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This paper deals with the susceptibility to radio frequency interference (RFI) of the analog front ends embedded in smart power integrated circuits. The case of RFI injected in the power transistor terminals that propagates to the analog front ends of the same integrated circuit is investigated using equivalent circuits that include the silicon substrate. The demodulation of the interference that takes place in the analog front-end circuits is evaluated by means of an approximate nonlinear method. In particular, it is shown that the demodulation of the substrate interference in a MOS differential input stage depends on the asymmetry of the circuits connected to the input terminals. Based on this, a simple method to increase the immunity to such interference is presented and its effectiveness is proved through computer simulations and through measurements carried out on a test chip.