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I'd like to build two filters:

  1. One that rejects all frequencies except the fundamental.
  2. One that responds to AC currents between 25 and 400Hz. This would be a band pass filter.

Even though I'm planning to learn how to do this by software, when it comes to op amps, how do I know when to use a first order or second order filter? Or multiple orders.

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  • \$\begingroup\$ Almost the same as using digital filters, the attenuation per decade. \$\endgroup\$ Mar 3 '20 at 23:28
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    \$\begingroup\$ You usually spend a LOT more time describing what you are doing, and why, and what's more important, and what's less important. Many of those things won't seem directly related, but are. And there are bandpass filters made from the same piece of cloth, so to speak, and other bandpass filters made from a lowpass and a highpass, with detailed required to decide which option to choose. \$\endgroup\$
    – jonk
    Mar 4 '20 at 7:08
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    \$\begingroup\$ 1. is impossible and 2. is under-constrained. When designing filters, you need to adopt the appropriate limitations in what they really can achieve. For instance in yout (1), a filter that only passes one frequency will take an infinite time to respond. \$\endgroup\$
    – Andy aka
    Mar 4 '20 at 8:19
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...when it comes to op amps...

Opamps really have nothing to do with the filter's order.

Using opamps is one of the ways that a filter can be implemented. There are also RC, LC, RLC and resonator based filters that use no opamps at all.

You should read a book on filter design and learn about designing a prototype filter based on a realistic specification ("reject all" isn't a realistic requirement). During the design of the prototype filter the order of the filter is determined.

When the prototype filter meets the specification then it can be translated into a design which can be implemented, perhaps using opamps.

If all this goes too far for you then find an existing design and re-build that.

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