Re: LTspice

From: Jim Thompson (thegreatone_at_example.com)
Date: 03/21/05 

Date: Mon, 21 Mar 2005 10:21:11 -0700

On Mon, 21 Mar 2005 09:03:38 -0800, ldg <asfd@hotmail.com> wrote:

>On Mon, 21 Mar 2005 08:13:40 -0700, Jim Thompson
><thegreatone@example.com> wrote:
>
>>Sure it can. In most Spices, it's done with subcircuits...
>>
>>xR node1 node2 <stray nodes> ModelName Params: L=100u W=2u M=4
>
>
>I used to do such things when I used pspice. It causes you to look
>for ways around its limitations. Unless doing a subcircuit adds
>accuracy to the simulation, it's difficult for me to understand how
>doing this is better.

A "resistor" is a NATIVE Spice device. If it has more than two
terminals, or has parameters, it's a "subcircuit".

>
>Once in a while I use a capacitor symbol, for instance, that calls a
>subcircuit to add stray to ground. This stray is constantly present
>in an IC and if you don't account for it, it can cause real problems.
>It isn't needed all the time, so I just change the pointer to another
>symbol in another directory and it netlists out normally.

Since I have 100's of clients and 100's of models, I create my own
symbol set with Templates appropriate to the process.

For instance I don't just place an NMOS device, I place an XFAB-NMOS,
so that its strays are properly computed per the process.

>
><snip>
>Done the same way as R's...
>
>xC node1 node2 <stray nodes> ModelName Params: L=100u W=20u (Some
>nutcases use an M here, I don't, it's confusing... for segmented R's I
>may adopt "S")
>
>***********
>
>It's less confusing to place the same capacitor over and over again?

Sorry, I went back and placed the modifier after the wrong device. I
use M= on capacitors all the time. I have so many different types of
capacitors (in silicon) that I probably have 20 different symbol
types.

I just find M= for resistors confusing... parallel?? or serial??

>If you say so.
>
>Lets say you were drawing a 6 bit switched capacitor mdac. This
>means you have to draw 1,2,4,8,16,and 32 cap arrays (in subcircuits
>perhaps?). Or you could do one of your workarounds and create the
>m=<M> function yourself I suppose. Then you'd have to figure out how
>to do an lvs netlist for layout.

Naaaah! You missed the BEAUTY of my schematic capture. The
simulation TEMPLATE and the LVS TEMPLATE are different.

For instance, MOS devices, in the LVS netlist have no AD, AS, PD, PS,
etc., but do in the simulation netlist.

>You also could simply make the caps
>sequentially larger, but I know you wouldn't do that because of
>matching issues and the chance of confusing the layout person.
>
>In any case, if I have a choice of simply adding m=32 on a cap, I'll
>do this instead. If the schematic capture supports iterated
>instances, I've also created these arrays by adding the appropriate
>instance name on the device. C[0:31]

I can do that too, but generally limit such notation to buses.

>
>Regards,
>Larry
>
>"nutcase"

I presume you mean, by "nutcase", that you've never really used a good
simulator ;-)

                                        ...Jim Thompson 

-- 
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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