Some Spice simulators have a CAPTAB option that allows you to print the total capacitance on a node.  You could then multiply that by the voltage (q=CV) ... but I guess that only works if your capacitance is mainly linear.  It's probably non-linear enough that you wouldn't be able to properly verify charge conservation.

Spectre's info analysis can output a captab. From spectre -h info:

Code:

1	 what=oppoint	What parameters should be printed.  Possible values
				  are none, inst, models, input, output, nodes, all,
				  terminals, oppoint, captab, parameters, primitives,
				  subckts, assert, or allparameters.
...
Captab parameters
9	 detail=node	 How detailed should the capacitance table be.
				  Possible values are node, nodetoground or
				  nodetonode.
10	sort=name	   How to sort the capacitance table.  Possible values
				  are name or value.
11	threshold=0 F     Threshold value for printing capacitances (ignore
				  capacitances smaller than this value). 

So:

Code:

captab info what=captab where=file 

In ADE, you can access this from the Options form on the DC analysis. There's a checkbox labelled "captab", and when enabled you can then set the other parameters listed above. Enable the DC operating point, and run the simulation. Some output appears in the log, but you can also do Results->Print->Capacitance Table afterwards.

Regards,

Andrew.


in your netlist would do it. There's also

Hmm, I see the captab stuff in the output log but the option results->print->capacitance table is greyed out?  Is there some setting I may be missing?

int_d tells me that there is a drain resistor between the output node and the channels of the MOS devices.  I think this is a fundamental limitation of the CAPTAP approach: it only counts capacitance connected directly to the node, and doesn't take into account even a huge capacitor on the other side of a small resistor.

You could, of course, manually add up the capacitances you know are "connected" but that assumes that you know whether to use Cdg or Cgd and to include the junction and overlap capacitances.

The other approach would be to attach an ac current source to the output node and measure the ac voltage on the node.  Then, use
 I = (G + jC) V
to solve for the effective capacitance.

int_d is the "internal drain node" of the MOS model.  The value in the CAPTAB for that node should be the sum of all the capacitances (channel, overlap, junction) of the MOS.  CAPTAB only ever reports capacitances directly connected to a node (it probably reports the diagonal matrix elements of the capacitance matrix).

You can get a similar result by testing this circuit:

r1 (s 0) resistor r=1k
rs (s sint) resistor r=0.5
cs (sint 0) capacitor c=1n
rch (sint dint) resistor r=20
cd (dint 0) capacitor c=1n
rd (dint output) resistor r=0.1

The captab should show capacitances (of 1nF) on sint and dint, but nothing on output.