Average resistance in a sampler

[yefj]

Hello, i Have here a sampler and i need to calculate its evarage resistance of the sampler.
The circuit of the sampler is shown bellow. how did they get the Rsw expression?
Thanks.

 

[vivekroy]

Replace Vdd by Vdd-Vin in the expression and some algebra later you will get the expression you are looking for.

 

[BigBoss]

It also depends on duty cycle of the sampling signal..