Influence of the Low-Pass Filter Order to the Attenuation Deviation at the High Frequencies

Abstract

The Sallen-Key low-pass biquad shows a decrease in attenuation at high frequencies because the real operational amplifier has a finite transit frequency. As a result, the course of the actual frequency response deviates from the ideal. Generally, this phenomenon occurs only in even-order filters in both Voltage Mode (VM) and Current Mode (CM) but is no longer observed for odd-order filters. When determining the order of the filter, it is always necessary to round the calculated value upwards, so it is discussed whether it is appropriate to round it up to the nearest odd number. Finally, this method of attenuation decrease correction is compared with the correction using another active element.