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Designing band pass filter using STLM358
I've used the Filter Pro and I'm having problems with cross over distortion using the values recommended by Filter Pro.
I'm desigining a Sallen Key Butterworth Bandpass
Corner frequency Attenuation: 16.998 dB
Stop Band Bandwidth: 24 kHz
Allowable passband ripple: 1dB
Stopband Attenuation:-21 dB
Center frequency: 17.5 kHz
Passband Bandwidth: 7kHz
I'm powering the LM358 using +/-12v supply.
Center frequency should be 17.5kHz but measures ~15kHz. I believe this is due to the cross over distortion interfering with bandpass?
Any help would be appreciated.
Cross over distortion would cause distortion, but shouldn't affect center frequency. What's at work here is inadequate gain bandwidth of the op amp.
I put your parameters into FilterPro and created a design - the design report is attached: 8182.bp.pdf
Note that the op amps require 17-22MHz of GBW product. An LM358 only has about 1MHz GBW. This will cause the center frequency to shift.
I built the design in TINA-TI v9, the circuit is attached: 6786.Sallen-Key 4th Order Butterworth BPF.tsc
Running this using an LM358 versus an OPA228 (which has about 30MHz GBW) results in this:
The center frequency using the LM358 is indeed around 15.5kHz, while the OPA228-based design places it right at 17.5kHz, as expected.
Make sure to look at the GBW requirement of op amps when designing active filters! As you can see here, it really does matter!
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