# Help understanding 60 Hz low pass filter behavior in oscilloscope

I have been testing this circuit in simulation

which is supposed to be a low pass 60Hz filter, as far as i am concerned the simulation works as intended in simulation starts filtering frequencies above 60Hz and after this the amplitude starts going down, i can use a bode plotter to confirm this and change the frequency of the function generator while i see how the sin wave gets less and less amplitude the higher the frequency

However when building this same circuit in the protoboard and using the oscilloscope at the output of the OPAMP something odd happens, i uploaded THIS VIDEO where you can see its behavior in the oscilloscope

The blue signal is the input while the yellow is the output

I start a with a 5v 30Hz signal, and slowly increase the frequency, after 60Hz the output starts increasing the voltage up to 10v and only after reaching 300Hz, it starts filtering the signal, why is this happening? is it normal?

• Do you know the value tolerances of the capacitors you are using in the real circuit? Many low cost capacitors (such as those commonly used for bypass filtering) will often have very wide tolerances. (For example a 0.1uf capacitor is one of the most popular values for use in bypass filtering.) Try your simulation with one capacitor value 20% higher and the other 20% lower.
– Nedd
Commented Mar 6, 2015 at 8:44

## 1 Answer

The values in the circuit produce a natural resonance at about 89Hz based on this formula: -

Natural resonance = $\dfrac{1}{2\pi \sqrt{R_1R_2C_1C_2}}$

This circuit can also produce what to the uninitiated might think of as strange behaviour - it can actually resonate i.e. produce a significnatly higher amplitude at a range of frequencies just when you might have expected it to be attenuating those frequencies. This is component value dependent.

I believe this is what you are seeing. I think you are seeing this because you have one of the capacitors at the wrong value. For instance, if the 47n were in fact a 1nF capacitor the circuit would resonate at about 600 Hz with a significant peak: -

I used an online sallen-key calculator and plugged in the R and C numbers to get this. Here is the link.

I would encourage you to check the capacitor and resistor values you used.

• i just did, and yeah apparently it was the tolerance of the cap, i used several until i got one that didnt made that effect, and i got it right this time, however its not filtering from 60Hz or at least the filtering seems to start from around 150Hz, i know its logaritmic but, how normal is this? i just fixed the problem 10v problem by replacing the cap for another one but now the practical part of this doesnt match with my calculations or simulation, im guessing now its the value of my caps or resistors Commented Mar 6, 2015 at 19:08
• i just did, and yeah apparently it was the tolerance of the cap, i used several until i got one that didnt made that effect, and i got it right this time, however its not filtering from 60Hz or at least the filtering seems to start from around 150Hz, i know its logaritmic but, how normal is this? i just fixed the problem 10v problem by replacing the cap for another one but now the practical part of this doesnt match with my calculations or simulation, im guessing now its the value of my caps or resistors Commented Mar 6, 2015 at 19:08
• It is an 89 Hz filter for starters. Next I don't believe it is a tolerance problem. Try measuring both resistors and buy some new 10% tolerance capacitors. Finally, what are you expectations for this filter? What are you expecting it to do? Commented Mar 6, 2015 at 19:15