The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Simulators >> Circuit Simulators >> Lookup Table - Voltage in/ Voltage Out - SPECTRE
https://designers-guide.org/forum/YaBB.pl?num=1280531140

Message started by kevinvinv on Jul 30^th, 2010, 4:05pm

Title: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by kevinvinv on 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?

Title: Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by sheldon on Jul 30^th, 2010, 7:30pm

Kevininv,

Use the Verilog-A table model.

Best Regards,

Sheldon

Title: Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by kevinvinv on 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!

Title: Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by Geoffrey_Coram on 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.

Title: Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by kevinvinv on 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!

Title: Re: Lookup Table - Voltage in/ Voltage Out - SPECTRE
Post by Ken Kundert on 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