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Questions on how to build a low-pass filter?

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Newbie level 4
Jul 26, 2007
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Sorry to bother you:

I just have problems when building a low-pass filter, the input signals are a 100 Hz 10V signal and a 5V 100k Hz one, they are combined at the input. I just need the 100 Hz signal at the output to turn on a microcontroller, (which I believe is low impedance). My questions are:

1. Do you have any recommened filter type: like second-oder, sallen-key or some other one? I just need as accurate as possible and not too complicated.

2. You have any recommended opamps? I need to build a through-hole one first.

3. as to bandwidth, it must follow the high freq signal or the low freq?

4. If the cap tolerance is a 20% one, is it a unstable factor? I don't want the 100 Hz signal to be eliminated at any time, it is an important signal.

Thank you very much.

here is a paper about how to build a very low cut-off frequency filter. maybe it will help you.

Since the 2 signals differ in frequency by a factor of 1000 and are about equal in amplitude, you probably don't need more than a single-pole RC filter. If the cut-off frequency is set at about 300 Hz, the filter will have almost no effect on the 100 Hz signal but will attentuate the 100 kHz signal by about 50 dB. Thus the output of the filter will be a 10V 100 Hz signal and a 15 millivolt 100 kHz signal. Almost any op amp will do since the frequencies are low and no gain is required. The op amp is only needed to buffer the signal from the assumed low impedance load. A tolerance of 20% on the capacitor will have little effect (the cut-off frequency will vary between 240 and 360 Hz, not enough to affect the 100 Hz signal level).

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