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Many of the AC characteristics of a switch are determined by the parasitic and external capacitances associated with the device. AC characteristics such as bandwidth, isolation and crosstalk are degraded by large capacitances. Bandwidth is specified as the frequency where the small signal gain is attenuated by -3dB. The bandwidth is a function of the switch output capacitance in combination with Cswitch and the load capacitance. The dominant effect is the switch output capacitance. To maximize bandwidth, the switch needs to be designed with low capacitance and low RON. It should be noted that all NMOS switches inherently have larger bandwidths than a parallel structure, as the capacitance of the switch is significantly reduced. (The PMOS is normally more than three times the area of NMOS). Off Isolation is a measure of the ability of the switch to block AC signals in the off state, also known as feed through. The dominating capacitance that is responsible for feed through is Cswitch. Cswitch is not directly specified in datasheets, but it is the difference between the on capacitance (Cs,Cd(on)) and the sum of the two off capacitances (Cs(off) + Cd(off)). Large values of Cswitch will produce large values of feed through, proportional to the input frequency. The bottom figure in the slide shown here illustrates the degradation in Off isolation as frequency increases. Therefore to maximize the Off isolation, choose a switch that has as small a Cswitch as possible.
