Reducing the EMI Susceptibility of a Kuijk Bandgap

In this paper, the susceptibility of a Kuijk bandgap voltage reference to electromagnetic interferences (EMIs) superimposed to the power supply is investigated. A model of the bandgap circuit is derived from experimental tests consisting of scattering parameters and susceptibility measurements on a test chip. The model is able to correctly predict the susceptibility of the circuit by means of SPICE transient simulations. The simulations identify the fundamental stray components responsible for EMI coupling that are usually not taken into account in postlayout analyses. From the EMI point of view, the rectification phenomenon of bipolar transistors, used in the bandgap cell, and the operational amplifier (op-amp) input distortion are shown to cause the voltage reference performance degradation. In particular, for the first time, the importance of the bandgap cell is pointed out, suggesting that the measures to reduce the susceptibility of the op-amp may not be sufficient to guarantee the bandgap immunity. Moreover, the analyses of main parasitic paths (from the power supply net to the more sensitive nodes) demonstrate the relevance of paths that are not commonly considered during the design phase and that may lead to an immunity degradation. Possible solutions to reduce the bandgap susceptibility are also explored, suggesting design criteria, filtering techniques, and layout variations. A second test chip is designed and manufactured to validate the suggested improvements and verify the critical role of the bandgap cell. Finally, a simplified theoretical analysis, which allows a fast bandgap susceptibility evaluation, is presented. This tool is used to point out the importance of the rectification phenomenon compared to the op-amp differential pair distortion.