parametric analysis

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Parametric analysis

OverviewThis course describes how to set up parametric andtemperature analyses. Parametric and temperature areboth simple multi-run analysis types.

COURSE 6

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Parametric analysis

Minimum requirements to run a parametric analysisMinimum circuit design requirementsSet up the circuit according to the swept variable type as listed in table 1 .Set up a DC sweep, AC sweep, or transient analysis.

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Parametric analysis

Minimum program setup requirements1 In the Simulation Settings dialog box, from the Analysis type list box, select Time Domain Transient).2 Under Options, select Parametric Sweep if it is not already enabled.3 Specify the required parameters for the sweep.

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Parametric analysis

Note: Do not specify a DC sweep and a parametric analysis for the same variable.

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Parametric analysis

Overview of parametric analysisParametric analysis performs multiple iterations of a specified standard analysis while varying a global parameter, model parameter, component value, or operational temperature. The effect is the same as running the circuit several times, once for each value of the swept variable.

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Temperature analysis

Minimum circuit design requirements None.Minimum program setup requirements1 In the Simulation Settings dialog box, from the Analysis type list box, select Time Domain (Transient).2 Under Options, select Temperature Sweep if it is not already enabled.3 Specify the required parameters for the sweep.

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Temperature analysis

For a temperature analysis, PSpice reruns standard analyses set in the Simulation Settings dialog box at different temperatures.You can specify zero or more temperatures. If no temperature is specified, the circuit is run at 27°C. If more than one temperature is listed, the simulation runs once for each temperature in the list. Setting the temperature to a value other than the default results in

recalculating the values of temperature-dependent devices.

Temperatures can also be achieved using parametric analysis .With parametric analysis, the temperatures can be specified either by list, or by range and increments within the range.

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Using global parameters and expressions for values

In addition to literal values, you can use global parameters and expressions to represent numeric values in your circuit design.

Global parameters

A global parameter is like a programming variable that represents a numeric value by name. Once you have defined a parameter (declared its name and given it a value), you can use it to represent circuit values anywhere in the design; this applies to any hierarchical level.Some ways that you can use parameters are as follows:Apply the same value to multiple part instances.Set up an analysis that sweeps a variable through arange of values (for example, DC sweep or parametric analysis).

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When multiple parts are set to the same value, global parameters provide a convenient way to change all of theirvalues for “what-if” analyses.

Example: If two independent sources have a value defined by the parameter VSUPPLY, then you can change bothsources to 10 volts by assigning the value once to VSUPPLY.

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Declaring and using a global parameterTo use a global parameter in your design, you need to:define the parameter using a PARAM part, anduse the parameter in place of a literal value somewhere in your design.

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To declare a global parameter1 Place a PARAM part in your design.2 Double-click the PARAM part to display the Parts spreadsheet, then click New.3 Declare up to three global parameters by doing the following for each global parameter:a Click New.b In the Property Name text box, enter NAMEn, then click OK. This creates a new property for the PARAM part,NAMEn in the spreadsheet.c Click in the cell below the NAMEn column and enter a default value for the parameter.d While this cell is still selected, click Display.e In the Display Format frame, select Name and Value, then click OK.

Example: To declare the global parameter VSUPPLY that will set the value of anindependent voltage source to 14 volts, place the PARAM part, and then create anew property named VSUPPLY with a value of 14v.

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To use the global parameter in your circuit1 Find the numeric value that you want to replace: a component value, model parameter value, or other property value.2 Replace the value with the name of the global parameter using the following syntax:{ global_parameter_name }The curly braces tell PSpice to evaluate the parameter and use its value.Example: To set the independent voltage source, VCC, to the value of the VSUPPLYparameter, set its DC property to {VSUPPLY}.

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ExpressionsAn expression is a mathematical relationship that you can use to define a numeric or boolean (TRUE/FALSE) value.PSpice evaluates the expression to a single value everytime:it reads in a new circuit, anda parameter value used within an expression changesduring an analysis.

Example: A parameter that changes with each step of a DC sweep or parametric

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Specifying expressionsTo use an expression in your circuit1 Find the numeric or boolean value you want to replace: a component value, model parameter value, other property value, or logic in an IF function test.2 Replace the value with an expression using the following syntax:

{ expression }where expression can contain any of the following:

standard operatorsbuilt-in functionsuser-defined functionssystem variables listeduser-defined global parametersliteral operandsThe curly braces tell PSpice to evaluate the expression and use its value.

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Example: Suppose you have declared a parameter named FACTOR (with a value of1.2) and want to scale a -10 V independent voltage source, VEE, by the value ofFACTOR. To do this, set the DC property of VEE to:

{-10*FACTOR}

PSpice evaluates this expression to:

(-10 * 1.2) or -12 volts

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Lab work - parametric analysis

Analyzing an amplifier’s settling time using performance analysis

Settling time is a key performance parameter for an amplifier. The standard simulation methodology to test for this parameter steps the input voltage over the relevant input range and measures the time taken for the output to settle to some defined value close to its steady state value. The defined value depends upon the resolution of the system. For example, a 12 bit system in a range of ten volts will probably need to settle to within 1.2 mV (1/2 lsb) of its final value.

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During the design of such an amplifier, many parameters are varied to optimize the settling time. It can become extremely tedious moving along the response curves to find the exact settling time. Performance Analysis by means of "goal function" definition can facilitate this investigation. To demonstrate the implementation of the relevant goal functions, the settling time of an LF411 in unity gain configuration will be computed as a function of load capacitance ( figure 1). A parametric sweep of the parameter "cload" over the range of 100pF to 700pF using 7pF steps will generate the data for the performance analysis.

http://www.pcb.cadence.com/Technical/Library/Schematic/ContentPage/tn/tn104.htm

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Figure 2 shows the response of the system for three different load capacitors, to a one volt step at the input. The method normally used to estimate the settling time from these curves is fairly straightforward. We simply start at the end point and scan backwards along the curve until we find a point where the response curve intersects the defined settled value


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The next step is to create a goal function to measure the settling time by choosing Goal Function in the Trace menu. It will be called "settle".

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The goal function definition ( Figure 4) performs the backwards search from the end of the run to where the defined value (1.01 volts in this case)

intersects the curve.


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Using this goal function, we can now examine the settling time versus load capacitance using Performance Analysis in the Trace menu.

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We’ll use the Wizard. The next step is to choose the goal function (settle).

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In the next step, we choose the name of the trace to search.

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Performance Analysis then evaluates the goal function for the entire family of waveforms (one for each step of cload). The result is shown in Figure 8.


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The curve in Figure 8 shows the trend, but several inconsistent discontinuities are noticeable. To appreciate where the discontinuities come from, we must first visualize the oscillation which intersects the defined level. With increasing load, this oscillation will increase in amplitude as will the cycle after it. At some point, however, the succeeding cycle will grow enough to intersect the defined level, giving a jump of half the oscillation period.

To smooth the curves even further, a more appropriate function could be defined. We first detect the peaks of the cycles in the neighborhood of the defined value. We can then fit a polynomial to these points and use this to predict the settling time. The goal functions to implement this are shown below as S1, S2, and S3, which are three components of the Lagrangian polynomial. In the example shown, the Lagrangian components are evaluated at a defined level of 1.01, which, when added together, will produce the settling time curve to 10 mV.


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Different settling time curves can be produced by modifying the goal functions for different defined levels. Figure 12 shows the curves for the settling time to 20 mV, 10 mV, and 5 mV, by setting the defined level at 1.02, 1.01, and 1.005, respectively. Note that both the "marked point expression" and the LEVEL function are modified for each distinct defined level. The resulting settling time can be defined as a macro expression (Macro in the Trace menu):


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parametric analysis

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