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how to print node-to-node capacitance?

flying

I'm trying to find a way in spectre to calculate the capacitance between every two terminals of a transistor (I hope it exists).

In Spectre users' Guide, info could be used to print node-to-node capacitance.

********************************************************************************
  Inputsweep1 sweep param=dc dev=vg start=Vgnd stop=Vdd step=0.05 {
  Inputsweep2 dc param=dc dev=vd start=Vgnd stop=Vdd step=0.05
  }
  sweepInfo info what=captab where=file file="capNodes" detail=nodetonode

*************************************************************************************8

I expected I could get a capacitance list for every vd and vg combination,however unfortunately I just got the following text:

My questions are:

1: what's the meaning of 'n1:dbnode',  'n1:dbnode', and 'n2:dbnode'? I didn't have instance 'n2' inthe netlist...

2: how to get the node-to-node capacitances with respect to terminal voltages?

**********************************************************

******************************
 3 Capacitance Table `sweepInfo':
 4 ******************************
 5 Capacitance values computed in info analysis `sweepInfo' at T = 27 C.
 6
 7 n2:dbnode : n1:dbnode   fixed=0           , variable=53.4296 a   ,
sum=53.4296 a   .
 8 n1:dbnode : 0   fixed=0           , variable=0           ,
sum=0           .
 9 n1:dbnode : n1:int_b    fixed=-0          , variable=0           ,
sum=-0          .
 10 n1:dbnode : vd  fixed=-0          , variable=53.4296 a   ,
sum=53.4296 a   .
 11 n1:dbnode : vss fixed=-0          , variable=0           ,
sum=-0          .
 12
 13 n1:int_b : n1:int_b     fixed=576e-21     , variable=8.87769 a   ,
sum=9.45369 a   .
 14 n1:int_b : 0    fixed=0           , variable=0           ,
sum=0           .
 15 n1:int_b : n1:dbnode    fixed=-0          , variable=0           ,
sum=-0          .
 16 n1:int_b : n1:int_g     fixed=576e-21     , variable=10.777 a    ,
sum=11.353 a    .
 17 n1:int_b : n1:sbnode    fixed=-0          , variable=0           ,
sum=-0          .
 18 n1:int_b : vd   fixed=-0          , variable=-423.98e-21 ,
sum=-423.98e-21 .
 19 n1:int_b : vg   fixed=-0          , variable=0           ,
sum=-0          .
 20 n1:int_b : vs   fixed=-0          , variable=-1.47531 a  ,
sum=-1.47531 a  .
 21 n1:int_b : vss  fixed=-0          , variable=0           ,
sum=-0          .
 22
 23 n1:int_g : n1:int_g     fixed=53.6817 a   , variable=91.0626 a   ,
sum=144.744 a   .
 24 n1:int_g : 0    fixed=0           , variable=0           ,
sum=0           .
 25 n1:int_g : n1:int_b     fixed=576e-21     , variable=247.452e-21 ,
sum=823.452e-21 .
 26 n1:int_g : vd   fixed=26.5528 a   , variable=16.6791 a   ,
sum=43.232 a    .
 27 n1:int_g : vg   fixed=-0          , variable=0           ,
sum=-0          .
 28 n1:int_g : vs   fixed=26.5528 a   , variable=74.136 a    ,
sum=100.689 a   .
 29
 30 n1:sbnode : n1:sbnode   fixed=0           , variable=113.911 a   ,
sum=113.911 a   .
 31 n1:sbnode : 0   fixed=0           , variable=0           ,
sum=0           .
 32 n1:sbnode : n1:int_b    fixed=-0          , variable=0           ,
sum=-0          .
 33 n1:sbnode : vs  fixed=-0          , variable=113.911 a   ,
sum=113.911 a   .
 34 n1:sbnode : vss fixed=-0          , variable=0           ,
sum=-0          .
 35
 36 vb : vb fixed=0           , variable=0           , sum=0           .
 37 vb : 0  fixed=0           , variable=0           , sum=0           .
 38
 39 vd : vd fixed=26.5528 a   , variable=72.2212 a   , sum=98.774 a    .
 40 vd : 0  fixed=0           , variable=0           , sum=0           .
 41 vd : n1:dbnode  fixed=-0          , variable=53.4296 a   ,
sum=53.4296 a   .
 42 vd : n1:int_b   fixed=-0          , variable=3.45385 a   ,
sum=3.45385 a   .
 43 vd : n1:int_g   fixed=26.5528 a   , variable=31.758 a    ,
sum=58.3108 a   .
 44 vd : vg fixed=-0          , variable=0           , sum=-0          .
 45 vd : vs fixed=-0          , variable=-16.4203 a  , sum=-16.4203 a  .
 46
 47 vdd : vdd       fixed=0           , variable=0           ,
sum=0           .
 48 vdd : 0 fixed=0           , variable=0           , sum=0           .
 49
 50 vg : vg fixed=0           , variable=0           , sum=0           .
 51 vg : 0  fixed=0           , variable=0           , sum=0           .
 52 vg : n1:int_b   fixed=-0          , variable=0           ,
sum=-0          .
 53 vg : n1:int_g   fixed=-0          , variable=0           ,
sum=-0          .
 54 vg : vd fixed=-0          , variable=0           , sum=-0          .
 55 vg : vs fixed=-0          , variable=0           , sum=-0          .
 56
 57 vs : vs fixed=26.5528 a   , variable=170.151 a   , sum=196.704 a   .
 58 vs : 0  fixed=0           , variable=0           , sum=0           .
 59 vs : n1:int_b   fixed=-0          , variable=5.17639 a   ,
sum=5.17639 a   .
 60 vs : n1:int_g   fixed=26.5528 a   , variable=48.5276 a   ,
sum=75.0804 a   .
 61 vs : n1:sbnode  fixed=-0          , variable=113.911 a   ,
sum=113.911 a   .

********************************************************************************** 

  •  I think it's doing what you want, you just need to sort out which node is which.  Try labelling the nodes with meaningful names.

    Keep in mind that if you have the terminals connected to VSS and VDD it will be looking at the entire capacitance between (for example) the gate and VSS or VDD, not just the source or drain.

    Also, if you're just looking for properties of the device itself, you can print the dc operating points to see cgs, cgd, cds, etc.

  • Thanks a lot for your reply!!!!

    if I connect all terminals to a voltage source, e.g., Source is connected to a voltage source with value 0 volts, can I get the real Cgs?

    In spectre, Cgs=dQg/dVs. but the cap between Gate and Source should be cgs=dQgs/dvgs. they are different.

    I just tried to use info to get multilpe cap tables, like Cgs table, Cgd tables, with respect to terminal voltages Vg and Vd. So I used sweep for Vg and Vd. But in the file, there is only one value for every type of nodel capacitance. is it possible to get this kind of tables? 

  •  

    If you just want to see the device capacitances of a single transistor, you don't need the capacitance table.  Just find the dc properties in the results browser.  You can plot sweep results this way too.

     I don't know what you mean about "Cgs=dQg/dVs" and "cgs=dQgs/dvgs".

  • Hi Andrew , Thanks a lot for your reply!!

    I use DC simulation, then I got the values of many caps of a transistor, like cgs,csg,cgd,cjs, etc., in the following webpage, you mentioned the cij in spectre is not realnode-to-node capacitance, so I'm wondering how to get node-node capacitance in a transistor, like gate-to-source capacitance. 

    I used captab, I found the node-node absolute capacitance is same value as DC simulation results. 

    If cij from DC simulation is indeed node i to node j capacitance, then I can use DCsweep to get cap tables. 

    http://www.designers-guide.org/Forum/YaBB.pl?num=1087411058/1

     

  •  Please note that I am not Andrew Beckett.  Glad I could be of some help though.

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