25-07-2012, 11:27 AM
Filter Design in Thirty Seconds
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Introduction
This document is intended for designers that do not have the time to check filter theory in old
college textbooks—and try to translate transfer equations into something that can be put into
production. This is like looking at the back of the textbook for the answer. Speaking of the back
of the book—Appendix B contains a brief introduction to the filter circuits given here, and the
limitations of this quickie approach to design.
To design a filter, four things must be known in advance:
• The power supplies available: positive / negative—or only positive (single supply)
• The frequencies that need to be passed, and those that need to be rejected.
• A transition frequency, the point at which the filter starts to work—or—a center frequency
around which the filter is symmetrical.
• An initial capacitor value—pick one somewhere from 100 pF for high frequencies to 0.1 μF
for low frequencies. If the resulting resistor values are too large or too small, pick another
capacitor value.
Wide Band Pass Filter
This is nothing more than cascaded Sallen-Key high pass and low pass filters. The high pass
comes first, so energy from it that stretches to infinite frequency will be low passed.
Notch Filter
This is the Fliege Filter topology, set to a Q of 10. The Q can be adjusted independently from
the center frequency by changing R1 and R2. Q is related to the center frequency set resistor
by the following:
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications,
enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
orders and should verify that such information is current and complete. All products are sold subject to TI’s terms
and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
[attachment=28823]
Introduction
This document is intended for designers that do not have the time to check filter theory in old
college textbooks—and try to translate transfer equations into something that can be put into
production. This is like looking at the back of the textbook for the answer. Speaking of the back
of the book—Appendix B contains a brief introduction to the filter circuits given here, and the
limitations of this quickie approach to design.
To design a filter, four things must be known in advance:
• The power supplies available: positive / negative—or only positive (single supply)
• The frequencies that need to be passed, and those that need to be rejected.
• A transition frequency, the point at which the filter starts to work—or—a center frequency
around which the filter is symmetrical.
• An initial capacitor value—pick one somewhere from 100 pF for high frequencies to 0.1 μF
for low frequencies. If the resulting resistor values are too large or too small, pick another
capacitor value.
Wide Band Pass Filter
This is nothing more than cascaded Sallen-Key high pass and low pass filters. The high pass
comes first, so energy from it that stretches to infinite frequency will be low passed.
Notch Filter
This is the Fliege Filter topology, set to a Q of 10. The Q can be adjusted independently from
the center frequency by changing R1 and R2. Q is related to the center frequency set resistor
by the following:
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications,
enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
orders and should verify that such information is current and complete. All products are sold subject to TI’s terms
and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.