# Part Tolerance Specification

In this section we will only cover the simple case of how to specify tolerances on devices at the schematic level for use with Monte Carlo, sensitivity and worst-case analyses. SIMetrix has much more comprehensive features for specifying tolerances. For complete documentation on tolerance specification please refer to the Simulator Reference Manual/Monte Carlo, Sensitivity and Worst-case/Specifying Tolerance.

In this topic:

## Setting Device Tolerances

To select individual device tolerances proceed as follows:

1. Select part or parts whose tolerances you wish to be the same. (You can individually select parts by holding the control key down and left clicking on each).
2. Select menu Monte Carlo > Set Selected Component Tolerances... and enter tolerance in the dialog box. You may use the '%' symbol here if you wish, so 5% and 0.05 have the same effect. (Note: this is the only place that '%' is recognised - you can't use it netlists or models).
If all the resistors or all the capacitors in a circuit are to have the same tolerance, select either Monte Carlo > Select All Capacitor Tolerances or Monte Carlo > Select All Resistor Tolerances.
Device tolerances can be applied to the following parts:
• Capacitors
• Resistors
• Inductors
• Fixed voltage sources
• Fixed current sources
• Voltage controlled voltage sources
• Voltage controlled current sources
• Current controlled voltage sources
• Current controlled current sources
• Lossless transmission lines (applied to Z0 parameter)

Device tolerance will be ignored for other devices.

## Model Tolerances

Tolerances may be applied to .MODEL statements using a distribution function. For example:
.MODEL NPN1 NPN IS=1.5e-15 BF={180*GAUSS(0.5)}

The GAUSS function above returns a random number base around 1.0 with a 3$\sigma$ Gaussian distribution of 0.5. (Gaussian distribution with standarad deviation of 0.5/3).

There are many other distribution functions including a user defined function which can be used for any distribution. In addition you can use parameter definitions to define random variables to create matched devices and parameters. Full details may be found in the Simulator Reference Manual/Monte Carlo, Sensitivity and Worst-case/Specifying Tolerances.

## Matching Devices

Some devices such as resistor networks are constructed in a manner that their tolerances track. Such devices often have two specifications one is an absolute tolerance and the other a matching tolerance. A thin film resistor network might have an absolute tolerance of 1% but a matching tolerance of 0.05%. This means that the resistors will vary over a +/-1% range but will always be within +/-0.05% of each other.

To specify matched devices for Monte Carlo analysis two pieces of information are required. Firstly, the parts that are matched to each other must be identified and secondly their matching tolerances need to be specified.

### To Identify Matched Devices

• Select the parts you wish to match to each other. (Use control key to select multiple parts.)
• Select menu item Monte Carlo > Match Selected Devices
• You must now supply a lot name which must be unique. You can use any alphanumeric name.

### Matching Tolerances

To specify device match tolerances, proceed as follows:

• Select the parts you wish to match to each other. (Use control key to select multiple parts.)
• Select menu item Monte Carlo > Set Match Tolerances
• Enter the desired tolerance.
If using device tolerance parameters, note that any absolute tolerance specified must be the same for all devices within the same lot. Any devices with the same lot name but different absolute tolerance will be treated as belonging to a different lot. For example if a circuit has four resistors all with lot name RN1 but two of them have an absolute tolerance of 1% and the other two have an absolute tolerance of 2%, the 1% resistors won't be matched to the 2% resistors. The 1% resistors will however be matched to each other as will the 2% resistors. This does not apply to match tolerances. It's perfectly OK to have devices with different match tolerances within the same lot.

## Random Distribution

The default distribution for device tolerances is Gaussian with the tolerance representing a $3\sigma$ spread. This can be changed to rectangular using two simulator options. These are:

 MC_ABSOLUTE_RECT If set absolute tolerances will have a rectangular distribution. MC_MATCH_RECT If set matching tolerances will have a rectangular distribution.

Distributions can be specified on a per part basis or even a per parameter basis by using distribution functions in an expression. See Simulator Reference Manual/Monte Carlo, Sensitivity and Worst-case/Specifying Tolerances for details.