Active low pass filters pass signals from low frequencies and reject signals from high frequencies. Like other types of active filters, they use active components to shape and stabilize the behavior of the filter, and to serve as a buffer interface for the driven component. There are two basic classes of active low pass 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 low pass filters have a continuous time operation. Other types of active low pass filters may also be available.
Filter characteristic is an important specification to consider when selecting active low pass filters. Choices include Bessel, Butterworth, Chebyshev (Cauer), elliptic, Gaussian, and Legendre. Bessel filters are active low pass 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 or Cauer 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.
Active low pass filters carry performance specifications for cutoff frequency, supply voltage, supply current, and operating temperature. Products also differ in terms of features and applications. For example, some active low pass filters feature protection against electrostatic discharge (ESD), short circuits, and over current conditions. Other devices are radiation-tolerant or can operate with only one power supply. In terms of applications, active low pass filters are often rated for general, audio/video, automotive, avionics, commercial, communications, computers, data acquisition, industrial, medical, or military applications. Some devices are also used in portable devices such as personal digital assistants (PDA) and cell phones.