EMI Susceptibility Model of Signal Conditioning Circuits Based on Operational Amplifiers

Circuits based on operational amplifiers (OPAMPs) are susceptible to electromagnetic interference (EMI) in the radio-frequency range due to the AM demodulation produced by nonlinearity of internal transistors. Such a phenomenon produces demodulated signals in the low-frequency range. This paper presents a model to predict the susceptibility of analog signal-conditioning circuits, based on a few test parameters, while ignoring the internal structure of OPAMPs involved in the circuit. The method has been experimentally tested in two applications: 1) test of simple instrumentation adapter circuits and 2) predictions of susceptibility of an amplifier for an electret microphone, tested according to EN55020.