# [SI-LIST] Re: non-negative off diagonal capacitive matrix elements ??

• From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Fri, 31 Aug 2001 12:40:09 -0500

```Ray,

The electrostatic induction matrix (note: this is NOT
the capacitance matrix) represents mutual capacitance
as negative numbers in the off-diagonal elements.
Accordingly, as Mick pointed out, the off-diagonal
elements must be non-negative.

For the one positive off-diagonal you do have, it is
five orders of magnitude lower than the self
capacitance, so I would tend to believe the error
was due to numerical round off.  You can try increasing
the accuracy of the tool, but the value is so low,
I recommond manually setting the value to zero.

Simply FYI: the on-diagonal elements of the electrostatic
induction matrix are not self-capacitance as some folks
tend to believe.  The on-diagonal values represent
the capacitance of that element assuming all
other elements were at ground
potential.  To obtain self capacitance, simply add all
the values in the matrix row (i.e., C11 = B11 + B21 + B31 + ..).

Pat

>
> I've just extracted the RLGC matrices for 5 coupled
> striplines using Apsimtech RLGC.
>
> As expected, the diagonal elements of the capacitive matrix are
> positive capacitance values. All the off-diagonal elements are
> negative EXCEPT one. Is this correct. It has been a while since I've
> off diag element were all negative. Anyone know for sure ??
>
> The element in column 1 row 5 is the one I question:
>
>
>    3.1600e-15
>   -4.9360e-16   3.2540e-15
>   -2.0080e-17  -4.8990e-16   3.2540e-15
>   -1.3380e-18  -1.9820e-17  -4.8990e-16   3.2540e-15
>    3.1280e-20  -1.3380e-18  -2.0080e-17  -4.9360e-16   3.1600e-15
>
>
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```