ADF4372 Lock detect

Dear Kudret,

I am now using 3 wire SPI with the ADF4372 so I can use the Mux out pin to output N/2. However I have discovered that the digital lock detect readback ( as determined by bit 0 of reg 7C)  does not work unless the MUX select ( bits 7:4 of reg 20) is set to 0001 .

  This is a real nuisance , as it means that I cannot use the Mux out     to output N/2 or R/2  without losing the lock detect readback. I want to use the N/2 output to drive an external analog phase detector.

Could you please confirm this, and suggest any register settings that would avoid it.

A further question: The default settings disable the bleed current, however the notes suggest that the bleed should be on. Presumably the bleed current is only appropriate in fractional mode. Should I switch on the bleed in fractional modes, or use the default?

Also, what is the BLEED_GATE  function.

Also, should I change the LPD vale ( reg 27) when changing from integer to fractional modes?


Cosmo Little

  • 0
    •  Analog Employees 
    on Jan 25, 2021 7:37 AM 1 month ago

    Hi Cosmo Little,

    Unfortunately, MUXOUT pin cannot output digital lock detect status and scaled divider outputs simultaneously. You can check the lock status by reading back the Reg0x7C[0] bit. 

    If you are using fractional divider, it is suggested to enable Bleed Current for better output PN performance. Similarly, LDP setting should reflect the operation mode, 0 for fractional and 1 for integer. 



  • Dear Kudret,

    I regret that you have misunderstood my problem. When the multiplex pin is set to output N/2,  the lock status read back always returns zero. The read back only works if the multiplex is set to output lock detect. Thus I cannot read the lock status when outputting N/2.

       This is a problem as my intentions were to switch over to a sampling ( analog) phase detector after  the loop had locked using the normal phase detector. I would simultaneously tri-state the charge pump. However as I cannot read the lock status when outputting N/2, this scheme seems to be impossible.


    Cosmo Little

  • 0
    •  Analog Employees 
    on Jan 25, 2021 11:29 AM 1 month ago in reply to cosmolittle

    Sorry for misunderstanding Cosmo Little. I will check it with the team and get back to you.