PLL Filter reference to VDD or GND?

Hi.

I am wondering why the PLL filters on e.g. the ADAU1451 and ADAU1701 are supposed to refer to VDD.

In both datasheets, the PLL Loop Filters are specified to refer to VDD (Digital or PLL VDD), but without any further explanation.
Could the filter not just be referring to the Digital or PLL GND instead?

On most layouts I suppose that the voltage reference level for the loop filter is more stable on ground compared to the 3.3 V supply.

I have experienced an issue with the PCM9211 from Texas Instruments, where the loop filter was also supposed to refer to a PLL VDD.
But, due to layout issues, the PLL was randomly unlocked. When I then tied the PLL Loop filter reference to GND, the PLL became extremely stable.

I hope someone can help me clarifying this.

Best regards,

Jesper Thørring Jensen

Audio Signal Processors

dsp

adau1451

filter

SigmaDSP Audio Processors

pll

adau1701

adau145

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