The Worst Case analysis is a statistical analysis that performs multiple consecutive simulations by varying, at random, the value of all the components for which a tolerance value has been specified. The Worst Case analysis allows to study the effect that the variations of the values of the circuit elements have on the performance of the circuit.
In the worst case analysis, the value of each component is changed either positively or negatively by its maximum tolerance value. For example: a resistor with a nominal value of 1000 ohms and a tolerance of 10% will assume a value of 900 or 1100 ohms for each step.
Unlike the Monte Carlo analysis where the number of passes must be specified, the worst case analysis automatically calculates the number of passes to cover all possible combinations of tolerances. The number of combinations is 2n, where n indicates the number of parameters that vary its value. For example: if two RA and RB resistors have a nominal value of 1000 and 2000 ohms, and their tolerance value is 5% and 10%, four combinations are obtained:
| RA | RB |
|---|---|
|
950 |
1800 |
|
1050 |
1800 |
|
950 |
2200 |
|
1050 |
2200 |
The tolerance values of the components can be specified as follows:
For resistors, capacitors, inductors, and DC generators, you can specify tolerance values directly in the SPICE attributes of the component by specifying a value for the TOLERANCE parameter. This mode is useful for including tolerance values directly in component libraries.
In the Worst Case analysis dialog box, you can directly specify tolerance values for component and model parameters.
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To perform a Worst Case Analysis, select Worst Case analysis in the Analysis Mode box in the Setup dialog box for one of the following analyses: AC, Operating Point, Transient, Transfer Function, Noise, Distortion. |
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A Worst Case analysis can produce a large amount of data causing an excessive use of memory and a lower speed of simulation. To reduce the amount of data that is stored during a simulation it may be convenient to specify the names of only the vectors of interest, in this case only the data related to the specified vectors will be stored. |
Categories
Lists the categories of components for which you can specify a tolerance value. If you select a category, the Tolerances box lists all components for which a tolerance value has been specified. You can specify a tolerance value for component parameters, model parameters, and global parameters.
Tolerances
In this box you can set tolerance values for component parameters, model parameters and global parameters. To add a new tolerance value, click on the Add button and then select the component, model or global parameter to be changed.
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The list also shows, in green, the tolerance values specified in the SPICE attributes of the components included in the schematic. |
Name
Specify the name of the component, model, or global parameter. For the following categories: resistors, capacitors, inductors, and DC generators, you can specify a default tolerance value (select *default* for the name) that will be used for all components in the specified category for which no tolerance value is specified.
Parameter
Specify the parameter to which the specified tolerance values are to be applied. The term VALUE refers to the value of resistors, capacitors, inductors and DC generators as specified in the value field of the component in the schematic.
Value
You can specify a new nominal value of the parameter, or select default to use the current value.
Tolerance values
You can specify either the DEV tolerance or the LOT tolerance or both. In the latter case, the tolerance values are calculated independently and added together. The combination of DEV and LOT tolerances is useful for situations where component variations are not completely correlated, but are not completely independent of each other either. For example, in a resistive network there may be a component related LOT tolerance and a smaller DEV tolerance related to the variations of individual resistors.
Each tolerance value is defined by the following two parameters:
Dev Tol or Lot Tol. It can be specified either as an absolute value or as a percentage by adding the suffix %. For example, for a 1000 ohms resistor if you specify a tolerance of 10 you will get a variation between 990 and 1010 ohms, while if you specify 10% you will get a variation between 900 and 1100 ohms.
Group. You must assign the same group number to multiple components when you want their tolerance changes to be correlated, that is, values increase or decrease jointly. If you specify the zero group then the value of the component varies independently of all other components. Groups from 1 to 15 are available.
Analysis results
A Worst Case analysis consists of a first simulation in which the values of the components assume the nominal value and in a number of statistical simulations equal to the calculated number of repetitions. If the number of repetitions is limited, the result of the analysis can be reported as a set of curves each of which corresponds to a certain combination of the set tolerance values. If the number of repetitions is high then it is convenient to compress the results into a single curve. The available options are as follows:
| Option | Description |
|---|---|
|
Individual |
The result is a series of curves, each of which corresponds to a different combination of tolerance values. |
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Maximum Value |
The result of the analysis is the nominal value curve and the maximum value curve. |
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Minimum Value |
The result of the analysis is the nominal value curve and the minimum value curve. |
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Greatest Difference |
The result of the analysis is the nominal value curve and the curve with the point of greatest difference from the nominal curve. |
Skip tolerance values specified in the symbols
Check this box to ignore the tolerance values specified in the SPICE attributes of the components. Checking this box will cause the analysis to be performed using only the tolerance values specified in the dialog box.