Active filters are electronic filers that use active components such as voltage amplifiers or operational amplifiers. The amplifier shapes and stabilizes the behavior of the filter, and serves as a buffer interface to the driven component. Specifications for active filters include filter type, filter order, filter characteristic, and standards or certifications. Typically, active filters are specified as first, second, third, fourth, fifth, sixth, seventh or eighth-order filters. There are two main European Union (EU) certifications for active filters: RoHS and ELV. Restriction of Hazardous Substances (RoHS) requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of specific hazardous substances. End of Life Vehicles (ELV) is a similar directive, but for automotive products.
There are five basic types of active filters: low pass, high pass, band pass, band reject, and universal or programmable. Low pass filters attenuate frequencies below a frequency called the cut-off frequency. High pass filters attenuate frequencies above the cut-off frequency. Bandpass filters are active filters that are used to attenuate frequencies above and below a range of frequencies (i.e., the bandwidth or passband of the filter). Any signal with a frequency within the bandpass range passes through the filter. Any signal with a frequency outside the bandpass is reduced. Band reject filters (band stop filters, notch filters) are used to attenuate a range of frequencies and allow all other frequencies to pass. Universal filters can be configured by the user to function as a low pass, high pass, band pass, or band stop filter.
Filter characteristic is an important specification to consider when selecting active filters. Choices include Bessel, Butterworth, Chebyshev, and elliptic. Bessel filters are active filters with a passband that maximizes the group delay at zero frequency, thus showing a constant group delay in the passband. Butterworth filters are designed so that the frequency response is flat in the passband. Chebyshev filters feature a very steep roll-off, but have ripples in the passband. Elliptic filters (Cauer filters) exhibit equalized ripple in both the passband and the stopband. Active filters with other filter characteristics are also available.