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Is using a second order bandpass filter less effective than using a second order high pass filter and a second order low pass filter one after the other?

I tried a bandpass filter to filter out frequencies between 0.5Hz and 15Hz, and found that there was still quite a bit of noise.

Then I used a second order high pass filter and a second order low pass filter one after the other and found that that eliminated more noise than a bandpass filter.

Why would a bandpass be worse than a high pass and low pass one after the other?

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  • \$\begingroup\$ Were these filters Butterworth, Bessel, Chebyshev, Elliptic? (If you don't know, they were likely Butterworth since those have the simplest characteristic to understand) \$\endgroup\$ – ThreePhaseEel Nov 28 '16 at 23:52
  • \$\begingroup\$ I believe they were Butterworth (I was taught that they have Sallen-Key configuration - same thing?) They look like the ones at this link: electronics-tutorials.ws/filter/second-order-filters.html \$\endgroup\$ – EmilyF Nov 29 '16 at 0:03
  • \$\begingroup\$ Yeah, that's a Butterworth response curve on the site, alright! \$\endgroup\$ – ThreePhaseEel Nov 29 '16 at 0:04
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A second order bandpass filter has a first order roll off rate on each side of the pass band. A second order low pass filter in series with a second order high pass filter has second order roll off rates on both sides of the pass band. The two are not equivalent.

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Then I used a second order high pass filter and a second order low pass filter one after the other and found that that eliminated more noise than a bandpass filter.

That now becomes equivalent to a 4th order bandpass filter and hence will generally be better than a 2nd order bandpass filter.

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