Investigation on the Susceptibility of Operational Amplifiers to Switching Noise

This work deals with the susceptibility of operational amplifiers to the noise generated by the fast switching of power transistors. The problem is of high importance for any high density electronic circuits comprising power and analog sections, as it is for power ASICs. In such systems, the switching noise generated by the power section couples with the analog one although they are kept isolated. This is due to the parasitic coupling of such circuits through the package and through the silicon substrate they share. Such disturbances, not only affect the nominal signals at high frequency, meaning at the switching frequency and its harmonics, but also within the bandwidth of the analog channel. This is due to the demodulation of such disturbances, which results from the nonlinear characteristics of the device the analog circuits are made of. Specifically, this work focuses on the operational amplifiers, which are widely used in analog design. An integrated system comprising of a buck converter and a feedback operational amplifier, both integrated in the same silicon die is considered with the aim of evaluating the disturbance affecting the operational amplifier at the input, then a DPI test bench comprising a wide band arbitrary waveform generator is used to inject the switching noise resulted from the transient analyses. The results of the DPI immunity tests carried out with such a disturbance are compared with those obtained by injecting CW RFI.