Is there a trade-off in using the charge pump high gain (HiK) mode on the HMC PLLs; e.g. HMC984?
The only drawback is that it requires an active inverting loop filter configuration because the CP pin must operate into a mid-rail (2.5V) bias for consistent loop dynamics.
The HiK feature does not use current sources. It is simply switching the CP pin between ground or supply rail (5V) (and Hi-Z of course). Hi-K mode will deliver 3.5mA when operating into a 2.5V bias.
If a passive loop filter is used, the effective CP current will vary with Vtune resulting in varying loop dynamics.
I'm looking into this for you. In the meantime you can get a basic idea of the impact by reviewing pages 6-4 through 6-7 and the section for "Charge Pump Gain" on page 35 of the HMC703LP4 datasheet (attached) as it has this functionality as well.
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