EMI Resisting Analog Input Circuits

Abstract

This chapter evaluates the impact of EMI injected into the input terminals of analog circuits by means of two case studies. The first case study shows that the immunity to EMI of an operational amplifier can significantly be increased by reducing the EMI induced offset generated in the input differential stage. After reviewing and studying existing EMI resisting differential input circuits as well as their respective advantages and disadvantages, a source-buffered differential pair is introduced exhibiting a high immunity to EMI compared to the classic differential pair. This source-buffered differential pair topology is evaluated with a test-IC, and measurements illustrate that the source-buffered differential pair generates a much smaller EMI induced offset compared to a classic differential pair. The second case study describes the effect of very large common-mode EMI signals which are conveyed to the inputs of an instrumentation amplifier. An input circuit with a high immunity to EMI as well as a higher insensitivity to mismatch is presented and evaluated. The calculations are verified using circuit simulations.