PSpice Application Notes

PSpice App Note_Analog Behavioral Modeling

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APPLICATION NOTE 2 Figure 2: Look-up Tables Sub-circuit The input to the table is the differential gain formula, but the look-up table has only two entries. The output of the table is interpolated between these two endpoints and clamped when the input exceeds the table's range. This is a convenient use of the table look-up form, available in PSpice. Designing Small Systems Small systems of behavioral models are easy to design using PSpice. For example, a true-RMS circuit can be built by decomposing the RMS function: 1. Square the signal 2. Integrate over time 3. Take the square-root of the time average These three operations can be bundled in a tiny sub-circuit for use as a module: Figure 3: RMS Sub-circuit .subckt RMS in out G1 0 1 VALUE {V(IN)*V(IN)} C1 1 0 1 R1 1 0 1G E1 out 0 VALUE {IF(TIME<=0, 0, SQRT(V(1)/TIME))} .ends The current source, G1, squares the signal, which is then integrated in the capacitor, C1. The voltage on the capacitor is time averaged, and the square-root is taken. The resistor is a dummy load that satisfies the algorithm. The voltage source E1 shows that the value of simulated time is available in Analog Behavioral Modeling, and may be used as a variable in a formula. Note the if-than-else function; If time is less than or equal to zero

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