While doing voltage and power output analysis using MultiSim 11 the power outputs and voltage outputs change when I measure with meters in the schematic and when I do a transient analysis an look at the outputs in the grapher.

[ronquan]

While doing voltage and power output analysis using MultiSim 11 the power outputs and voltage outputs change when I measure with meters in the schematic and when I do a transient analysis and look at the outputs in the grapher.  What is causing this discrepancy.  How can I correct it?  Which way yields that correct answer?

[_user32]

ronquan,

There are often times variations on how the instruments measure and how measurements are calculated from raw voltage/current data on the grapher view.

For current readings instruments typically insert a very small R value and measure current (I) based on the voltage across R (such as with the Multimeter).   Also there could be other factors as to how the measurement is being calculated (average power, peak power, apparent power, etc...).

If possible, please send us the circuit in question, so that we can take a look.

Regards,

Pat Noonan

National Instruments

[ronquan]

Thank you for the reply.  I don't believe I'm allowed to attach the circuit due to company policy,  But I've been reading about a Spice problem called trapezoidal oscillations in a book by Ron Kielkowski and I'm pretty sure I've seen something like this occurring in our circuit simulations.  So far I haven't seen how to change integration methods.  So if you know how to set what were called options in some of the Spice2 versions that may help me get to a solution to these and possibly the voltage shift problems.

Thanks,

ronquan 

[_user32]

ronquan,

I completely understand on the circuit sharing.   Perhaps you can simplify the circuit to just show the issue, perhaps that would allow you to show the specific issue without sharing any company IP.

If you need to access the integration method (for something like switching signals or where you suspect there might be trapezoidal oscillations) you can adjust the integration method from trapezoidal to gear in the Multisim settings.   If you are doing interactive simulation, you would go to:

Simulate -> Interactive Simulation Settings -> Analysis Options (tab) -> click on 'Custom Settings' and 'Customize...'

Within the Transient (tab) you can set the Integration method.   Also in the Global (tab) settings, it is common for users to adjust the RELTOL and RSHUNT...   Also in the first window 'Interactive Simulation Settings' under the Defaults (tab) you can select the maximum time step (TMAX) which is another common SPICE engine setting to change that would adjust simulation accuracy.

Multisim first attempts to select adequate settings for TMAX, but for precision work or to circumvent convergence or accuracy issues, usually changing a combination of these settings will produce more desireable results (change 1 at a time and test the result rather than changing them all is recommended).  If the settings are too wide or too narrow you will likely experience a convergence issue (convergence wizard can sometimes help).

Good luck and please report back if any of these changed settings helped your simulation accuracy.

Regards,

Pat Noonan

National Instruments