Lab0: Introduction to LTSpice
for Electronics – I
By Naeem Maroof
Venue: Electronics Lab
EE Department, CIIT Islamabad
Why SPICE?
• It is: Simulation Program with Integrated Circuit
Emphasis
• Analog circuit simulation has been inseparable from
analog IC design.
• SPICE simulators are the only way to test circuitry prior
to integration onto a chip.
• SPICE simulation allows measurements of currents and
voltages that are virtually impossible to do
LTSpice
• LTSpice is intended to be used as a general purpose
schematic capture program with an integrated SPICE
simulator.
• The program includes an integrated hierarchical
schematic capture program that allows users to design
new circuits.
• There is a built-in database for most of Linear
Technology's power ICs and many passive components.
• And most importantly its Free and Robust
LTSpice
• An integrated waveform viewer displays
the simulated waveforms and allows
further analysis of the simulation data.
• The device database, schematic
editing, simulation control and
waveform analysis are integrated into
one program.
Tutorial
• What will you people learn in this
tutorial?
– Drawing the circuit
– Simulation
– Simulation Commands
Drawing the circuit
• Begin a new circuit
– File\New Schematic, or
– click on the "New Schematic" icon on the toolbar
• Drawing the circuit:
– Adding a GND
• This is very important. You cannot do any simulation on
the circuit if you don't have a ground. To place a ground,
you can
– press the 'g' key, or
– use the ground icon on the toolbar or
– get it from the 'Edit' menu.
• If you aren't sure where to put it, place it near the bottom of
the drawing.
Drawing the circuit
• Getting the other Parts:
– The next thing that you have to do is get some or all
of the parts you need.
– This can be done by
• Clicking on the icon for a specific component;
(This is good for common components such as resistors,
capacitors, etc.)
• Clicking on the 'component' button OR
• Pressing "F2" OR
• Going to "Edit" and selecting "Component..."
Drawing the circuit
• Once this box is open, select a part that you want in your circuit. This can
be done by typing in the name or scrolling down the list until you find it.
• z
Drawing the circuit
• Some common parts are:
– res - resistor
– cap - capacitor
– ind - inductor
– diode - diode
– voltage - any kind of power supply or battery
• Anything in [ ] is a library, which contains many parts.
• Each type of part can be placed multiple times
• Automatically named / numbered
• Right click or press ‘Esc’ to stop placing a particular part
Drawing the circuit
• To rotate parts: "Ctrl+R"
• To reflect (or 'Mirror') the part: "Ctrl+E".
• You can rotate / mirror before placing
• OR you can select and then rotate / mirror
• You can move / drag parts. So Don't worry about
putting it in exactly the right place
Drawing the circuit
• Placing / Arranging the Parts:
– You should have most of the parts that you need by now
– All you need is to put them in the places that make
sense (usually a rectangle works well for simple circuits).
– To move parts, click on the 'move' icon, and then you
may select parts and drag them where you want them.
– When you have a part selected for a move, you can
rotate or reflect it as well
Drawing the circuit
• Connecting the Circuit:
– Now that your parts are arranged well, you'll have to
attach them with wires.
– Go up to the tool bar and select the "Draw Wire"
button OR
– Press F3 OR
– go to "Edit" and select "Draw Wire"
– With the pencil looking pointer, click on one end of a
part, when you move your mouse around, you should
see crossed lines appear. Attach the other end of your
wire to the next part in the circuit.
Drawing the circuit
– Repeat this until your circuit is completely wired.
– If you want to make a node (to make a wire go
more then one place), click somewhere on the
wire and then click to the part
– To get rid of the pencil, right click or Press ‘Esc’
Drawing the circuit
• Changing the Name of the Part:
– You probably don't want to keep the names C1, C2 etc., especially if
you didn't put the parts in the most logical order. To change the name,
right click on the present name (C1, or R1 or whatever your part is),
then a box will pop up (Enter New Reference Designator), where you
can type in the name you want the part to have.
– Please note that if you double click on the part or its value, no box will
appear.
Drawing the circuit
• Changing the Value of the Part:
– If you only want to change the value of the part, you can right click on the
part, (not the name), and a box title by the part name (such as "Resistor") will
appear. The number of fields in the box will depend on the type of part it is.
Type in the new value and press OK. Use u for micro as in uF = microFarad.
z
Drawing the circuit
• Using Net (Node) Labels:
– These are important if you want to use your
own identifiers for points in the network where
you want to determine voltages rather than
having to work with the node numbers that
LTSpice assigns.
– To add net labels
• Press "F4", or
• Click on the "Label Net" icon
• select "Edit/Label Net" from the menu.
Drawing the circuit
– When you do this, a window will pop up where
you assign the label you want to use for the
net.
Drawing the circuit
• Saving:
– To save the circuit, use the save button on
the tool bar or any other method you would
normally use to save files.
• Printing:
– To print, you may use the menu or the print
icon
Simulation
• Before you do the simulation:
– Have your circuit properly drawn and saved.
– No floating parts on your page (i.e.
unattached devices).
– You should make sure that all parts have the
values that you want.
– There are no extra wires.
– It is essential that you have a ground in
your circuit.
Simulation
• Choosing a simulation:
– Click on the Simulate button on the tool bar
or use the "Simulate/Edit Simulation Cmd"
command.
Simulation
• Enable whatever type(s) of analysis you
want using the Edit Simulation Command
window. The last one you choose is the
one which will be done when you simulate.
• It will check to make sure you don't have
any errors. If you do have errors, correct
them.
Simulation
• Graphing:
– Go to the "View" menu:
Simulation
• Adding/Deleting Traces:
– From View menu use "Visible Traces" or click
on the toolbar
Simulation
• The add traces window allows you to choose various
signals from the circuit, or to create mathematical
expressions involving them.
• To delete a trace, select its title on the graph and press
"Delete".
• Waveform Arithmetic
– In Visible Traces, there are functions that can be performed,
these will add/subtract (or whatever you choose)
– Select the signal(s) that you wish to have displayed.
– There are many functions here that may or may not be useful. If
you want to know how to use them, you can use LTSpice's Help
Menu.
Simulation
• Labelling:
– Click on Text Label on top toolbar.
– Type in what you want to write and Click OK
– You can move this around by single clicking
and dragging.
• Finding Points:
– Click on the name of the trace you want to
look at and then a cursor window will appear,
showing information about the point currently
selected.
Simulation
Simulation
• You can use the cursor keys to move back and
forth through the data points.
Simulation
• Saving:
– To save your probe you need to go into the
tools menu and click display, this will open up
a menu which will allow you to name the
probe file and choose where to save it. You
can also open previously saved plots from
here as well.
• Printing:
– Select Print in Edit or on the toolbar
Simulation Commands
• DC Operating Point
– This is a simple, but incredibly useful analysis. It will not give you anything
to plot, but it will indicate the voltages at all nodes and currents through
all devices in the circuit.
• -
Simulation Commands
• Transient
Simulation Commands
• The transient analysis is probably the most
important analysis you can run in LTSpice, and it
computes various values of your circuit over
time.
• Two very important parameters in the transient
analysis are:
– Stop Time.
– Time to Start Saving Data
– Maximum Timestep
– various other checkboxes....
Simulation Commands
• The ratio of Stop Time to Maximum Timestep
determines how many calculations LTSpice
must make to plot a wave form.
• LTSpice always defaults the start time to zero
seconds and going until it reaches the user
defined final time.
• It is incredibly important that you think about
what timestep you should use before running the
simulation
Simulation Commands
• If you make the timestep too small the probe
screen will be cluttered with unnecessary points
making it hard to read, and taking extreme
amounts of time for LTSpice to calculate.
• However, on the other hand is the problem that if
you set the timestep too high you might miss
important phenomenon that are occurring over
very short periods of time in the circuit.
Therefore play with step time to see what works
best for your circuit.
Simulation Commands
• AC Analysis
Simulation Commands
• The AC analysis allows you to plot
magnitude and/or phase versus frequency
for different inputs in your circuit.
• Type of Sweep
In the AC analysis menu you have the
choice of three types of analysis:
– Linear,
– Octave and
– Decade.
Simulation Commands
• DC Sweep
– The DC sweep allows you to do various different
sweeps of your circuit to see how it responds to
various conditions.
– For all the possible sweeps,
• voltage,
• current,
• temperature, and
• parameter and global
– You need to specify a start value, an end value, and
the number of points you wish to calculate.
Simulation Commands
• DC Sweep
– For example you can sweep your circuit over a
voltage range from 0 to 12 volts. The main two
sweeps that will be most important to us at this stage
are the voltage sweep and the current sweep. For
these two, you need to indicate to LTSpice what
component you wish to sweep, for example V1 or V2.
– Another excellent feature of the DC sweep in
LTSpice, is the ability to do a nested sweep.
– A nested sweep allows you to run two simultaneous
sweeps to see how changes in two different DC
sources will affect your circuit.
Types of Sources
• Voltage Sources
– A voltage source can be configured in many
possible ways. Right clicking on one will bring
up the "Independent Voltage Source" window.
The options which show up in the window will
change as the function selected changes.
– (none)
• This is your basic DC voltage source that
simulates a simple battery and allows you to
specify the voltage value.
Types of Sources
• SINE
– A few things to note about the alternating
current source. First, there are two possible
analyses which can be done and so there are
two sets of parameters.
– For an ac analysis, the parameters are:
• AC Amplitude which is the peak value of the
voltage.
• AC Phase which is the phase angle of the voltage
Types of Sources
• For a transient analysis, the parameters are:
• DC offset is the DC offset voltage. It should be
set to zero if you need a pure sinusoid.
• Amplitude is the undamped amplitude of the
sinusoid; i.e., the peak value measured from
zero no DC offset value.
• Freq is the frequency in Hz of the sinusoid.
• Tdelay is the time delay in seconds. Set this to
zero for the normal sinusoid.
Types of Sources
• Theta is the damping factor. (Not the phase angle!)
Also set this to zero for the normal sinusoid.
This is used to apply an exponential decay to the
sinusoid; theta is the decay constant in 1/seconds.
• PHI is the phase advance in degrees. Set this to 90 if
you need a cosine wave form.
• Ncycles is the number of cycles of the pulse that should
happen. Leave it as zero if you want ongoing pulses.
• For this analysis, LTSpice takes it to be a sine source, so
if you want to simulate a cosine wave you need to add
(or subtract) a 90° phase shift. Note that the phase angle
if left unspecified will be set by default to 0°
Types of Sources
• PULSE
– PULSE is often used for a transient simulation
of a circuit where we want to make it act like a
square wave source. It should never be used
in a frequency response study because
LTSpice assumes it is in the time domain, and
therefore your probe plot will give you
inaccurate results.
Types of Sources
• Vinitial is the value when the pulse is not "on." So for a
square wave, the value when the wave is 'low'. This can
be zero or negative as required. For a pulsed current
source, the units would be "amps" instead of "volts."
• Von is the value when the pulse is fully turned 'on'. This
can also be zero or negative. (Obviously, V1 and V2
should not be equal.) Again, the units would be "amps" if
this were a current pulse.
• Tdelay is the time delay. The default units are seconds.
The time delay may be zero, but not negative.
• Trise is the rise time of the pulse. LTSpice allows this
value to be zero, but zero rise time may cause
convergence problems in some transient analysis
simulations. The default units are seconds
Types of Sources
• Tfall is the fall time in seconds of the pulse.
• Ton is the pulse width. This is the time in seconds that
the pulse is fully on.
• Tperiod is the period and is the total time in seconds of
the pulse.
• Ncycles is the number of cycles of the pulse that should
happen. Leave it as zero if you want ongoing pulses.
• This is a very important source for us because we do a
lot of work with the square wave on the wave generator
to see how various components and circuits respond to
it.
Reference
• http://denethor.wlu.ca/ltspice
