Active band reject filters are tuned circuits that prevent the passage of signals within a specified band of frequencies. These devices are also known as bandstop or notch filters. There are two basic classes of active band reject filters: switched capacitor and continuous. Each type of device is available with first, second, third, fourth, fifth, sixth, seventh or eighth filter order. Switched capacitor filters are clocked devices. The input signal is sampled at a high rate and processed discretely instead of on a continuous-time basis. Continuous active band reject filters have a continuous time operation. Other types of active band reject filters are also be available.
Active band reject filters carry performance specifications for cutoff or center frequency, bandwidth, power dissipation, operating temperature, supply voltage and supply current. Like other types of active filters, they also differ in terms of features and applications. For example, some active band reject filters feature protection against electrostatic discharge (ESD), short circuits, and over current conditions. Others are radiation-tolerant or can operate with only one power supply. In terms of applications, active band reject filters may be rated for audio/video, automotive, avionics, commercial, communications, computers, data acquisition, general industrial, medical, or military applications. Some devices are also used in portable devices such as cell phones and personal digital assistants (PDA).
Filter characteristic is an important specification to consider when selecting active band reject filters. Choices include Bessel, Butterworth, Chebyshev, elliptic, Gaussian, and Legendre. Bessel filters are active band reject 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 exhibit equalized ripple in both the passband and the stopband. Gaussian filters produce no overshoot in response to an input step. Instead, 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.