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Strange filter response
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Seán:
Hello,

I have a question about an op-amp filter response. My filter is a multiple feedback filter with a gain of 30 V/V and a cutoff of around 10kHz.

What, if anything could result in a flat passband response at say 30 dB up to about 5kHz, which then drops to about 25 dB staying flat at the level up to around 10kHz, given that the response should be flat at 30 dB? It seems to have a double dip.

I have heard people say that the component sizes can cause issues in designs? In the sense that you should use kOhms instead of hundreds of Ohms for resistors and so on. But I am not sure what impact this could have, if any.

Thanks in advance for any thoughts or suggestions.

Seán

rstofer:
A schematic would help.

Are you running up against the gain-bandwidth product of the op amp?  Check the datasheet.
Seán:

--- Quote from: rstofer on June 09, 2018, 09:11:45 pm ---A schematic would help.

Are you running up against the gain-bandwidth product of the op amp?  Check the datasheet.

--- End quote ---

Hi,

I put a picture of the schematic in the original question. The simulated response looks fine, the measured response seems to have the issue.

The gain bandwidth of the op-amp is 5.5MHz, so I don't think that is the issue?

Thanks,
Seán
bson:
Your component values would indicate a gain of 3 rather than 30.

When I calculate it I get about 36ohm, not 360ohm for R1 and R4 given an R2 of 1.1k, gain of 30, w0 at 2*pi*10kHz and Q=0.707.  (Actually 35.4 and 36.6 for the resistors.)  I also get 630nF for C1 and 10.2nF for C2; again your values would suggest a gain of 3.

MFB/Rauch filters aren't that sensitive to component tolerances and values, so I doubt that's the problem.

At an actual gain of 30x you might be pushing the envelope on phase margin and introducing oscillation...
Seán:
Isn't the gain set by -R2/R1, so 11k/360 = 30.5.
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