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Bandpass filter OP07CP help!

RachelAnne

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The band-pass filter filter (0.05HZ~100HZ) made with OP07CP as the op amp, got a strange waveform as shown in the figure. What is the reason for this?
1624605397027.png

1624605518099.png
 

BradtheRad

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The spikey waveform happens when square waves go through a series capacitor. It performs the math function known as differentiation. I think your input is the sine wave? So how can it get converted to a square wave? Running it through an op amp with extremely high gain.
 

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You have a lowpass filter at the input then feed almost no high frequencies to the highpass filter.
The highpass filter should be at the input and its output should feed the lowpass filter.
EDIT: Your highpass filter uses extremely high value 47uF capacitors causing its cutoff frequency to be extremely low at about 0.03Hz!
 
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danadakk

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The sims look OK from both transient and bode, although lot of phase
activity. Maybe you have Inv and NonInv switched on one of the OpAmps ?

1624625119113.png






1624625166358.png


1624625254876.png



Regards, Dana.
 

Audioguru

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I think you want Butterworth highpass and lowpass filters.
Then instead of wrongly amplifying the highpass opamp, add a resistor to make the capacitor ratios 2:1.
Then instead of attenuating the input and wrongly amplifying the lowpass opamp, add a capacitor to make the capacitor ratios 2:1.
 

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danadakk

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Order of LPF and HPF considerations -



LPF first can reduce noise overall, a HPF first can reduce DC offsets......


Regards, Dana.
 
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Audioguru

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I checked both filter sequences and you were correct with the highpass filter at the input so that the lowpass filter at the output cut hiss. I was wrong and here is why:
 

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danadakk

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Not sure I follow what you did here.

If passive done noise sources, the Rs, would contribute symmetrical to topology, so
no change. Same for S&K done with unity G followers. But if done with VCVS, different
G's, then order would make a difference ? Or am I missing something here ?

Regards, Dana.
 

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