![]() ![]() ![]() With parametric variation, we easily can simulate pots and switches, or simply play "what if." The formula can be written a number of ways. This is how we do parametric variation in SPICE. Note that the Gain resistor, Rgain, has its value written as a math formula and that formula is enclosed in curly braces. The only restrictions on component values is none of them can be zero and you have to adhere to the syntax when you use multipliers like "k," "u," "Meg," etc. You can enter component values or part numbers by right-clicking on the text field you want to edit. The power supply (Vbatt) is also found in the COMPONENT list, it's called VOLTAGE. To find a MOSFET, click on the COMPONENT tool (looks like a logic gate). Every part must have a unique ref des or else LTSpice will give you a nice error message when you hit the RUN button. You can change them manually if you like. LTSpice will automatically set the ref designators sequentially. Don't worry about setting their values yet, just get all of the parts on the schematic and connect them. You can use the mouse's scroll wheel to zoom in and out of the drawing.Īdd parts one at a time, in any order, and use the Wire tool connect them. Hover the cursor over each icon to see what it does. Before you start drawing, get familiar with the Tool Bar. Click File -> New Schematic to create a blank sheet. When you start LTSpice, there is no schematic or even a sheet on which to draw it. For this example, I invite you to draw something similar to the picture below. We'll start with something simple, the Zvex SHO. Although current versions of SPICE have a graphics-based front-end, the first thing SPICE does when you hit the Run button is convert the schematic file to a text file because SPICE uses that text file as the input to the analysis engine. Enter the modern age with GUI and schematic input. The was also a list of SPICE directives (commands) that told SPICE what to do. The circuit file contained a list of the parts, their values and how they were connected. Originally, SPICE circuit files were purely text. Used together, sim and test can get you to a good design or through a troubleshooting session with the least amount of consternation. That being said, I am constantly impressed with how accurately a good circuit sim matches the test data. Testing is the best way to confirm that your circuit sim is telling the truth. Simulation and test will each show you things that the other can't. I do not rely on either testing or simulation alone, I use them in tandem. Not everything that comes out of a simulation is "good." You have to evaluate and interpret the simulation results to see if they make sense. A circuit simulation is only as accurate as the models and the user's abilities. The old adage "garbage in - garbage out" is definitely applicable here. Simulations are a powerful tool when used properly. It can be obtained for free from Analog Devices.īefore we get started, a few words about circuit simulations. You must have already installed LTSpice on your computer. You should have some very basic circuit theory under your belt, like Ohms's Law and some understanding of what the various components do. ![]() It's very helpful to already have some experience with drawing schematics using a CAD tool such as Eagle, DIPtrace or Tin圜AD. It is not a course in circuit analysis, although I will provide some tips along the way. ![]() The purpose of this tutorial is to assist you in getting up the learning curve with LTSpice. ![]()
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