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Author Topic: Active RC filter design  (Read 1000 times)
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promach
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« on: March 25, 2016, 03:10:42 15:10 »

I have a school project on active RC filter design.

I am posting it here just to get some suggestion on how to proceed with my design.

Since they are active filters, I actually have my own transistor-level opamp to work on.
I am starting from scratch to get my filter schematics to work according to the spec given.

I am actually learning how to design butterworth LP filter.

Besides, how would I include the signal source impedance in my filter design calculation ?

Any snippets from books would be much appreciated.

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Signal
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« Reply #1 on: March 25, 2016, 08:50:02 20:50 »

Besides, how would I include the signal source impedance in my filter design calculation ?
When finally you will get that secret schema of Sallen-Key low pass filter you will be smart enough to handle a pair of connected resistors. believe in yourself Wink
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Gallymimu
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« Reply #2 on: April 03, 2016, 04:12:56 04:12 »

source impedance is just a series resistor on the input.  i.e. idea voltage source with resistor in series with output.
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johnf
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« Reply #3 on: April 19, 2016, 04:58:37 04:58 »

The best book I ever had on active filter design. Get it if you can.
"Active Filter Cookbook" by Don Lancaster, ISBN 0-672-21168-8

I bought mine back in the early 1980's and it taught me everything I have ever used about active filters. It's now dog eared and full of annotations. It's easy to follow and the maths is well presented and easy for most to understand.

TI offer a good active filter design application for free called FilterPro. Use it to check your designs, or even to do the designs if you are in a hurry.
http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-filterdesignervanity-en

Hope this helps.
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PICker
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« Reply #4 on: April 19, 2016, 07:14:56 07:14 »

You can try to design your own active filter by using online software like webench filter designer:
http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-filterdesignervanity-en
or offline software TI FilterPro:
http://www.ti.com/filterpro-dt
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witsanukai
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« Reply #5 on: April 19, 2016, 10:54:52 10:54 »

You can try to study from here, then draw the schematic in any simulation program ex.proteus
Then try to give an input and sweep frequesncy to check the output response will make you more imagine and understanding.
Input impedance base on your resistor termination at input.
 Smiley

https://en.wikipedia.org/wiki/Butterworth_filter

http://www.electronics-tutorials.ws/filter/filter_8.html
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crunx
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« Reply #6 on: April 23, 2016, 08:52:34 20:52 »

I have used with success the filter calculators on:

http://sim.okawa-denshi.jp/en/Fkeisan.htm

Those might be helpful in this case, too.

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UncleBog
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« Reply #7 on: April 24, 2016, 06:52:32 18:52 »

Analog Devices online design tools, including an analog filter wizard are here: http://www.analog.com/en/design-center/design-tools-and-calculators.html.
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promach
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« Reply #8 on: April 30, 2016, 08:01:51 08:01 »

I have a bandpass filter as follows:



But I would like calculate the low pass corner frequency which I am stuck at right now.

The transfer function I got for the low pass sallen-key is

2484375/(s^2 + 1762.5 + 1562500)

after normalizing the nominator and last term of the denominator to "1", I found that it is not an Butterworth filter design.

I would like to seek your advice to find out the LP corner frequency.

Thanks!
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bigtoy
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« Reply #9 on: April 30, 2016, 11:47:20 23:47 »

To my experience, there are 2 ways to determine this. The easy way & the hard way.

For me, the hard way is to calculate it all mathmatically. I've forgotten much of my college math for this stuff.

The easy way (for me, I do this quite often) is to quickly put the circuit in Linear Technologies free LTspice program & simulate it. Then not only can you see the corner frequency, you can also see the entire frequency response (which for a multistage filter like this one is not always a simple "linear" response), you can see attenuation or gain levels, you can easily adjust component values (ie tolerances) and see the effect of that, etc.
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Gallymimu
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« Reply #10 on: May 01, 2016, 06:54:38 06:54 »

I have a bandpass filter as follows:


But I would like calculate the low pass corner frequency which I am stuck at right now.

The transfer function I got for the low pass sallen-key is

2484375/(s^2 + 1762.5 + 1562500)

after normalizing the nominator and last term of the denominator to "1", I found that it is not an Butterworth filter design.

I would like to seek your advice to find out the LP corner frequency.

Thanks!

your equation looks wrong. do you mean 2484375/(s^2 + 1762.5s + 1562500)

Posted on: May 01, 2016, 06:41:25 06:41 - Automerged

I have a bandpass filter as follows:


But I would like calculate the low pass corner frequency which I am stuck at right now.

The transfer function I got for the low pass sallen-key is

2484375/(s^2 + 1762.5 + 1562500)

after normalizing the nominator and last term of the denominator to "1", I found that it is not an Butterworth filter design.

I would like to seek your advice to find out the LP corner frequency.

Thanks!

This is what I came up with from you numbers:

numerator = pi/2 * 1562500
sqrt(1562500) = 1250 = omega^2
fc = 200Hz
alpha = 442 for Q = 1/sqrt(2) for a butterworth
alpha = 881 your alpha

where sallen key equation =

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2N5109
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« Reply #11 on: May 05, 2016, 04:00:21 16:00 »

Greetings promach:  

The transfer function you put has complex conjugate poles at:  f=140+j141 Hz and 140-j141 Hz,  calculated from solving quadratic equation in denominator.*  Complex poles are close to origin so response being lowpass and has 3 dB down from peak (occurring at 141 Hz) at ~sqrt(2)*141 = 200 Hz.  

Advice:   Active filter is easy to design but beware the noise  added.   Is not good for low noise applications.  Get very good prediction of output noise spectrum using LTSPICE.  Need a discrete JFET or NPN LNA ahead of the filtering stages so  is good to keep the filter gains low.

--2N5109

* quadratic formula is x1, x2 = ( -b +/- sqrt(b^2 - 4*a*c)) / (2*a) gives roots of a*x^2 + b*x  + c,
i.e.,  a*x^2 + b*x  + c = (x-x1)*(x-x2).  

« Last Edit: May 05, 2016, 04:31:34 16:31 by 2N5109 » Logged
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