How to understand charge sharing in charge pump of PLL?

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djallon

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The circuit of charge pump is showed in the picture. What I am confused is that if there is not a unit gain opamp, voltage of node N1 and N2 would not change too much , so charge sharing is not terrible. But some papers realize charge pump with the unit gain opamp to reduce charge sharing. Why the circuit need unit gain opamp? And how the opamp reduce charge sharing? Plz help me to solve this problem. Thanks very much!
 

if there is not a unit gain opamp, voltage of node N1 and N2 would not change too much , so charge sharing is not terrible.
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N1 and N2 are floating up to the supply rails minus current source saturation voltage, the parasitic charge transferred to the loop capacitor might be "terrible" enough, CS saturation possibly causes additional unwanted effects.

The operation of the suggested clamp circuit is simple enough, I'm sure you can figure it out (or re-read the citation explaining the figure).

See also https://www.ece.tamu.edu/~spalermo/ecen620/lecture11_ee620_charge_pumps.pdf
 

thank you very much! There is also parasitic charge transferred to the loop capacitor though there is unit gain amplifier.How to explain this?
Then, "CS saturation",what do you mean?
 

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