I am using LTspice to simulate voltage measurements from a circuit. I noticed, with exactly the same circuit components, each time I obtain different results. I assume it is due to thermal noise. (is that right?)

Then I wonder is there a way I can set seed to the SPICE simulation so that my results are reproducible.

To provide more detail: I did a Transient analysis. I am running a circuit with amplifiers, multipliers, resistors and capacitance for 200mS. I run the same circuit with nothing changed (I literally quit LTspice, open again, and hit Run button for the same schema), I found the voltage measurements at one node for the two runs are different. They stay the same for the initial ~30mS, then start to have a lag, then go on completely different tracks. Since nothing about the circuit has changed, I suspect it is caused by thermal noise. Please let me know if there could be other reasons.

Many thanks!

  • 1
    \$\begingroup\$ You need to supply more information. What is your circuit? Are you doing a DC or AC simulation? How different are the results? Thermal noise is not likely to be causing your problem but without more information, no one can tell. \$\endgroup\$
    – Barry
    May 16, 2017 at 23:28
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    \$\begingroup\$ AC Spice doesn't model noise sources directly, but it is configurable for solution precision. You can also run afoul of floating point precision. A circuit with multiplier is non-linear, so AC analysis is not meaningful - use a Transient (voltage vs time) analysis. \$\endgroup\$
    – glen_geek
    May 17, 2017 at 2:10
  • \$\begingroup\$ @glen_geek Thanks for the suggestion. Transient analysis gives the same observation that two runs give somewhat different results with nothing in the circuit changed. The two runs track for a while then start to have a lag, then go completely un-sync. \$\endgroup\$
    – user75006
    May 17, 2017 at 2:20
  • \$\begingroup\$ In your transient run, under simulate menu, choose edit simulation cmd and set a small value for maximum time step. See if that helps. Many oscillators have trouble starting from nothing (because noise isn't present). \$\endgroup\$
    – glen_geek
    May 17, 2017 at 2:40
  • \$\begingroup\$ By any chance, did you move any components, rewire some nodes, but kept plotting the same v(Nxxx)? Because LTspice changes the names of the nodes dynamically, unless they are explicitly named. \$\endgroup\$ May 17, 2017 at 5:56

1 Answer 1


LT spice should be consistent from run to run. Thermal noise is not simulated in a normal transient simulation. If your seeing differences from run to run they could most likely be explained from a numerical solver standpoint and differences on rounding, which some circuits can be quite sensitive to.

Another explanation may be that the models you are using have b-sources with a .rand statement in it, but this is unlikely.

If you are using a .rand statement, there isn't a way to set the seed (documented anywhere that I know of). If you need a .rand section to be repeated from run to run, use a PWL source, record the data as a .wav or PWL file and use that to make the runs consistent.

  • \$\begingroup\$ Thank you very much for explaining. Just one thing: this source link analyses noise. So noise is only simulated in the .noise mode, but not present in the measurements from the .tran mode? \$\endgroup\$
    – user75006
    May 17, 2017 at 21:01
  • \$\begingroup\$ In addition, I don't understand where the randomness of numerical solvers come from; and how LTspice chooses rounding for each run. Why would those factors be random given exactly the same circuit and initial condition? (Please pardon me if the question looks weird; I am new to spice simulation.) \$\endgroup\$
    – user75006
    May 17, 2017 at 21:14
  • \$\begingroup\$ Its not inherent to spice simulations, it is inherent to computing more specifically floating point calculations. theochem.uni-stuttgart.de/~knizia/random_rounding Numerical solvers can be very sensitive depending on the circuit being simulated. Without knowing more about your circuit and simulation I can only speculate and generalize \$\endgroup\$
    – Voltage Spike
    May 17, 2017 at 22:00

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