How does the sallen key filter work?

Nanaiah

Apr 8, 2017
7
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

• circuit1 (1).asc
6.6 KB · Views: 45

BobK

Jan 5, 2010
7,682
If I understand you, you want the filter to basically pass a DC level with some ripple. Ignoring the Sallen-Key question for now, you would do that by using a low pass filter with a corner point well below the frequency of the input.

What you have as input is full wave rectified sine wave. You would simply put a capacitor across the output in order to get DC with ripple as shown in your second diagram.

Bob

Nanaiah

Apr 8, 2017
7
If I understand you, you want the filter to basically pass a DC level with some ripple. Ignoring the Sallen-Key question for now, you would do that by using a low pass filter with a corner point well below the frequency of the input.

What you have as input is full wave rectified sine wave. You would simply put a capacitor across the output in order to get DC with ripple as shown in your second diagram.

Bob
Thank you for the reply sir. How do i select the capacitor value that i am going to use across the output? i tried using a basic low pass RCfilter. I am able to get the ripple but i need to increase the gain so that i can shift the waveform. How do i do this?

BobK

Jan 5, 2010
7,682
What are you trying to do? Tell us the source of the waveform amd the characteristics of the load.

Bob

Nanaiah

Apr 8, 2017
7
What are you trying to do? Tell us the source of the waveform amd the characteristics of the load.

Bob
2 waves one which is variable (0 degrees phase shift from Cx) and another which is kept constant (180 degrees out of phase from Cref) are amplified and fed to a multiplier where it is multiplied and then fed to a low pass filter which should provide me with a direct voltage at the output. The direct voltage at the output must vary as the variable Cx varies. This is the motive behind this circuit sir.

Arouse1973

Dec 18, 2013
5,178
Where does the pulsing or ripple requirements come from. I am a little confused in respect to your requirements.
Thanks

Nanaiah

Apr 8, 2017
7
Where does the pulsing or ripple requirements come from. I am a little confused in respect to your requirements.
Thanks
According to the paper attached below, I must get a direct voltage at the output after the filter sir.

Attachments

• A capacotance based solids concentration transducer with high immunity - Copy.pdf
679.2 KB · Views: 81

AnalogKid

Jun 10, 2015
2,893
Since the mixer is driven by a square wave on one input, it might be functioning as a full-wave rectifier. But it isn't a diode, so there would still need to be one before whatever kind of filter. This is assuming that the DC level he wants represents the peak value. If the RMS value would be better, then that would be driven without a series diode.

or something like that.

ak

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