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How does the sallen key filter work?

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Nanaiah

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I need to design a sallen key filter such that the output of the filter is a pulsating DC or ripple output when compared to the input. I tried implementing the below circuit but the output is not satisfactory. I am getting a sine wave instead of a pulsating dc or a ripple.
Frequency at the input of the filter is 20Khz.
Note: The input of the filter is the result of multiplication of sine and square wave.
Blue wave form - input of filter
Green wave form - output of the filter
and the desired ripple output image is also posted below.
 

Attachments

  • input and output waves.JPG
    input and output waves.JPG
    223.7 KB · Views: 121
  • desired output.JPG
    desired output.JPG
    17.2 KB · Views: 66

You forgot to mention the filter specification. I guess, fourier analysis clarifies why you only see 20 kHz (and a bit of 10 kHz) due to rectifier asymmetry, but not the n*20 kHz harmonics that make the actual waveform.
 

I have designed the filter for 20Khz frequency, with a gain of 2. Also i am not getting the desired output and the output i am getting at present is also not exactly twice than the input as it should be since the gain is 2. Do you think the filter output and the component values used are right?
 

Hi,

you just need to lower the cutoff frequency.

I assume you calculated with a cutoff frequency of 20kHz, but this give only poor attenuation.

If your input ripple is about 1.8V pp and - for example - you want an output ripple of 20mVpp then:
Expected attenuation factor is: 1.8V / 0.02V = 90
Now that you have a second order filter: take the square root of 90. Let´s say this is about 10.

This means your cutoff frequency needs to be 20kHz/10 which is 2kHz.

--> re calculate your filter values...then start a new simulation.


Klaus

- - - Updated - - -

Hi,

so you don´t want to smooth it?
You want to see the ripple?

Could you please define your expected output signal, wavefrom, amplitude.. maybe with a sketch..

Klaus

- - - Updated - - -

Hi,

to your question: "How does the sallen key filter work?"

Sallen Key is just a topology (circuit, hardware).

Sallen key doesn´t specify the filter type nor the filter characteristic...
You may build
* high pass filters
* low pass filters
* band pass filters
* band stop filters
*....

And you may build different filter characteristics with a sallen key filter, like:
* Butterworth
* Bessel,
* Linkwitz-Riley
*...

Klaus
 

KlausST said:
Hi,

you just need to lower the cutoff frequency.

I assume you calculated with a cutoff frequency of 20kHz, but this give only poor attenuation.

If your input ripple is about 1.8V pp and - for example - you want an output ripple of 20mVpp then:
Expected attenuation factor is: 1.8V / 0.02V = 90
Now that you have a second order filter: take the square root of 90. Let´s say this is about 10.

This means your cutoff frequency needs to be 20kHz/10 which is 2kHz.

--> re calculate your filter values...then start a new simulation.


Klaus

- - - Updated - - -

Hi,

so you don´t want to smooth it?
You want to see the ripple?

Could you please define your expected output signal, wavefrom, amplitude.. maybe with a sketch..

Klaus

- - - Updated - - -

Hi,

to your question: "How does the sallen key filter work?"

Sallen Key is just a topology (circuit, hardware).

Sallen key doesn´t specify the filter type nor the filter characteristic...
You may build
* high pass filters
* low pass filters
* band pass filters
* band stop filters
*....

And you may build different filter characteristics with a sallen key filter, like:
* Butterworth
* Bessel,
* Linkwitz-Riley
*...

Klaus
Click to expand...

Thank you for your reply sir. I want ripple output as posted in the picture.
 

Attachments

  • desired output.JPG
    desired output.JPG
    17.2 KB · Views: 59

The amplitude of the output varies with variable capacitance Cx at the input. I need to first get a ripple as shown in the uploaded image and then find a mathematical relationship between the input and the output of the filter.

I am using this topology for low pass filter sir. The AD633 multiplier and the low pass filter will act as a demodulation circuit which should provide me with a direct voltage at the output.
 

Hi,

Thank you for your reply sir. I want ripple output as posted in the picture.
Click to expand...
In the picture are two lines. Which one do you want?

You have been asked already to specify your desired signal.

If you want the red line: Then you need to specify the rate dU/ dt of the falling signal.

The red line is not made with a usual filter. You can not generate this with a Sallen Key topology.
It is just fed through an (ideal) diode
and the cathode of the diode is connected to a capacitor (other side to GND).
Then a constant discharge current gives the red signal shape.

Klaus
 

I'm confused. If the pictured waveform is what you actually want then there is one way to get it: rectify into a cap and provide a fixed load with a drain resistor. It's a non-linear waveform so it depends on the diodes and no filter type is going to provide that as an output.

(Beaten by Klaus)
 

As others have noted, a filter cannot give the waveform you want.

Here's an LTspice simulation of a peak hold circuit that gives a waveform similar to the one you show:
It consists of high speed op amp follower driving a peak hold circuit.
Diode D2 in the feedback circuit provides voltage compensation for D1.

Putting the D1 in the loop to generate an ideal rectifier was problematic due to oscillations from the capacitive load C1, so I went with a compensated open loop circuit.


Capture.PNG
 
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