Feedthrough capacitor in analog switches
The FETs (MOSFETS) used in analog multiplexors have two series capacitors:

1) source-bulk (we assume there is no channel --DC path)

2) bulk-drain

These two capacitors in series set the high-frequency behavior IF the bulk is allowed to move freely.

Holding the BULK quiet is partly your responsibility, based on how firmly, how strongly, you control the GND and the VDD pins on the silicon.

Note I said silicon.

Different packages will have difference inductance, and even different pinouts and even different routing patterns (in the case of BGA, etc).

View your high frequency analog mux model as







simulate this circuit &ndash; Schematic created using CircuitLab