I'm designing an ADC with 2 separated grounds: AGND and DGND - one for analog and other for digital domain. I'm using the QRC to perform a parasitic extraction of my final layout. In the QRC's "Extraction" tab, there is a "Ref Node" field. In this case, is necessary to fill "Ref Node" field with AGND, DGND or short both in layout and rename the net?
I usually short them. That said - I have a one extra hierarchy level for LVS where I short the Analog and Digital grounds together (as if on PCB) and during QRC my reference s the external ground connection.
That said - I do not think it makes much sense to run QRC on digital. With propper library you should not have to.
After placing them in a new layout and shorting them for extraction AGND and DGND will be GND, but when you will see the report of QRC ( which comes from layout ) if a analog net has 25pF with GND means net is coupled with AGND same with Digital nets .