SAR sampling switch mosfet problem

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yefj

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Hello,Mgnd is a NMOS .during the proccesing stepI need to keep Vgs<Vt to keep the charge.
The SAR is 7 bit with a certain capacitance.The general formula for V+ (Source of the mosfet) is shown bellow.
I need to peak the minimal Vg needed for the most problematice source (V+).

Our V+ could be a negative voltage thus source could turn on ,and source will be the right side as shown bellow.
Vin=1V for the formula and Vref=1
Using the formula bellow what is the most problematic V+ ? and why?
Thanks.


 

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