Output Waveform of Sallen-Key Bandpass Filter

Thread Starter

glitzy_dust

Joined Mar 16, 2015
55
Hi everyone,

I have a question with regards to the output of an SK bandpass filter I'm building.

I have built the bandpass filter by cascading a low-pass filter and high-pass filter, both of which are of the SK-type, using the op-amp, LM358. This is shown in the schematic I have drawn below, together with the values of the capacitors and resistors:

https://ibb.co/fWSR2b


However, the output waveform is weird as seen below after feeding in a sine wave of amplitude 4.3V from a signal generator. I suspect that it may be because of the high capacitance value but I am not very sure if this is the case. May I ask if anyone can advise me on how to improve the shape of the output waveform?

https://ibb.co/bJbx8G


Thank you very much. :)
 

dl324

Joined Mar 30, 2015
8,891
What is the amplitude of the input and output waveforms?

With a 5V supply, the input voltage range for LM358 can be surprisingly small. When exceeded, the output may experience inversion.

Positive output voltage range could also be quite limited.

Both are limited to about a couple volts from the positive supply.
 

Thread Starter

glitzy_dust

Joined Mar 16, 2015
55
What is the amplitude of the input and output waveforms?

With a 5V supply, the input voltage range for LM358 can be surprisingly small. When exceeded, the output may experience inversion.

Positive output voltage range could also be quite limited.

Both are limited to about a couple volts from the positive supply.
Thank you so much for your reply! :)

I fed in a 4.3V amplitude sine wave from a function generator though on hindsight, I think it might be an overkill. The output amplitude is 110mV as seen from the oscilloscope. I think this may signal some issue with my setup~ ><
 

Thread Starter

glitzy_dust

Joined Mar 16, 2015
55
Just an update on my project:

I have decided to try using a higher valued capacitor, e.g. 0.1uF electrolytic capacitor as opposed to using a 1000pF ceramic capacitor, because I feel that the switching waveform may be due to the capacitor discharging and charging up too quickly.

I will try to switch the order of the filtering as well, e.g. LPF --> HPF and HPF --> LPF.

I will update everyone again and thank you so much for your help! ^^
 

dl324

Joined Mar 30, 2015
8,891
It would be helpful if you posted the input and output waveforms simultaneously:
clip3.jpg

I still think you're having problems with the input voltage range of the opamp. It is only guaranteed to be about 2V from the positive supply.
 

Thread Starter

glitzy_dust

Joined Mar 16, 2015
55
It would be helpful if you posted the input and output waveforms simultaneously:
View attachment 138755

I still think you're having problems with the input voltage range of the opamp. It is only guaranteed to be about 2V from the positive supply.
I see. May I ask if there is any way to improve the input voltage range of the opamp? :)

I am sorry that I haven't been able to test my BPF for some time. I managed to plot the Bode magnitude response plot and everything seems fine. I have not checked the transient response~~ Hopefully everything will be okay :) Will update asap too!
 

Thread Starter

glitzy_dust

Joined Mar 16, 2015
55
Increase the supply voltage or go to a rail-rail type op amp.
I see. Thank you for your reply. I am currently using an LM358 opamp for my project.

May I ask if you mean that I should increase Vcc (+ of opamp)?

I am currently supplying : Vcc = 5V Vee = GND and the rest are tied to a +2.5V input, as in my drawing above. May I ask if I increase Vcc, I should increase my +2.5V too, for example, if I have Vcc = 6V, my voltage should be +3V right?

Thank you.
 
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