Analysis and Design of Analog Integrated Circuits
Analysis and Design of Analog Integrated Circuits
EMI-resistant CMOS differential input stages
IEEE Transactions on Circuits and Systems Part I: Regular Papers
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In negative-feedback amplifier design, electro-magnetic interference (EMI) behavior is usually completely disregarded. EMI can, e.g., result in detection of low-frequency envelope variations of the usually high-frequency interfering signals. If the detected signals end up in the pass band of the negative-feedback amplifier, they cannot be distinguished from the intended signal any longer, so the signal-to-error ratio (SER) is reduced. Several measures can be taken to prevent unacceptable reduction of the SER, like applying filters, chokes, etc. In this paper, however, circuit design aspects are investigated. It is assumed that interference reaches the amplifier input and that the SER has to be assured by a proper design of the negative-feedback amplifier. Since EMI is related to nonlinear distortion, it is a function of the loop gain of the negative-feedback amplifier. For a given electromagnetic (EM) environment it is therefore possible to calculate the minimum loop gain required to reduce EMI to acceptable levels without filtering. To illustrate this systematic design method a transimpedance amplifier is designed and built to properly function in interfering field strengths up to 30 V/m. Experimental results are in good agreement with theory.