PSpice Application Notes

PSpice App Note_Analog Behavioral Modeling

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APPLICATION NOTE 1 Introduction Behavioral Modeling is the process of developing a model for a device or a system component representing the behavior rather than from a microscopic description. You can use Behavioral Modeling in the domain of analog simulation to model new device types and for black-box modeling of complex systems. In this document, examples are used to show how the Analog Behavioral Modeling feature of PSpice can be used to: Calculate square roots Use ideal non-linearity from look-up tables Design small systems Pass parameters to sub-circuits Calculating Square Roots Assume that you need to create a signal whose voltage is the square root of another signal's voltage. A simple solution is to use a feedback circuit to calculate square roots. But this technique fails if the reference signal goes negative. The solution then is to use the functional form of Analog Behavioral Modeling: Esqrt out_hi out_lo value={sqrt(abs(v(input)))} Figure 1: Square Roots Sub-circuit This model takes the absolute value of the ground-referenced signal input before evaluating the square-root function. The absolute-value function is a nonlinear function. Note: You can also use a floating signal-pair in the model; for example, replace v(input) with v(in_hi)- v(in_lo) or v(in_hi,in_lo). Using Ideal Non-Linearity from Look-up Tables You can introduce ideal non-linearity using the table look-up form of Analog Behavioral Modeling. For example, the following one-line, ideal OpAmp model has high gain, but its output is clamped between ±15 volts.: Eamp out 0 table {200K*(v(in_hi)-v(in_lo))}= + (-15,-15) (15,15)

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