iamxo
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eecs4ever said:"As we all know, in pipeline ADC the capacitor size could be scaled down in latter stages together with the opamp current ( however the GBW should be constant). "
Is your question about scaling ?
the feedback factor , f = Cf/(Cf+Cs+Cp) , when you scale everything by a factor K, f remains the same.
iamxo said:uh, it sounds reasonable, however, we need to maintain the opamp GBW, so the Cp is not scaled the same as Cs or Cf. In a word, my problem is that i can not use low current to achieve high closed-loop GBW mainly due to large input parasitic cap. I do not know how to solve it.
timof said:iamxo said:uh, it sounds reasonable, however, we need to maintain the opamp GBW, so the Cp is not scaled the same as Cs or Cf. In a word, my problem is that i can not use low current to achieve high closed-loop GBW mainly due to large input parasitic cap. I do not know how to solve it.
Can you change the metal routing in order to decrease parasitic capacitances?
Usman Hai said:iamxo, I think you are missing very important point here. ft of an opamp can be given as ft=β gm/Cl. When you scale the stages, the load feedback factor remain constant however, the load capacitance is reduced half (assuming you scaling down by 2 approx and not taking into account unscaled parasitics like CMFB etc etc ). So, when Cl is halved, you need half the current to keep the same gm and half the input transistors to keep the same current density (or Vdsat). Therefore, later stages opamp must use half the current.
In one of my papers (which is about to publish soon) I derived the relationship between the ft, Cl and Cip(input parasitics) as well in to make the designing of later stages more systematic. I can send you the paper once it will be published. I can tell you that there is an optimal size for input cap to get the max efficiency and minimum power. In other words if your input parasitics are more or less than that optimal value, you will be burning more power to get the desired ft.
Usman Hai said:In one of my papers (which is about to publish soon) I derived the relationship between the ft, Cl and Cip(input parasitics) as well in to make the designing of later stages more systematic. I can send you the paper once it will be published...
PaloAlto said:Usman Hai said:In one of my papers (which is about to publish soon) I derived the relationship between the ft, Cl and Cip(input parasitics) as well in to make the designing of later stages more systematic. I can send you the paper once it will be published...
I'd love to see that paper. Can you send it to me once it's ready? Just reply here, or as pm and I can give you my email
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