Post Layout simulation accuracy

For very low DC current levels, your simulation results may be affected by parameter "gmin" - the default is (I think) 1e-12 S (or 1e12 Ohm - i.e. 1pA for 1V voltage).

"gmin" is a resistor (conductance) that SPICE simulators connect between every node and the ground (to avoid nasty floating node problems, to have a deterministic and converging solution).

Just set it to a different value - e.g. 1e-18 - and check if you a difference in your simulation results.

Maxim

Thanks for replying Maxim. I did it but still, I see the unexpected output after extraction. I have also changed the iabstol to 1-15 ( it doesn't converge for lower iabstol). So in terms of accuracy, if I use these low Gmin and iabstol values, the accuracy of the results should be fine, right? Is there any other parameter that I should be aware of for a circuit with very low current inputs? As for my findings, the mom caps after extraction cause the problem. So now I need to be sure I have the simulation setup and parameters completely right.

Thanks

Dear Fatima65,

Fatima65 said:

f I use these low Gmin and iabstol values, the accuracy of the results should be fine, right? Is there any other parameter that I should be aware of for a circuit with very low current inputs? As for my findings, the mom caps after extraction cause the problem. So now I need to be sure I have the simulation setup and parameters completely right.

A few comments if I may....

1. Are you sure your models support this level of accuracy? I would be very surprised i your PDK vendor guarantees a fA level of current accuracy. At the fA level, the presence of even ambient light will impact the currents and I can't imagine how the vendor could qualify any level of accuracy to this degree.  Have you inqured?

2. You might examine the limits of SPICE accuracy in terms of its parameters at URL:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjP8aTkj_rxAhXLFVkFHU58B60QFjAGegQIDxAD&url=https%3A%2F%2Fdesigners-guide.org%2Fanalysis%2Fdg-spice%2FchA.pdf&usg=AOvVaw2r19Ksg3wDbO2TO5zaXaTP

(section of Ken Kundert's book)

Section A.2.1.1 speaks tp iabstol and its limits for a desired accuracy.

3. One approach might be to run your simulation at a much higher temperature to determine if your currents are showing signficant increases, I assume the relationship with temperature is exponential.

Shawn

Dear Fatima65,

Fatima65 said:

f I use these low Gmin and iabstol values, the accuracy of the results should be fine, right? Is there any other parameter that I should be aware of for a circuit with very low current inputs? As for my findings, the mom caps after extraction cause the problem. So now I need to be sure I have the simulation setup and parameters completely right.

A few comments if I may....

1. Are you sure your models support this level of accuracy? I would be very surprised i your PDK vendor guarantees a fA level of current accuracy. At the fA level, the presence of even ambient light will impact the currents and I can't imagine how the vendor could qualify any level of accuracy to this degree.  Have you inqured?

2. You might examine the limits of SPICE accuracy in terms of its parameters at URL:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjP8aTkj_rxAhXLFVkFHU58B60QFjAGegQIDxAD&url=https%3A%2F%2Fdesigners-guide.org%2Fanalysis%2Fdg-spice%2FchA.pdf&usg=AOvVaw2r19Ksg3wDbO2TO5zaXaTP

(section of Ken Kundert's book)

Section A.2.1.1 speaks tp iabstol and its limits for a desired accuracy.

3. One approach might be to run your simulation at a much higher temperature to determine if your currents are showing signficant increases, I assume the relationship with temperature is exponential.

Shawn