Lowpass and Highpass Filter Calculator
These are parts value calculators I wrote to help design low and high pass analog active filters. They are unity gain op-amp based filters and are most useful in the audio frequency range. RF filters are best made with passive LC components. These parts calculators are based on formulas and tables from the book "Electronic Filter Design Handbook" by Arthur B. Williams. If you need a bandpass filter click here.
Paul Falsted has a great online interactive Java program to help visualize filter responses with different poles and response types. Use it to see how a particular filter will perform.
There are both 2 and 3 pole filter calculators. The 3 pole filters require 2 extra parts but have a steeper roll off in the stop band than 2 pole filters. In other words, they are better at rejecting the unwanted frequency range.
The low pass filter attenuates frequencies above the specified cut off value and the high pass attenuates frequencies below the cut off freq.
Select the desired filter type from the drop down menu. Butterworth is optimized for flat frequency response in the pass band. Chebyshev sacrifices flatness for a steeper roll off in the stop band. This version has 0.1 dB of passband ripple. Bessel filters sacrifice both flatness and roll off for linear phase in the pass band.
Next enter the desired value for the resistors or capacitors. All the resistors in the low pass filter are equal values. The high pass filters have equal value capacitors.
Enter the 3db cut off frequency and click the COMPUTE button and read the part values in the right hand panel. If the resulting parts values aren't optimum, try different values for the equal value resistors or capacitors and try again.
Op-Amps
The circuits below assume classic dual supply op-amps that use plus and minus power such as the LM348. If you use a single supply op-amp (eg: LM324) you will need to connect the grounds to a virtual ground, usually half way between real ground and Vcc. One way to do this is connect two 1K resistors in series between Vcc and ground. Connect a 10uF capacitor from the junction of the two resistors to ground. The junction of the two resistors is a virtual ground at Vcc/2 .
I like to use single supply quad cmos op-amps such as LMC660 or LMC6484 because of their rail to rail output swing and wide bandwidth. The dual version is LMC6032.
The source code is here. Report bugs via my contact page.
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