Where exactly appears the channel capacitance and Ron when switch on DG442 switches
The on capacitance is across the switch analog terminals.

From the schematic in the datasheet, I have added where it exists:



The capacitor exists across the analog switch channel (which was not identified in the schematic) so the A and B analog terminals are where I have put the capacitor.

This is also the primary cause for off isolation degrading with frequency coupled with the isolation resistance and load; as you can see it is a classic single pole with 20dB/decade characteristics.


When I did a quick calculation, I got a value of about 15pF for the source to drain capacitance, close enough for confirmation.

For a really good overview of analog switches, I recommend this application note.