User manual ORCAD PSPICE

[. . . ] OrCAD PSpice® User's Guide Copyright © 1998 OrCAD, Inc. Trademarks OrCAD, OrCAD Layout, OrCAD Express, OrCAD Capture, OrCAD PSpice, and OrCAD PSpice A/D are registered trademarks of OrCAD, Inc. OrCAD Capture CIS, and OrCAD Express CIS are trademarks of OrCAD, Inc. Microsoft, Visual Basic, Windows, Windows NT, and other names of Microsoft products referenced herein are trademarks or registered trademarks of Microsoft Corporation. [. . . ] The value of XFORM at a frequency is calculated by substituting j·w for s, where w is 2p·frequency. For transient analysis, the value of EXPR is evaluated at each time point. The output is then the convolution of the past values of EXPR with the impulse response of XFORM. We recommend looking at one or more of the references cited in Frequency-domain device models on page 6-181 for more information. Example The input to the Laplace transform is the voltage across the input pins, or V(%IN+, %IN-). The EXPR property may be edited to include constants or functions, as with other parts. The transform, 1/(1+. 001·s), describes a simple, lossy integrator with a time constant of 1 millisecond. This can be implemented with an RC pair that has a time constant of 1 millisecond. Using the part editor, you would define the XFORM and EXPR properties as follows: XFORM = 1/(1+. 001*s) EXPR = V(%IN+, %IN-) The default template remains (appears on one line): TEMPLATE= E^@REFDES %OUT+ %OUT- LAPLACE {@EXPR}= (@XFORM) After netlist substitution of the template, the resulting transfer function would become: V(%OUT+, %OUT-) = LAPLACE {V(%IN+, %IN-)}= (1/1+. 001*s)) The output is a voltage and is applied between pins %OUT+ and %OUT-. For DC, the output is simply equal to the input, since the gain at s = 0 is 1. 182 PSpice-equivalent parts For AC analysis, the gain is found by substituting j· for s. This gives a flat response out to a corner frequency of 1000/(2) = 159 Hz and a roll-off of 6 dB per octave after 159 Hz. The gain and phase characteristic is the same as that shown for the equivalent control system part example using the LAPLACE part (see Figure 41 on page 6-165). For transient analysis, the output is the convolution of the input waveform with the impulse response of 1/(1+. 001·s). The impulse response is a decaying exponential with a time constant of 1 millisecond. This means that the output is the "lossy integral" of the input, where the loss has a time constant of 1 millisecond. This will produce a PSpice netlist declaration similar to: ERC 5 0 LAPLACE {V(10)} = {1/(1+. 001*s)} Frequency response tables (EFREQ and GFREQ) The EFREQ and GFREQ parts are described by a table of frequency responses in either the magnitude/phase domain or complex number domain. The entire table is read in and converted to magnitude in dB and phase in degrees. Phase is interpolated linearly; magnitude is interpolated logarithmically. For frequencies outside the table's range, 0 (zero) magnitude is used. EFREQ and GFREQ properties are defined as follows: EXPR TABLE value used for table lookup; defaults to V(%IN+, %IN-) if left blank. series of either (input frequency, magnitude, phase) triplets, or (input frequency, real part, imaginary part) triplets describing a complex value; defaults to (0, 0, 0) (1Meg, -10, 90) if left blank. 183 Chapter 6 Analog behavioral modeling DELAY R_I group delay increment; defaults to 0 if left blank. [. . . ] A good resistor value to use in parallel is then R = 2**100e3*. 001 = 628 ohms. Below the roll-off frequency the inductor dominates; above it the resistor does. This keeps the width of spikes from becoming unreasonably narrow. Bipolar transistors substrate junction The UC Berkeley SPICE contains an unfortunate convention for the substrate node of bipolar transistors. If the capacitance model parameters are specified (e. g. , CJS) then the junction has (voltage-dependent) capacitance but no DC current. [. . . ]