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CMOS Switch Modeling

Question asked by MatF on Jun 8, 2018
Latest reply on Jun 14, 2018 by MatF

Hi,

 

I'm performing a Spice simulation including a switch for which no Spice model is provided. To evaluate the transfere funtion of my circuit I'm modeling the switch by a simplyfied PI - network. So I use the on-resistance and the capacitances, stated in the datasheet.

 

I'm modeling the ADG4612, supplied with +-5V. The off capacitances Cs(off) and Cd(off) should be clear (see attached figure (a)). I have read the tutorial MT-088, but I have still problems by interpreting the channel's on-capacitances (maybe I missed something in the tutorial).  

 

According to the datasheet "Cd(on) and Cs(on) represent on switch capacitances, which are measured with reference to ground". There is no corresponding test circuit shown in the datasheet, so I assume that one pin (Drain or Source) is connected to ground, and the remaining one is the "input".

 

Because the quotation above mentions capacitances (PLURAL), I'm not sure how to model the on-capacitance. From my point of view, Cs(on) and Cd(on) is one capacitance (the channels on capacitance), which I would model by split it into two equal capacitances (see Fig. (b)).

 

Is my assumption correct?

 

BR

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