# LT SPICE Montecarlo Analysis Clarifications required

#### hoyyoth

Joined Mar 21, 2020
249
Dear Team,

I am learning about LTSPICE MC analysis from this site. I understood almost all the things except the below one.

"Then make another spice directive to execute a monte carlo run as below
.step param run 1 100 1
The above syntax means that we are going to run the simulation for 100 times with starting point at 1 and the increment is 1.

"
My Question here is ,

What we are incrementing with a value of one. Here all the tolerances are fixed.(Please correct me if I am wrong).

Is this something like in the 1st iteration we take resistor value as R = R+Tolerance and in 2nd it is like R-Tolerance,something like that.

Are they running 100 permutation and combination like this

Regards
HARI

#### ericgibbs

Joined Jan 29, 2010
17,416
hiHARI
The param is 'run' you are running the simulation 100 times.
E

#### hoyyoth

Joined Mar 21, 2020
249
hiHARI
The param is 'run' you are running the simulation 100 times.
E
Hi Eric,

My doubt is Why to run 100 times.Two runs are enough right? One with values of all resistors and voltage source to minimum
and the the other when all these values are maximum.

Regards
HARI

#### ericgibbs

Joined Jan 29, 2010
17,416
hi Hari,
Consider the effect of the components either being +/-tolerance in a 'mix' of the tolerances.
The effect of every component +/- tolerances possible in a combinational mix should be considered for the Monty Carlo.
For example, on one 'run' R1 may be considered as R1 +tol and on the next run R1 -tol and so on for all the components marked for {mc,......}

The V3 voltage step by the DC analysis command.

E
I will post a simplified version of a circuit.

#### hoyyoth

Joined Mar 21, 2020
249
hi Hari,
Consider the effect of the components either being +/-tolerance in a 'mix' of the tolerances.
The effect of every component +/- tolerances possible in a combinational mix should be considered for the Monty Carlo.
For example, on one 'run' R1 may be considered as R1 +tol and on the next run R1 -tol and so on for all the components marked for {mc,......}

The V3 voltage step by the DC analysis command.

E
I will post a simplified version of a circuit.
Hi Eric,

Thank you.
Can I say that they are iterating with difference combinations of "tol".

Regards
HARI

#### ericgibbs

Joined Jan 29, 2010
17,416
Hi HARI.
A simple voltage divider, both 1K, with tolerance limit set to 0.05, run just 100 times in .trans.
Vo is shown for all the possible values of both resistor from tol 0.00 thru +/-0.05

Added the asc file

E

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#### hoyyoth

Joined Mar 21, 2020
249
Thank you Eric.
What I understood is given below.

A 100 ohms resistor with 10% tolerance may have values ranging from 90 ohms to 110 ohms.
The mc function generate random values between 90 and 110 ohms.If number of iterations are more
the number of random values also will be more.
Please correct me if I am wrong

#### ericgibbs

Joined Jan 29, 2010
17,416
Hi HARI,
Not random values, but the Tolerance value divided by the run time intervals, added and subtracted from the stated resistance value.
Try adjusting the Tol value and number of run times to see the effect
Using .tran shows some interesting results, the vr and vr2 are the limits expect for reference only.
E

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#### hoyyoth

Joined Mar 21, 2020
249
Hi Eric,

Thank you .Got it.

Regards
HARI