Implementation of low pass filters with real opamps: Where is the limit?

Abstract

Active filters based on RC components together with operational amplifiers are quite common functional blocks to be met across different branches of electronics design. During the past, their topology has settled on several well-known and effective solutions. As an example, Sallen-Key or Multiple Feedback topology can be named. The designers can find a huge number of “cookbooks”. According to them they can find help with designing their filters. However, the theory described in these cookbooks usually fits ideal operational amplifiers in a good manner, but, unfortunately, the real parameters of the operational amplifiers may spoil the real behaviour of the design significantly. This problem occurs especially at very high frequencies that are typical for injection of electromagnetic interferences into low frequency signal lines. The authors of this paper have tested several antialiasing (low-pass) filter designs and provide results of measurement at real filters compared to the simulations. Moreover, they provide a discussion on why the results of the simulation do not fit the real behaviour of the filters and suggest improvements in the design of the low pass filters in that way so their utility value was improved. © 2019, Danube Adria Association for Automation and Manufacturing, DAAAM. All rights reserved.