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I have to design a Band Pass Filter for a project. The filter should let frequencies between 100 and 1000 Hz with a gain of -3db and should damp frequencies about 4 KHz. I am opting for two cascaded Sallen-Key Filters of second order.One is 100Hz highpass and the other is 1000Hz lowpass .Am I on the right track ? I am unable to decide how to choose the Resistance and Capacitor in order to achieve the desired result.Can someone please help? :)

Have a nice day!

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  • \$\begingroup\$ Cut-off frequency: 1/(2piRC) \$\endgroup\$ – Mast Jan 30 '17 at 8:36
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    \$\begingroup\$ I use this sim.okawa-denshi.jp/en/Sallen3tool.php online calculator. I found it with google. Have you tried google? It's a very useful resource, which allows you to find other useful resources on the interwebs. As a general rule (for typical opamps) resistors between 3k and 300k are good, apply thought before using resistors outside that range. \$\endgroup\$ – Neil_UK Jan 30 '17 at 8:36
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    \$\begingroup\$ What about frequencies BELOW 100 Hz? How much attenuation above 4 kHz? \$\endgroup\$ – LvW Jan 30 '17 at 8:45
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    \$\begingroup\$ Hmm, the extra frequency (100 Hz) value does not automatically imply a lowpass filter specification, rather a bandpass filter. It should be 0Hz for lowpass. If you want the response to be flat between these two frequencies then how flat? \$\endgroup\$ – KalleMP Jan 30 '17 at 8:46
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Yes, you can combine low pass and high pass filters to make a band pass filter.

At your relatively low frequencies, you can get better performance and accuracy by doing it digitally. Let's say you sample at about 5 kHz. Using a 4096 sample buffer gives you long tails on the filter kernel, and will yield a much sharper filter than you can reasonably do in analog. The convolution would require 5000 x 4096 = 20.5 M multiply-accumulates per second. There are many small DSPs or microcontrollers that can easily do this. Take a look at the Microchip dsPIC series, for example. A dsPIC EP family part can do 70 M MACCs/second.

What you need isn't pushing any limits of readily available parts, and you can even increase the sample rate if you have a aliasing problem, or widen the filter kernel for sharper pass band to stop band transitions.

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