HMC439 loop filter op-amp for wide bandwidth PLL


I am designing a wide bandwidth PLL (fout 38.4GHz, external divider-by-32, kVCO 0.6GHz/V, f_pfd=1.2GHz, bandwidth 150MHz) with HMC439 as phase-frequency detector. Can you put some suggestions on how to select the loop filter op-amp? Which op-amp parameters are important (GBW, supply voltage, slew rate)? Or can you suggest one op-amp for my design?

Many thanks and regards,


  • 0
    •  Analog Employees 
    on Jul 10, 2018 7:50 PM over 2 years ago

    Hi Anh,

    We've noticed that voltage feedback op amps are sometimes easier to implement active loops with phase frequency detectors like the HMC439 and HMC3716 that output voltage pulses. Next, you need to decide whether or not you want (or need) an op amp that plays rail to rail, look at the output voltage swing spec if you're unsure. Once you've determined this, look for an op amp with low voltage noise (less impact on phase noise), low current noise (less impact on phase noise), low input bias current (better spurious) and acceptable gain bandwidth product (high enough to achieve the desired loop bandwidth but not excessively high as Delta Sigma Modulator (DSM) noise may not be attenuated as well.  

    If you're planning on using the HMC439 evaluation board for analysis you'll need a pin-to-pin compatible op amp. The AD8021 (rail to rail), AD829, AD8065, ADA4084-1 (rail to rail) and ADA4898-1 (very low input bias current) are good options.

    The AD8065 or the new ADA4625 are good choices and included in ADISimPLL so loop design may be more predictable if you choose these as a starting point. The ADA4625 one of our applications engineers looked at these two specifically and found that the AD4625 had better rail to rail performance but phase margin for a similar loop bandwidth was lower than compared to AD8065.

     Finally, not sure what's driving the use of the HMC439 but you might consider the HMC3716 which has similar performance but includes invert and lock detect capability and is available in a smaller package with better thermal properties. It also incorporates some of the external resistors required on the HMC439 which further reduces the overall footprint.

    Best Regards, 


  • Dear Marty,
    Many thanks for your detailed answer, especially for pointing out the importance of low input bias current for low spurious. I have some follow up questions:
    - Can you elaborate on the DSM noise? I think it is important in fractional-N PLL. Given my design is an integer-N one, is this noise still important? 
    - How do you decide the gain of the op-amp to select a good GBW product? The op-amp's feedback impedance is an R-C circuit which is frequency-dependent. Which frequency do you look at to decide the op-amp's gain?