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I need to implement a simple lowpass filter for a data acquisition circuit (using stretch conductive fabric as a resistive sensor). Various sources have recommended that the best way to go about this is to use filter design software.

I haven't found a filter design tool (calculator) that I like yet. The online ones I've found have been buggy/obnoxious. The best solution would be one with a nice GUI that works with Mac OS X. Alternatively a Unix/Linux solution could work. Windows is a no-go for me.

I found this list: http://www.circuitsage.com/filter.html

EDIT: I'm using this web-based one right now, specifically for active filters (which are more applicable for sensors as they are more stable with low frequencies).

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  • \$\begingroup\$ You should make it clear whether you are interested in designing digital filters or analog filters; and, for analog filters, active or passive. \$\endgroup\$ – nibot Feb 14 '12 at 13:34
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MATLAB is a pretty good place to start for filter simulation and design. There is a filter design toolbox that is pretty useful. However it does come at a cost.

For sensor applications Butterworth filters are generally better as they have a maximum flat passband (at the expense of phase response and roll off). That means that your signal amplitude will be flat throughout the frequency range.

Stay away from implementing a Sallen-Key topology active filter, it is very difficult to get all the components to be matched and maintain good accuracy, try doing a Monte Carlo simulation on a Sallen-Key circuit to get a better understanding.

Switched capacitor filters are good for steep roll-off and these are available in Butterworth filters. These do need a single pole before and after them to remove an aliasing due to the clock signal of the switching which occurs anywhere from 50 - 100 times greater than your cut-off.

Alternatively use a simple single pole RC filter (active or passive) and feed into a higher speed ADC and then you can use digital signal processing on an embedded platform or PC to perform decimation and analysis. This shifts cost and complexity from analog components to software and processing requirements.

Most importantly - ensure you comply with the Nyquist criteria and that you are sampling at at least twice the highest frequency, in practice, this means sample at four - 10 times your highest frequency, to allow for filter roll off well below your ADC resolution level at the Nyquist rate.

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    \$\begingroup\$ SciPy can do a lot of the same things as MATLAB, but for free. scipy.org/doc/api_docs/SciPy.signal.filter_design.html There's also more direct clones like Octave and FreeMat, but I don't know if they have all the filter design stuff built-in. \$\endgroup\$ – endolith Oct 16 '10 at 13:23
  • \$\begingroup\$ I have a copy of MATLAB – will check it out, though I'm not sure if it yields electrical diagrams. \$\endgroup\$ – terrace Oct 16 '10 at 18:30
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    \$\begingroup\$ you will not get electrical diagrams into or out of matlab. You have a few options, one of them being the filter design toolkit, which allows you to model filters directly, or perform a laplace transform of your circuit and model and simulate using laplace s parameter, then convert that back into electrical diagrams. Matlab will model the actual filter polynomials, and not the circuit, as filters are unviersal, and cross a few domains, where as eletrical circuits are exactly that. They can however realise filter polynomials! \$\endgroup\$ – smashtastic Oct 16 '10 at 19:16
  • \$\begingroup\$ there is also octave which free/open source and is (mostly) compatible with matlab code for filter design: gnu.org/software/octave \$\endgroup\$ – Mark Feb 10 '11 at 6:15
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Try Scilab, it comes with FIR and IIR filter design and simulation functions. It's free! There are Linux and MAC versions.

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Depends what you mean by "simple lowpass filter" and what you need the software to do. If you just want the -3 dB frequency for an LC filter:

Google Calculator

Wolfram Alpha

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Analog Devices has a filter wizard:

http://www.analog.com/en/amplifiers-and-comparators/products/dt-adisim-design-sim-tool/Filter_Wizard/resources/fca.html

It designs Sallen Key, Multiple Feedback, State Variable, and Biquad active filters. The attenuation of a low-pass filter is about 12 dB per octave per 2nd order stage (buffer between stages).

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