Hello.

This is my first attempt with electronic simulators, and I have been struggling with understanding how MPLAB MINDI (Simetrix) implements initial conditions in capacitors.

I am trying to simulate two basic Op-Amp circuit configurations, one is an inverting amplifier with a 10x gain, while the other is the same circuit with a capacitor added in parallel to the feedback resistor, making it an LPF with 10x gain in the passband. I am referencing the non-inverting terminal to mid-supply, and it is decoupled through a heavy capacitor, in order to suppress the Johnson noise. The simulations run pretty nicely, just as expected, when I do not put any initial condition on this capacitor. As soon as I assert any initial condition on this decoupling capacitor, the simulation fails miserably.

Here are the two schematics and their responses.





Please note that the dotted lines are the frequency responses of the circuits with Initial Conditions for CR1 set to Initial voltage (with the voltage and options set as depicted in the below image), while the solid lines are the frequency responses of the circuits with Initial Conditions set to Open circuit (all the options are automatically disabled at this setting).




I would be grateful if someone would like to share their experiences with Simetrix, and providing me with a little guidance in understanding how it handles its stuff.


Warm Regards,
Abbas.

 

Hello.

This is my first attempt with electronic simulators, and I have been struggling with understanding how MPLAB MINDI (Simetrix) implements initial conditions in capacitors.

I am trying to simulate two basic Op-Amp circuit configurations, one is an inverting amplifier with a 10x gain, while the other is the same circuit with a capacitor added in parallel to the feedback resistor, making it an LPF with 10x gain in the passband. I am referencing the non-inverting terminal to mid-supply, and it is decoupled through a heavy capacitor, in order to suppress the Johnson noise. The simulations run pretty nicely, just as expected, when I do not put any initial condition on this capacitor. As soon as I assert any initial condition on this decoupling capacitor, the simulation fails miserably.

Here are the two schematics and their responses.
View attachment 181769 View attachment 181770

View attachment 181771 View attachment 181772


Please note that the dotted lines are the responses of the circuits with Initial Conditions for CR1 set to Initial voltage (with the voltage and options set as depicted in the below image), while the solid lines are the responses of the circuits with Initial Conditions set to Open circuit (all the options are automatically disabled at this setting).

View attachment 181773


I would be grateful if someone would like to share their experiences with Simetrix, and providing me with a little guidance in understanding how it handles its stuff.


Warm Regards,
Abbas.

I have never used Simetrix but I question why there is a "+" at the grounded end of CR1. If you are showing a transient simulation, what indicates that the results shown are invalid? The circuit appears initially disturbed but eventually settles to the same result.

 

Hello TeeKay6.

why there is a "+" at the grounded end of CR1

That "+" appears when the initial condition is enabled.

It seems you missed some details in my original post.

If you are showing a transient simulation,

The plot represents the circuits' frequency response, not the transient response (Please see the Y-axis labels in the plots).

what indicates that the results shown are invalid?

The fact that merely changing the initial conditions on a capacitor might cause some deflections in the responses, but does not change the characteristic behaviour of the circuit. If you are hard-wiring a circuit for a certain gain with/without frequency dependance, there might be some deviations in the amount of gains, but the gain would still be there!

The circuit appears initially disturbed but eventually settles to the same result.

They is no initial/transient/steady-state. The plot represents the circuits' responses to the varying frequency of the input signal, irrespective of time.

 

Hello TeeKay6.


That "+" appears when the initial condition is enabled.

It seems you missed some details in my original post.


The plot represents the circuits' frequency response, not the transient response (Please see the Y-axis labels in the plots).


The fact that merely changing the initial conditions on a capacitor might cause some deflections in the responses, but does not change the characteristic behaviour of the circuit. If you are hard-wiring a circuit for a certain gain with/without frequency dependance, there might be some deviations in the amount of gains, but the gain would still be there!


They is no initial/transient/steady-state. The plot represents the circuits' responses to the varying frequency of the input signal, irrespective of time.

Thank you for clarifying details of your post. I do agree that one does not expect initial conditions to alter frequency response. Beyond that I fear I have nothing to offer you. I suggest that you might try much simpler circuits to get a better feel for how initial conditions are handled. For example, start with a simple RC. You could also contact Microchip support with your question.

 

Thank you for clarifying details of your post. I do agree that one does not expect initial conditions to alter frequency response. Beyond that I fear I have nothing to offer you. I suggest that you might try much simpler circuits to get a better feel for how initial conditions are handled. For example, start with a simple RC. You could also contact Microchip support with your question.

Thanks alot, TeeKay6.
I really appreciate that.