Key Concepts

This section compares the gain-versus-frequency responses of the basic low-pass, high-pass, and band-pass filters. It also describes and analyzes the three basic filter response characteristics and other filter parameters.

• Low-pass filter (LPF)
• Passive Low-Pass Filters
• Active Low-Pass Filter (LPF)
• Active High-Pass Filter (HPF)
• Active Band-Pass Filter (BPF)
• Microchip FilterLab® Filter Design Software
• Butterworth Filter Topology
• Chebyshev Filter Topology
• Bessel Filter Topology

Important Pointers

• The critical frequency of a filter is called the cut-off frequency and defines the pass band of the filter at the point where the output reaches -3 dB of the maximum.
• Feedback or bypass circuits are used to determine the frequency of operation, usually a simple R-C network. Each network is called a pole.
• The number of poles determines how rapidly the output decreases after hitting the critical frequency. This is called the roll-off rate.
• Butterworth response provides a very flat amplitude response, referred to as a maximally flat response.
• Chebyshev response provides a rapid roll-off and is characterized by variations or ripples in the pass band.
• Bessel response exhibits a linear phase characteristic and is used for filtering pulse waveforms due to minimal distortion.
• Order of a filter refers to the number of poles. (2nd order = 2 poles)
• A high-pass filter passes frequencies above the cut-off frequency.
• A band-stop filter is used to reject a range of frequencies.
• A band-stop filter is also referred to as a notch filter.