The Designer's Guide Community Forum - Lookup Table - Voltage in/ Voltage Out

kevinvinv Junior Member Offline Posts: 10	Lookup Table - Voltage in/ Voltage Out - SPECTRE Jul 30^th, 2010, 4:05pm What would be the best way for me to implement a voltage in, voltage out lookup table in spectre?
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sheldon Community Fellow Offline Posts: 485	Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE Reply #1 - Jul 30^th, 2010, 7:30pm Kevininv, Use the Verilog-A table model. Best Regards, Sheldon
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kevinvinv Junior Member Offline Posts: 10	Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE Reply #2 - Jul 31^st, 2010, 11:36am I was hoping for something other than Verilog A... Simulating without verilog-A is like living life without a stomach ache. Almost every time I have convergence issues- I trace it to a verilogA block. Anyhow- if that is my only option- I guess I'll give it a go. Thanks!
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Geoffrey_Coram

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Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Reply #3 - Aug 3^rd, 2010, 5:16am

trace it to a ... poorly-written? ... verilogA block?

There are lots of examples of poorly-written Verilog-A modules here. You do sort of need a basic understanding of the mechanics of Spice -- Newton-Raphson, so the module had better have continuous derivatives, etc.

Be careful when you build your $table_model -- some splines can give strange values when extrapolating, or even when interpolating, such that the Newton solver will get confused.

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kevinvinv

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Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Reply #4 - Aug 3^rd, 2010, 5:33am

You know I've heard that "You just need to write it correctly" sort of thing. I've written a lot of VerilogA. I've used examples from the Cadence ahdl Lib unmodified, I've read books and taken classes. Bottom line- it more often than not is the cause of my convergence problems. I have MUCH better luck using analogLib components to model behavior.

I suppose I am still doing something wrong

Thanks for the comments! I appreciate it!

I got my table model working- did indeed see strange interpolation but when I switched to order "1" - I got what I wanted- just standard PWL.

Thanks again!

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Ken Kundert

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Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Reply #5 - Aug 3^rd, 2010, 11:04am

Kevin,
There are a series of very common mistakes that people make with Verilog-A that tend to cause most of the problems. Once you understand the language well enough not to make those, life with Verilog-A gets much easier. Next time Verilog-A causes you grief, consider posting your model to this forum. Perhaps we can point out your mistake.

-Ken

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