Active high pass filters pass signals from high frequencies and reject signals from low frequencies. There are two basic types of products: switched capacitor and continuous. Both are available with first to eighth order filtering, and may meet requirements from European Union (EU) directives such as Restriction of Hazardous Substances (RoHS) and End of Life Vehicle (ELV). Switched capacitor active high pass filters are clocked devices. The input signal is sampled at a high rate and processed discretely instead of on a continuous-time basis. By contrast, continuous filters have a continuous time operation. Other types of active high pass filters are also be available.
Active high pass filters carry performance specifications such as operating temperature, cutoff or center frequency, supply voltage and supply current. Like other types of active filters, they also differ in terms of features and applications. For example, some active high pass filters feature protection against short circuits, electrostatic discharge (ESD), or over current conditions. Others are radiation-tolerant or can operate with only one power supply. In terms of applications, active high pass filters are often rated for audio/video, automotive, avionics, commercial, communications, computers, data acquisition, general, industrial, medical, or military applications. Some active high pass filters are also used in portable devices such as digital cameras and cell phones.
Filter characteristic is an important specification to consider when selecting active high pass filters. Bessel, Butterworth, Chebyshev, elliptic, Gaussian, and Legendre are common choices. Bessel filters are active high pass filters whose passband 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. Gaussian filters produce no overshoot in response to an input step. They optimize the rise and fall times. Legendre filters are designed to produce the maximum roll-off rate for a given order and a flat frequency response in the passband. Active high pass filters with other, unlisted filter characteristics are also available.