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How to get rid of peaking in analog filter example?

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T3sl4co1l:

--- Quote from: Yuu on October 03, 2023, 11:22:16 pm ---
--- Quote from: T3sl4co1l on October 03, 2023, 08:09:23 pm ---Well, try adding some resistance (realistic, or intentional).  What's source resistance?  If it's zero, note it has a direct connection to the output at high frequency.  Op-amps are both inductive at the output (impedance rises with frequency) and lose gain at HF, doubly skewing the transfer function you might've expected.  Consider the effect of this against the capacitors, and ground-return paths (that is, ground by AC superposition, so, including the source itself, or intended-but-not-ideal virtual grounds of the amp inputs, etc.).

--- End quote ---

Hmmm okay so you're saying there's effectively a series inductor on the op-amp output which likely forms an oscillator with the four capacitors used in filter?

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Maybe not an oscillator outright, but a typical way that peaking shows up, particularly for capacitive loads.

The inductive characteristic is also in part a product of the higher poles within the amp, so these aren't exclusive explanations, but different aspects of the same underlying thing (transfer and output characteristics of the amp itself).

Tim

Yuu:

--- Quote from: BillyO on October 04, 2023, 12:21:26 am ---You are never going to make it perfect.

What is your required BW?

Have you tried putting in low pass filtering that kicks in around 1MHz?

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Ah not a bad idea. Modeling some source resistance (even just 5 ohms) and putting a capacitor to ground in LP config helped reduce those peaks below unity gain AFTER the desired filter response.
Also, I think the virtual grounds of the op-amps were doing some interesting business and I can kind of see that now like what T3sl4co1l was talking about.
This is cool. I'm learning from this.

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