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Design procedure for integrated MFB low pass filter

FvM

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I'm claiming factor 10 as an arbitary value, which gives 5.7 degree deviation from 90 degree phase. You can choose any different values according to your Q requirements. But that's not the point. Post #35, with the clarification in post #37, claims an A0 effect independent of the frequency ratio, which makes absolutely no sense, I think.
Qactual ~ Qideal(1-2Qideal/Ao)
 

sutapanaki

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I don't think post #35 and #37 claim that. What they say is that if instead of having an ideal integrator we have an opamp with finite DC gain, then that DC gain will impose a dominant pole in the otherwise ideal -20dB/dec slope of the integrator. That dominant pole will cause a phase shift that will be felt few decades beyond the dominant pole frequency and if the biquad poles are within that frequency range it will start affecting the filter parameters more or less.
I think we are on the same page, just talking about things differently.
 

Shashankgowda

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12 bit would suggest a distortion going into the ADC of at least 72dB, better - 78dB. You don't have much choice here but use opamp-RC filters. Gm-C will not give you that kind of distortion because it uses open loop integrators. If you want 2nd order you may go by with Sallen-Key filter but it becomes too sensitive to component variations for higher orders. If it is 4th order, then you'd be better off with a cascade of biquads. If you need more than 4-th order, then you should better use ladder filter architecture.
In any case you will have to design opamps with UGBW of around 100MHz. I would suggest before even starting the transistor level design, create a model for the opamp, that would have a first pole at low frequencies and a second pole at above UGBW - 3-4x higher than UGBW. Would also need to have of course DC gain modeled as well as noise (very important). Then, build your filter, whichever one you decide to use, with this amplifier model. You can change all parameters of the model and study how they influence the overall filter characteristic. You will not be able to see distortion with that model, though, unless some distortion comes from the resistors/capacitors you use, but it is not going to be the whole picture.
I don't think the resolution is going to matter to the LPF design that severely because if the noise is concerned then all the blocks before the LPF would give out much noise and LPF is there to remove them with some contribution of its own. but keep in mind that the 12-bit ADC should have an INL and DNL of less than 0.5 bit to work without an error. A 5MHZ cutoff LPF (anti-aliasing filter) could be achieved with first or second-order filter without that much noise added to the circuitry. if the model is of differential signals then it should furthermore reduce the noise.
 

Dominik Przyborowski

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One of the AAF main purpose is an image rejection, so ADC resolution gives desired number for it as well. There are a lot of spec numbers here, not only noise
 

sutapanaki

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I don't think the resolution is going to matter to the LPF design that severely because if the noise is concerned then all the blocks before the LPF would give out much noise and LPF is there to remove them with some contribution of its own. but keep in mind that the 12-bit ADC should have an INL and DNL of less than 0.5 bit to work without an error. A 5MHZ cutoff LPF (anti-aliasing filter) could be achieved with first or second-order filter without that much noise added to the circuitry. if the model is of differential signals then it should furthermore reduce the noise.
It is not a question of noise (only). It is mainly a question of distortion and also rejecting out of band frequency components that can alias back in-band.
 

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