buddypoor
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Hi rfmagic,
in your evaluation of the filter software simulation results you shouldn't forget that all simulations assume ideal opamp properties - as far as input and output impedances are concerned. Because of this, you did not detect the following disadvantage of the S+K topology: Above a certain frequency the attenuation will again decrease (the filter transfer curve is rising). This effect is due to the feedback capacitor: For higher frequencies there is a remarkable and rising portion of the input signal that reaches the opamp output DIRECTLY (not via the internal amplifier chain). This portion creates at the finite opamp output a signal voltage that increases with frequency. You can observe this effect via circuit simulation based on real amplifier macro models. Example: opamp type LM741, second order lowpass with Qp=1 and 3-dB frequency 50 kHz. As a result, the attenuation will be not better than -35 dB (at app. 400 kHz) and then will decrease again.
Regarding Monte Carlo simulation: May be you didn't get the right picture because - in some cases - tolerance variations can cancel or reduce each other. The worst case for S+K topology is variation of one of the gain fixing resistors alone! Try it - it's really bad! (Obviously, this is not the case for unity gain design - however, this alternative has other disadvantages).
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