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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> a basic question about SC filter https://designers-guide.org/forum/YaBB.pl?num=1246555498 Message started by subgold on Jul 2nd, 2009, 10:24am |
Title: a basic question about SC filter Post by subgold on Jul 2nd, 2009, 10:24am i have a simple switched cap filter as in the attachment. since the information is only captured at the end of phase 1, do i need to take care about if the opamp settles at the end of phase 2? |
Title: Re: a basic question about SC filter Post by vivkr on Jul 2nd, 2009, 11:48pm As you have a capacitance which is permanently connected in the feedback loop (you seem to be realizing a lossy integrator with this), it does matter that the opamp settles well in both phases. By the way, is this diagram complete, or are there some more switches which you have omitted for sake of simplicity? Regards, Vivek |
Title: Re: a basic question about SC filter Post by raja.cedt on Jul 3rd, 2009, 12:46am hi, whats this ckt actually, because its like variable gain amplifier not filter.... Thanks, Rajasekhar. |
Title: Re: a basic question about SC filter Post by buddypoor on Jul 3rd, 2009, 1:05am As already mentioned by VIVEK, I donīt know what the task of the shown circuit could be. A filter ? An integrator ? I donīt think so. |
Title: Re: a basic question about SC filter Post by manodipan on Jul 3rd, 2009, 2:43am Hi, I think it's a high pass filter with cutoff defined by feedback components and gain at infinity defined by unswitched capacitors.now it's important that opamp settles at the end of charge transfer phase otherwise output is erroneaos and will lead to several problems like nonlinearity related distortion etc.you can start with a behavioral model and check filter performance with nonidealities in opamp like finite gain and UGB. |
Title: Re: a basic question about SC filter Post by thechopper on Jul 5th, 2009, 4:56pm manodipan wrote on Jul 3rd, 2009, 2:43am:
In order for this ckt to be a HPF, should not the input capacitance have a switch in series with the opamp input clocked with CK1? Tosei |
Title: Re: a basic question about SC filter Post by buddypoor on Jul 5th, 2009, 10:33pm Hello SUBGOLD ! Some days ago you have asked a question concerning the function of a specific circuitry - and five community members made up their mind to find the task of the circuit. Donīt you think now itīs your turn to tell us something about the purpose or the origin of the circuit ? Or are you not interested anymore ? Regards |
Title: Re: a basic question about SC filter Post by subgold on Jul 6th, 2009, 5:48am buddypoor wrote on Jul 5th, 2009, 10:33pm:
hi, sorry for the late reply because i didn't get online during the weekend. as already pointed out by manodipan, i don't see why this circuit cannot be used as a HPF. if the input cap is also a switched cap as Tosei suggested, then it seems to be a LPF. |
Title: Re: a basic question about SC filter Post by thechopper on Jul 7th, 2009, 2:32pm Hi subgold, Actually I was not suggesting the input cap was a switched cap. I was only suggesting a switch in series with it and the input of the opamp. that would provide sampling on your input cap and still perform a HPF action. Since there would be no reset during ck2 on such cap then it would not act as a resistor in average (in which case then you would get a LPF). I agree that not switching the input cap performs HPF action, but without a sampled input (as opposed to what I was suggesting). Therefore you have a continuous-time output and I do not see why the opamp needs special settling requirements during ck2 (reset phase). Regards Tosei |
Title: Re: a basic question about SC filter Post by buddypoor on Jul 8th, 2009, 12:29am Besides the question of transfer characteristics, I am afraid the circuit as shown will exhibit severe bias point problems due to the 2 non-switched capacitors. Regards |
Title: Re: a basic question about SC filter Post by subgold on Jul 9th, 2009, 2:45am HdrChopper wrote on Jul 7th, 2009, 2:32pm:
Hi Tosei, that's exactly my question. do you mean for the shown schematic (no sampling on the input cap), it is not necessary to settle in ck2? other replies of this topic suggested it should, which is also what i thought at first. because although the information is only sampled in the end of ck1, the capacitor should also get the correct charge in ck2, in order to further process the information in ck1. |
Title: Re: a basic question about SC filter Post by subgold on Jul 9th, 2009, 2:59am buddypoor wrote on Jul 8th, 2009, 12:29am:
hi, could you please explain your doubt about the transfer function? i think it is a very basic building block for active filters. of course it is a little simplified in order to illustrated the question. in many other cases (e.g. biquad or higher oder filters), there might be feedback caps coming from the following stage, but i thought the conceptual background concerning the settling requirement should be the same. as for the bias problem, i think we always need some strategies to guarantee the correct biasing for a pure capacitive feedback network, there is nothing special about this circuit compared with other switched cap circuits. |
Title: Re: a basic question about SC filter Post by thechopper on Jul 9th, 2009, 2:32pm Hi subgold, What I meant is that: since you are supposed to fulfil the settling requirements for clock phase 1, then settling requirements for clock phase 2 will be already covered since there is no sampling nor charge redistribution during that phase. Therefore the requirements are not as stringent as during clock phase 1. Hope this clarify my viewpoint. Tosei |
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