ACWWong
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Hi Prabhu, Yes it doesn't really surprise me that Cs1 or Cs2 didn't increase as the spacing was reduced. This is because the pi network is NOT a physical model but a lumped represenatation of the spiral of the 2-port parameters computed at a particular frequency. The impact of reducing the spacing is likely to be reflected in ASITIC for your differential inductors in the way that the self inductance increases rapidly as the simulation frequency approaches a lowered SRF, that is that the interwinding capacitance is captured in the values and changes in L at higher frequencies rather than in the values of Cs1 and Cs2 (which reflect substrate losses). Also even in differential symetrical spiral, i don't think the interwinding is dominant over the substate capacitance, because each segments capacitance does still sum as series capacitances between the two ports (although it is larger than in a single ended spiral), but it all depends on your technology and inductor size. Do a google or IEEE search for papers by John R. Long on the topic. Anyway, in summary I would still recommend ASITIC above just using Assura-RCX in trying to build up a spiral inductor model, be it single of differential. It would interesting if you redid your modeling in a 3D simulator (Sonnet etc.) for your differential spirals to see the difference in the model performance between ASITIC and a full EM solver. Cheers aw
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