Signal conditioning II: analog filters

Abstract

In a previous article we studied how to adapt the main control signals of a power converter to make them suitable for an analog-to-digital transformation. However, coupled to these signals, interferences, spikes, or noise generated by electronic devices such as the own operational amplifiers (op amps) of the adaptation stage, can be encountered. The signal-to-noise ratio (S/N) expresses the importance of noise with respect to the main signal. This S/N parameter is recommended to be high. This way, noise can be almost neglected. There is a system that, among other features, increases this ratio and it is known as filter. Filters can be passive or active, analog or digital, but in the end, they have key purposes: eliminate frequency margins in the signal that may lead to wrong interpretation of information (modulation techniques, aliasing, etc.) and reduce the signal noise as much as possible. In this article the reader will find a study about signal filtering in power converters, focusing on active filters made of op amps. We will see different options and simulations regarding the design of an analog filter and how to choose one circuit over another. Moreover, the concept of synchronous sampling will be introduced, which will help us to understand the importance of a proper selection of the corner frequency of the filter.