Filter Design in Thirty Seconds

Bruce Carter, Texas Instruments
ABSTRACT
Need a filter ı fast? No theory, very little math ı just working filter designs, and in a hurry? This is the right article!

INTRODUCTION

This document is intended for designers that don't have the time to chase down filter theory in old college textbooks - and try to translate those ugly 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 ı 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 ı 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 ı or ı a center frequency around which the filter is symmetrical.
An initial capacitor value ı pick one somewhere from 100 pF for high frequencies to 0.1 mF for low frequencies. If the resulting resistor values are too large or too small, pick another capacitor value.

Ready? Let's design the filter. Pick the frequencies that need to be passed (shaded area) from one of the 6 choices that follow:

Figure 1. Low Pass Filter ı Go to Section 2

Figure 2. High Pass Filter ı Go To Section 3

Figure 3. Narrow (Single Frequency) Band Pass ı Go To Section 4

Figure 4. Wide Band Pass ı Go To Section 5

Figure 5. Notch Filter ı Single Frequency Rejection

Figure 6. Band Reject Filter

Low Pass Filter

Figure 7. Low Pass Filter for ıSupplies

Figure 8. Low Pass Filter for a Single Supply

Design Procedure:
- Pick C1________
- Calculate C2 = C1 * 2: __________
- Calculate R1 and R2 =: __________ (pick a standard value from Appendix A).
For Single Supply Case Only:
- Calculate Cin = Cout = 100 to 1000 times C1 (not critical): __________
DONE!
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