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# CMOS Switch Modeling

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

• Sorry I made a mistake in the previous post:

"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"."

One pin (Drain or Source) should be floating, and the remaining one is the "input".

BR

• Hi Matthias,

Let me first explain the model of the switch when it is “off”.

Coff (13pF from the datasheet) can be seen at each input and output (source and drain) pins of the switch referenced to ground. So your figure a is correct.

For Cd/Cs on, your figure b is also correct and should be also referenced to ground. The switch can be modeled as an RC low-pass filter. When the switch is “on”, Cs off and Cd off is in parallel combination, thus would sum to the value Con = 50pF.

In addition, you might want to have a block that chooses which value of capacitor (13pF or 25pF) to reflect according to the status (ON/OFF) of the switch.

Let me know if this helps.

Best Regards,

May

• Hi May,