How to have HSPICE ignore open drain/source MOSFETs?

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timyu1477

Joined: 18 Aug 2004
Posts: 3
Location: Sunnyvale, CA

18 Aug 2004 19:12 How to have HSPICE ignore open drain/source MOSFETs?

I am new to this board. I am wondering if someone knows how to have HSPICE ignore any MOSFET with open drain/source ( only 1 connection to the node). It seems open drain/source causes majority of convergence problems. Help or comments are appreciated.
All those open drain/soure MOSFET are extra devices in design, reserved for future circuit changes.

flatulent

Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth

18 Aug 2004 20:41 Re: How to have HSPICE ignore open drain/source MOSFETs?

Attach a large value resistor between the node an ground. 1MEGohm for example.

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bastos4321

Joined: 01 Jan 1970
Posts: 341
Helped: 24
Location: space

18 Aug 2004 23:59 Re: How to have HSPICE ignore open drain/source MOSFETs?

Use

.options GSHUNT=1e-12

This will put a condutance of value GSHUNT in evry node to 0.

"Conductance added from each node to ground. The
default value is zero. Adding a small GSHUNT to each
node can solve some “internal timestep too small”
problems caused by high frequency oscillations or by
numerical noise. "

Bastos

flyinspace

Joined: 01 Jul 2003
Posts: 145

19 Aug 2004 2:50 How to have HSPICE ignore open drain/source MOSFETs?

Hi Bastos,
I am wondering if the option "GSHUNT=1e-12" can affect the accuracy of the simulation, especially the high speed cirucit.
Thanks

timyu1477

Joined: 18 Aug 2004
Posts: 3
Location: Sunnyvale, CA

19 Aug 2004 2:56 Re: How to have HSPICE ignore open drain/source MOSFETs?

I am almost going to write some script to comment out each one of those open drain/soure MOSFET. I think besides some capacitance concern, those devices do not affect circuit performance. It may be tricky to write the script though.

tecsiun

Joined: 20 Aug 2004
Posts: 9

20 Aug 2004 14:21 How to have HSPICE ignore open drain/source MOSFETs?

All those open drain/soure MOSFET are usefull, if not, how can not set them NG?

timyu1477

Joined: 18 Aug 2004
Posts: 3
Location: Sunnyvale, CA

24 Aug 2004 16:07 Re: How to have HSPICE ignore open drain/source MOSFETs?

to tecsiun
What is NG? In my case, all those open drain/source in design are just backup devices. They are there in case of future revision such as changing the current mirror ratio, inserting offset etc. They are, in general tends to case covergense issue in HSPICE. So, if there is any option to comment out those devices, it will be helpful.

yeechyan

Joined: 14 Jun 2004
Posts: 111
Helped: 1

25 Aug 2004 3:29 Re: How to have HSPICE ignore open drain/source MOSFETs?

Do you mean that those open source/drain transistors are not in use in the circuit? Try ground all the terminals of the transistor if they're not currently in use in the circuit.

Hughes

Joined: 10 Jun 2003
Posts: 718
Helped: 87

25 Aug 2004 5:56 How to have HSPICE ignore open drain/source MOSFETs?

In schematic view, you can easily remove those transistors for simulation. For layout, a mark layer can be used to prevent those transistors being extracted by LPE tools.