# Sallen Key Low pass filter wrong simulation !

#### desk7

Joined Jun 10, 2017
18
I realized the Sallen Key low pass filter in Orcad pspice but the magnitude bode plot I obtain from simulation is very different from the bode plot of transfer function. Can you help me understand what's wrong with this?

#### Attachments

• 91.5 KB Views: 3
• 53.9 KB Views: 4

#### desk7

Joined Jun 10, 2017
18
Ok, I realized 1/RC is pulsation not frequency. but I don't understand why at high frequency the response raises. Can you explain me this?

#### crutschow

Joined Mar 14, 2008
25,123
• the cut off frequency should be 1/RC= 333 Hz while from simulation seems to be lower. Why?
Shouldn't it be closer to 1/2πRC?
• at high frequency the response raises. why?
The response raises at higher frequencies where the high frequency gain of the op amp is not sufficient to provide proper feedback and the signal goes directly to the output through C1.
This is particularly noticeable if you are trying to filter a squarewave, as the fast edges come through as spikes.

If you don't want that to happen, you can add a passive RC low-pass filter at the input to make it a 3-pole filter, as seen here.
That provide sufficient passive rolloff that the response will not start to rise at higher frequencies (although the rate of rolloff may reduce to 6dB/octave).
That also eliminates any output spikes from a squarewave input.

#### RBR1317

Joined Nov 13, 2010
551
why at high frequency the response raises. Can you explain me this?
Check out reference #3 in the Wikipedia article provided by @shteii01.

#### desk7

Joined Jun 10, 2017
18
Reading here and there, I've found the sentence "At the frequency where the amplifier’s output impedance is greater than the impedance of the resistor (R1), the feedback looks inductive and the response increases at a rate of 20 dB/decade."
What does it mean "looks inductive"? How is it possible?

#### crutschow

Joined Mar 14, 2008
25,123
Reading here and there, I've found the sentence "At the frequency where the amplifier’s output impedance is greater than the impedance of the resistor (R1), the feedback looks inductive and the response increases at a rate of 20 dB/decade."
What does it mean "looks inductive"? How is it possible?
Not sure.

#### OBW0549

Joined Mar 2, 2015
3,452
What does it mean "looks inductive"? How is it possible?
When we say something "looks inductive" we mean that its impedance increases with increasing frequency, just like an inductor.

This is common in op amp circuits; see here, here and here.

Last edited: