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.
New June 25 2012: Added 2 pole low and high pass filters with gain. They are the last two.
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. Click here for an active filter tutorial at Electronics Tutorials.
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.
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 signal 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 . Click here to see an example of a virtual ground. Just remember the op-amp signal inputs need to be biased somewhere between ground and Vcc. For example if you use 5 volts for Vcc the virtual ground should be in the 2 to 3 volt range. The unity gain low pass filters don't need an explicit virtual ground but the input signal must be confined to the a range between ground and Vcc.
June 25, 2012.
The next two filter calculators are new. They are inverting filters with gain. I've built several test circuits with the values computed and performance was as expected. The highpass version may require a small resistor in series with C1 if your op-amp doesn't tolerate capacitive loads and tries to oscillate.
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