Effects of Radio Frequency Interference in OPAMP Differential Input Stages

The paper deals with the problem of RFI induced output dc offset in operational amplifiers. In particular, it presents an extended analysis of the effect of the input differential stage nonlinearity in the generation of the output voltage offset. Differently from previously presented analyses, the proposed approach allows to take into account the effects of non purely square-law device models, e.g. including also velocity saturation effects. The paper also analyzes the common emitter differential input pair the analysis results can be easily extended to represent sub-threshold biased MOSFET differential stages. Based on the proposed analytical approach the paper discusses two very simple design provisions that mitigate the RFI effects. Design trade-offs related to these solutions are discussed. The proposed solutions are validated by simulations.