Warning : A virtual resistance of 1Tohms was added to each node

[Martin02]

Hi everyone
I am running simulation of power amplifier on ADS2013 there is no error but the following warning pop up : A virtual resistance of 1Tohms was added to each node during AC/SP analysis
I don"t know whether the error is related to my CREE CGH40010F transistor because the model has 4 pins whereas only 3 pins are used and one left or the warning is due to other issue. Thx

 

[andre_luis]

Warning : A virtual resistance of 1Tohms was added to each node

As reported at the title described on this thread, it is not properly an error, but just a warning, notifying that the original model of the component needed to be changed so that the simulation could occur ( probably for using some MNA algorithm, which requires no floating nodes for work ).

Once this resistance value is physically extremely high if compared to other impedances, no relevant effect is expected due to this "insertion".


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[Darktrax]

This is something that has to happen with all circuit simulators to prevent division by zero at some point in the calculation.

There was a time when using various SPICE netlists that one had to take care to manually add in an extreme value resistance across a capacitor, and add a trivially low value in series with an inductance to avoid zero or infinite currents in the matrix. 10^18 and 10^-18 were the usual values. The values are too extreme to alter the outcome in any meaningful way

The rule is that all circuit loops in the net must be able to complete a current. There must not be any parts of the circuit "stranded" with no path to ground, and there must not be any loops with zero resistance.

Modern solvers are slick enough to detect the condition and add it in for you.

 

[Martin02]

Ok thx. In my circuit there are 2 resistors with 0 Ohms. Is the warning due to that? How can I solve it? Or the warning is not affecting the result?
thx

 

[andre_luis]

...In my circuit there are 2 resistors with 0 Ohms. Is the warning due to that? How can I solve it?

No, the warning oppositely refers to an "almost infinite" resistor added mathematically at the component model just for ensure proper calculations requirements.

Or the warning is not affecting the result?

As stated above, for regular conditions, no significant effect is expected to be felt on final result.


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[Darktrax]

Ok thx. In my circuit there are 2 resistors with 0 Ohms. Is the warning due to that? How can I solve it? Or the warning is not affecting the result?
thx

To be clear - if you have resistors of 0 ohms, you use a value that is very LOW, like 1E-18 Ohms, not TeraOhms high value . Then the warning should not happen.

If you do just use values like 0 Ohms, clearly your software will do the warning, and substitute the very low value for you.

The times you would encounter very high values is in things like modeling a switch or relay or circuits with simple model capacitors. The software is not allowed to accept INFINITY Ohms. Either you supply the TeraOhms resistor in parallel, or the software will do it for you - with a warning. Here is where you add a value that is very HIGH.

Capacitor models are almost always implemented with a TeraOhms resistor across them eg. 1E+18 Ohms. Consider the example of a filter with capacitor coupled input. You might have a circuit where both ends of a component have no DC path to the 0V ground. Some simulators might well give warnings if this happens, and then add 1 TeraOhms to each node so that real calculate-able numbers can happen in the math without significantly affecting your circuit result.

 

[andre_luis]

Just as an assumption, I believe that such fake "almost infinite resistance" component insertions beyond original models can even be considered not as a problem, but oppositely as coherent with the real world model, once a minimal internal leakage current is expected to occur.






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