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How to Create a Low-Pass Filter

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AnandY
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Re:Cost of software
AnandY   5/7/2013 7:24:46 AM
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Jon, thanks for the informative post. By entering our requirements, we will be provided with the filter design along with the schematic, component values, spice. Is this FilterLab software available for free download? If yes, then it will be a great help for self training projects.

mtripoli3
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More software...
mtripoli3   5/7/2013 9:56:38 AM
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Texas Instruments has a great filter design program called FilterPro Desktop. Incredibly well done and totally free (you will have to login to download). I've used it many times in the past; not only does it do "theoretical" but also has options for "real world" values. http://www.ti.com/tool/FILTERPRO

If you want something even more advanced and don't mind spending a few dollars ($49 to be exact) there is Filter Wiz Pro from Schematica. It's like the TI program on steroids (http://www.schematica.com/active_filters/fwpro.html). More choices for filter designs than you will ever need. 

The Microchip offering is good for what it costs (free) but you will find the other two much more useful as genuine tools.

Analog Bill
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When Filters Deviate from Models
Analog Bill   5/8/2013 12:43:37 PM
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Your filter plot highlights a common mistake in filter design. The deviation shown at 5 kHz on the plot may not be due just to the "protoboard" you used. Unless your modeling software contains a comprehensive model of the op-amps used, it may assume they're rather ideal ... especially with regard to gain-bandwidth or GBW. Since most filters, especially "anti-alias" filters, are useful because of their high attenuation beyond their cutoff frequency, this is an especially important consideration. Note that the first capacitor in each section has one end tied to an op-amp output. If the op-amp has infinite GBW, its output impedance will remain very low - even at high frequencies in the "cutoff region" of the filter's response. But if there's not enough GBW, this impedance will allow the first capacitor to feed signal around the stage, causing the "leveling out" of passband attenuation hinted at by the plot.

Jon Titus
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Re:Cost of software
Jon Titus   5/15/2013 5:41:54 PM
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The Microchip software is free.

Jon Titus
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Re: When Filters Deviate from Models
Jon Titus   5/15/2013 5:44:41 PM
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Hi, AnalogBill.  I would assume that Microchip has taken non-ideal operations into consideration in the FilterLab software because the program uses Microchip op amps and the company wants engineers to get a good model so they will buy Microchip devices.  Good point, though, about watching out for models that do not reflect real-world op-amp characteristics.  Thanks for your comments.

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