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You can estimate ON resistance of Analog Switch by Vout/Iout, but it depends on the structure of Analog Switch.
For example, if Analog Switch is composed of Transmission Gate, the ON resistance is :
[Vds(pmos)/Ids(pmos)]//[Vds(nmos)/Ids(nmos)] , or
Vab/Iab, where Vab is the ON Voltage of TG, Iab is the ON current of TG.
The switch comprises of two TG , an inverter for control and a pulldown transistor. The setup provided to us is, a bias voltage connected to input pin, input voltage source which need to be varied. So is that we should measure current at out and vout at outpin to calculate RON.