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I want to design a very low frequency (<1Hz) high pass filter. I was thinking of using a 2 pole sallen-key opamp design with a pair of r's and c's. Is there anything special I need to consider when choosing component values/types. It looks as though the caps will need to be in the 100's of µF range.

Thanks

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  • \$\begingroup\$ You can make the caps bigger or you can make the resistors bigger \$\endgroup\$
    – endolith
    Feb 10, 2011 at 16:53
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    \$\begingroup\$ Not sure where the output of your filter leads, but could you use a digital filter? \$\endgroup\$
    – Nick T
    Feb 10, 2011 at 18:21

3 Answers 3

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It depends on how accurately you are expecting to control your cutoff frequency. A few point come to mind ...

  • Tolerance

High value electrolytic capacitors have wide tolerances, indeed cheap ones can be as wide as +100%/-50%. You won't get much better than ±10% and stability could still be an issue. Solid electrolyte (aluminium/tantalum) have better stability but will be much more expensive.

  • Leakage

Electrolytic capacitors will have a finite leakage current which will produce dc offsets given that your resistor values will also be high.

  • Polarization

Make sure that your circuit biasing keeps capacitors correcty polarized.

  • Charge/Discharge

High value capacitors will have to charge & discharge somehow if there is a non-zero dc bias (ie single rail). This will cause turn-on 'thumps' as the circuit settles-down which may take a many (tens of) seconds. At turn-off, the capacitors may discharge into the op-amp causing damage although given that your resistor values will also be large, this is less likely to be a problem.

The lowest frequency filter I have ever built was a 5Hz ±20% two-stage S&K (4 pole) maximally flat design which worked perfectly well.

You might also want to look at a Gyrator circuit to simulate a high value inductor.

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I would vote for 10-100uF ceramic capacitor with large resistor + temperature controlled chamber OR ceramic cap + programmable resistor to compensate for temperature changes.

This should be more stable over time, but less stable over temperature. Also, ceramic caps should have way less leakage.

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It's very difficult (or even impossible) to do such filter with a simple 2-pole analog filter, depending on your requirements (fp, fs, As). Download and install FilterPro (from Texas Instruments) and test some configurations. That's a very good and simple program. You enter the requirements of your filter, click on a button and then you get the circuit and the values for resistors and capacitors automatically.

Probably, with a digital filter you can achieve much better results. Take a look at IIR filters. What's the application of your high pass filter? Maybe DC removal? If so, google for IIR+DC removal, you'll get plenty of results.

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