Charge injection in a switch or multeplexer

Charge injection in analog switches and multiplexers is a level change caused by stray capacitance associated with the NMOS and PMOS transistors that make up the analog switch. The Figure below models the structure of an analog switch and the stray capacitance associated with such an implementation.The structure basically consists of an NMOS and PMOS device in parallel. This arrangement produces the familiar "bathtub" resistance profile for bipolar input signals.The equivalent circuit shows the main parasitic capacitances that contribute to the charge injection effect, C_GDN (NMOS gate to drain) and C_GDP (PMOS gate to drain).The gate-drain capacitance associated with the PMOS device is about twice that of the NMOS device, because for both devices to have the same on-resistance, the PMOS device has about twice the area of the NMOS. Hence the associated stray capacitance is approximately twice that of the NMOS device for typical switches found in the marketplace.

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