Second order passive low pass filter design

Audioguru again

Joined Oct 21, 2019
5,432
You need an active second-order lowpass Butterworth filter. The inductor in your passive filter will pickup all kinds of electrical hum and other interference.
 

crutschow

Joined Mar 14, 2008
31,150
Below is the LTspice simulation of a single opamp, 20kHz, 3-pole Sallen-Key type filter with a Butterworth response.
The circuit adds an addition resistor and capacitor at the input to a normal Sallen-Key filter configuration to give a 3-pole, 18dB/octave rolloff above 20kHz for improved noise rejection.

1630687905117.png
 

Ian0

Joined Aug 7, 2020
6,733
Below is the LTspice simulation of a single opamp, 20kHz, 3-pole Sallen-Key type filter with a Butterworth response.
The circuit adds an addition resistor and capacitor at the input to a normal Sallen-Key filter configuration to give a 3-pole, 18dB/octave rolloff above 20kHz for improved noise rejection.
He said second order. Don't give him any more phase shift!
My guess is that he's hoping to make a switched-mode supply with the feedback controlled by a processor.
 

MrAl

Joined Jun 17, 2014
9,638
Sorry above I have attached wrong one. Now you can see above post. Yes I have uohm at the source but I used high resistance value to reduces the peak energy at the red curve. If I keep uohm, then see how it will be the graph.
View attachment 247191
Hello,

What exactly are you trying to accomplish here?

If you are dealing with an ADC where you need an input filter then one of the most important things is the settling time. From the other thread, here are some examples of how the settling time relates to the damping factor and ringing.

The first pic (04) shows the time responses for various damping factors, the second pic (07) shows the normalized (to a gain of 1 in amplitude and to a frequency of w=1) frequency responses where the plot with the narrowest width is the one with the smallest damping factor (0.01).

From these two plots you can see the time domain comparisons make it easier to judge what you can expect with a filter on the input of an analog to digital converter, but the frequency response with the narrowest bandwidth is the same filter as the time domain plot with the wildest ringing.
So sharper bandwidth, more ringing, wider bandwidth, less ringing. You can also judge the difference in settling times from the time domain plots.

Note that the damping factor changes with load and with the driving point impedance, so it is best to specify those two first before designing the filter. The two are interactive so it is best to spec them both first.
 

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Ian0

Joined Aug 7, 2020
6,733
We did explain different types of filter as early as post #2, and 47 posts later he still hasn't told us, nor what his application is.
 

Papabravo

Joined Feb 24, 2006
19,617
We did explain different types of filter as early as post #2, and 47 posts later he still hasn't told us, nor what his application is.
Is that because we haven't been told about the double secret NDA we are all supposed to have signed in order for those important details to be revealed?
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
318
I also posed a question and your response would seem to indicate that you have none. I could be wrong, maybe you know or suspect something less than obvious.
Since I'm very beginner to electronics. When you asking me more detail of the project I couldn't provide you much information because I don't have any circuits to show you, still now I have a over view of the project only. You guys are asking every time show the circuit. How can I show you if I don't have any circuit to share with you. I understand that you guys are trying to help me but I'm feeling bad since I'm unable to provide you the information. Sorry for the inconvenience guys.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
318
Below is the LTspice simulation of a single opamp, 20kHz, 3-pole Sallen-Key type filter with a Butterworth response.
The circuit adds an addition resistor and capacitor at the input to a normal Sallen-Key filter configuration to give a 3-pole, 18dB/octave rolloff above 20kHz for improved noise rejection.

View attachment 247197
May I know why your showing the curve at -3db. Do we need to measure always at -3db?.
 
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