD and all other AVDD in the evaluation board are tied together. Therefore, only one connection to any of them needs power for proper functionality. The voltage needed is 1.8V. I'm also attaching the user guide of the AD9958 evaluation board.

    Best regards,

    Mark

  • 0
    •  Analog Employees 
    on Mar 9, 2021 1:52 PM in reply to mcee

    A note of caution...

    Although the AD9958/59 Evaluation Board uses 0Ω jumpers to short the various supply lines together, this does not necessarily mean there are internal connections between supplies within the device. Granted, in the case of the Evaluation Board, only one supply is necessary to feed the board connectors.

    However, in the case of a custom design, the PCB power plane must connect to ALL the supply pins of the device (a common internal connection may not exist). Even if the supplies are internally connected, connecting only one or two of them to the power plane would not be optimal, as it creates a choke point for current flow.

    Furthermore, the supply and ground pins are a significant path for heat transfer from the device to the board. So, from a thermal perspective, the more connections to power and ground planes, the better.