filter design in thirty seconds
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Application ReportSLOA093 December 2001
1
Filter Design in Thirty SecondsBruce Carter High Performance Analog
ABSTRACT
Need a filterfast? No theory, very little mathjust working filter designs, and in a hurry?This is the right document.
Contents1 Introduction ..................................................................................................................................22 Low Pass Filter .............................................................................................................................43 High Pass Filter ............................................................................................................................54 Narrow (Single Frequency) Band Pass Filter..............................................................................65 Wide Band Pass Filter ..................................................................................................................76 Notch (Single Frequency Rejection) Filter..................................................................................87 Band Reject Filter .........................................................................................................................9
Appendix AStandard Resistor and Capacitor Values.................................................................12Appendix BFilter Notes (for the More Technically Minded) ........................................................13
FiguresFigure 1. Low Pass Filter.................................................................................................................2Figure 2. High Pass Filter................................................................................................................2Figure 3. Narrow (Single Frequency) Band Pass...........................................................................3Figure 4. Wide Band Pass ...............................................................................................................3Figure 5. Notch FilterSingle Frequency Rejection .....................................................................3Figure 6. Band Reject Filter ............................................................................................................4Figure 7. Low Pass Filter for Supplies ........................................................................................4Figure 8. Low Pass Filter for a Single Supply................................................................................4Figure 9. High Pass Filter for Supplies .......................................................................................5Figure 10. High Pass Filter for a Single Supply...............................................................................5Figure 11. Narrow Band Pass Filter for Supplies .........................................................................6Figure 12. Narrow Band Pass Filter for a Single Supply.................................................................6Figure 13. Wide Band Pass Filter for Supplies .............................................................................7Figure 14. Wide Band Pass Filter for a Single Supply.....................................................................7Figure 15. Narrow Band Pass Filter for Supplies .........................................................................8Figure 16. Narrow Band Pass Filter for a Single Supply.................................................................9Figure 17. Band Reject Filter for Supplies ..................................................................................10Figure 18. Band Reject Filter for a Single Supply..........................................................................10
SLOA093
2 Filter Design in Thirty Seconds
1 IntroductionThis document is intended for designers that do not have the time to check filter theory in oldcollege textbooksand try to translate transfer equations into something that can be put intoproduction. This is like looking at the back of the textbook for the answer. Speaking of the backof the bookAppendix B contains a brief introduction to the filter circuits given here, and thelimitations of this quickie approach to design.
To design a filter, four things must be known in advance:
The power supplies available: positive / negativeor 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 workora center frequencyaround which the filter is symmetrical.
An initial capacitor valuepick one somewhere from 100 pF for high frequencies to 0.1 Ffor low frequencies. If the resulting resistor values are too large or too small, pick anothercapacitor value.
Ready? Lets design the filter. Pick the filter type from one of the following 6 options thatrepresents the frequencies to be passed (shaded area):
Figure 1. Low Pass FilterGo to Section 2
Figure 2. High Pass FilterGo to Section 3
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Filter Design in Thirty Seconds 3
Figure 3. Narrow (Single Frequency) Band PassGo to Section 4
Figure 4. Wide Band Pass Go to Section 5
Figure 5. Notch FilterSingle Frequency Rejection
SLOA093
4 Filter Design in Thirty Seconds
Figure 6. Band Reject Filter
2 Low Pass Filter
R2Vin
Supply
C1
C2
+
R1
+Supply
Vout
Figure 7. Low Pass Filter for Supplies
+Supply

+
+Supply
R4 100 k
R3 100 k
Cout Vin
C1Cin
C2
Vout R2R1
Figure 8. Low Pass Filter for a Single Supply
Design Procedure:
Pick C1: __________
Calculate C2 = C1 * 2: __________
SLOA093
Filter Design in Thirty Seconds 5
Calculate R1 and R2 = Frequency*C** 122
1
: __________ (pick a standard value from
Appendix A).
For the single supply case only:
Calculate Cin = Cout = 100 to 1000 times C1 (not critical): __________
DONE
3 High Pass Filter
VoutC2
R1
C1Vin
Supply
+
+SupplyR2
Figure 9. High Pass Filter for Supplies
R1
R2 +Supply
C1 Vout+
C2
Cout
+Supply / 2
Vin
Figure 10. High Pass Filter for a Single Supply
Design Procedure:
Pick C1 = C2: __________
Calculate R1: Frequency*C** 12
1
: ___________ (pick a standard value from
Appendix A).
SLOA093
6 Filter Design in Thirty Seconds
Calculate R2: Frequency*C** 122
1
: __________ (pick a standard value from
Appendix A).
For the single supply case only:
Calculate Cout = 100 to 1000 times C1 (not critical): __________
DONE
4 Narrow (Single Frequency) Band Pass FilterNOTE: These circuits include a gain of 10 (20 dB) at the center frequency.
R4
C2
+Supply
Supply
R1C1
+
R2
Vin
Vout
R3
Figure 11. Narrow Band Pass Filter for Supplies
R3Cout
+
CinC2
R1
R4
C1R2
Vout
+Supply
+Supply / 2
Vin
Figure 12. Narrow Band Pass Filter for a Single Supply
Design Procedure:
Pick C1 = C2: __________
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Filter Design in Thirty Seconds 7
Calculate R1 = R4: Frequency*C** 121
: ___________ (pick a standard value from
Appendix A).
Calculate R3 = 19 * R1
Calculate R2 = 19
1R
For the single supply case only:
Calculate Cin = Cout = 100 to 1000 times C1 (not critical): __________
DONE
5 Wide Band Pass FilterNOTE: The start and ending frequencies of the band should be at least five times different.
R2
+Supply
SupplySupply
+
C1
C2
R2
C1Vin
C2
+
R1
R1
+Supply
Vout
Figure 13. Wide Band Pass Filter for Supplies
C1
Cout
C2
R1 R2
R1
C2
C1
+
+Supply / 2
+Vin
R2 +Supply +Supply
Vout
Figure 14. Wide Band Pass Filter for a Single Supply
Design Procedure:
Go to Section 3, and design a high pass filter for the low end of the band.
SLOA093
8 Filter Design in Thirty Seconds
Go to Section 2, and design a low pass filter for the high end of the band.
For the single supply case only:
Calculate Cin = Cout = 100 to 1000 times C1 in the low pass filter section (not critical):__________
DONE
6 Notch (Single Frequency Rejection) Filter
R3
+
R2
Vout
Supply
R4
R1
Supply
+
C2
R6 100 k
R5 100 k
C1
+Supply
Vin
+Supply
Figure 15. Narrow Band Pass Filter for Supplies
SLOA093
Filter Design in Thirty Seconds 9
R4
+
R3
+Supply / 2
R1
+Supply
R2
C1
+Supply
R5 100 k
C2
Vin Vout
+
Cin Cout
R6 100 k
Figure 16. Narrow Band Pass Filter for a Single Supply
Design Procedure:
Pick C1 = C2: __________
Calculate R3 = R4: Frequency*C** 121
: ___________ (pick a standard value from
Appendix A).
Calculate R1 = R2 = 20 * R3
For the single supply case only:
Calculate Cin = Cout = 100 to 1000 times C1 (not critical): __________
DONE
7 Band Reject FilterNOTE: The start and ending frequencies of the band to be rejected should be at least fifty timesdifferent.
SLOA093
10 Filter Design in Thirty Seconds
+SupplyR2
100 k
+
C1 +Supply
R1 Vi
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