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Simulators >> Circuit Simulators >> Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
https://designers-guide.org/forum/YaBB.pl?num=1513514660

Message started by repah on Dec 17^th, 2017, 4:44am

Title: Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
Post by repah on Dec 17^th, 2017, 4:44am

Hello,

I am trying to do system level behavioral simulations of discrete time delta sigma ADC as given below.

My issues are with transient convergence in spectre - do I need to set the CMIN value - I set it to 1fF.

Also do I need to set the time step of the transient simulation - I was told by those with expertise to set it to 20*Unity gain bandwidth of the opamp - does that sound correct and if so how do I change the time step of transient simulations in spectre.

Thank you.

Title: Re: Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
Post by sheldon on Dec 17^th, 2017, 6:54pm

Repah,

There is no reason why a top level verification should not converge.
I have run similar simulations in the past, see the attached example.
The primary difference appears to be in the switching. It does look like
you are crowbarring the reference voltages that might be problematic.

Title: Re: Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
Post by sheldon on Dec 17^th, 2017, 6:54pm

The output waveforms.

Title: Re: Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
Post by sheldon on Dec 17^th, 2017, 7:02pm

Repah,

There should be no need to set the time step for this simulation.
The appropriate way to control the time step is to let the simulator
set the time step by setting the accuracy control, error preset. Start
with an error preset of moderate. If you need more accuracy try
using the Conservative error preset.

The reason you might want to set the time step is to force the
simulator to take a time step is to force the time step for the DFT.
It should not be required for this simulation.

Sheldon

Title: Re: Convergence for Transient Simulation of Discrete Time Delta Sigmal ADC
Post by Ken Kundert on Dec 18^th, 2017, 1:48am

If you would like help, you should give the error messages.

Also, you should show us the switch models.

-Ken