ltspice

LTspice Circuit Simulation Guide

SPICE Fundamentals

Netlist Structure

* Title line (first line, always a comment)
<components>
<directives>
.END

• .END must be last line. No statements after it.
• + at start of line continues previous statement.
• Comments: * (full line) or ; (inline).

Component Syntax

<ref> <node+> <node-> <value>
R1 in out 10k
C1 out 0 100n
V1 in 0 AC 1 PULSE(0 5 0 1n 1n 0.5m 1m)

Value Notation — CRITICAL

Suffix	Meaning	Value
f	femto	1e-15
p	pico	1e-12
n	nano	1e-9
u	micro	1e-6
m	milli	1e-3
k	kilo	1e3
MEG	mega	1e6
G	giga	1e9
T	tera	1e12

M means MILLI, not mega. Use MEG for 1e6. This is the #1 SPICE mistake. 1M = 0.001, not 1000000. Unrecognized suffix letters are silently ignored — no error, just wrong value.

Waveform Sources

PULSE(Vinitial Vpulse Tdelay Trise Tfall Ton Tperiod Ncycles)
SINE(Voffset Vamp Freq Td Theta Phi Ncycles)
EXP(V1 V2 Td1 Tau1 Td2 Tau2)
SFFM(Voff Vamp Fcar MDI Fsig)
PWL(t1 v1 t2 v2 ...)
PWL file=<filename>

PWL extras (LTspice-specific):

• Relative time: PWL(0 1 +1 2 +1 3) — times become 0, 1, 2
• Repetition: REPEAT FOR n (...) ENDREPEAT or REPEAT FOREVER (...) ENDREPEAT
• Scaling: VALUE_SCALE_FACTOR=x, TIME_SCALE_FACTOR=x
• Trigger: TRIGGER <expression> — output stuck at first value when expression is false

Directives

.tran 5m                          ; transient, 5ms stop
.tran 0 5m 0 10u                  ; tstep, tstop, tstart, tmaxstep
.tran 0 5m 0 10u startup          ; LTspice-only: ramp sources from zero
.ac dec 200 10 100k               ; AC sweep, 200pts/decade, 10Hz-100kHz
.dc V1 0 5 0.01                   ; DC sweep V1, 0-5V, 10mV step
.op                               ; DC operating point
.noise V(out) V1 dec 200 10 100k  ; noise analysis
.tf V(out) V1                     ; DC transfer function
.include /path/to/model.lib       ; include library
.ic V(node)=1.5                   ; initial conditions (used with UIC)
.nodeset V(node)=1.5              ; hint for DC operating point solver

.ic forces node voltages at t=0 (use with .tran ... UIC). .nodeset is a suggestion to help the OP solver converge — the solver can override it. Mixing them up causes wrong initial states or convergence failures.

.MEAS Syntax

.meas TRAN vmax MAX V(out)
.meas TRAN vpp PP V(out)
.meas TRAN trise TRIG V(out) VAL=0.1 RISE=1 TARG V(out) VAL=0.9 RISE=1
.meas AC fc WHEN mag(V(out)/V(in))=0.707
.meas AC gain_1k FIND mag(V(out)) AT=1k
.meas TRAN avg_out AVG V(out) FROM=1m TO=5m
.meas TRAN energy INTEG V(out)*I(R1)

Gotchas:

• RISE/FALL/CROSS numbering starts at 1, not 0.
• If TRIG event never occurs, measurement silently fails (no error, no warning).
• Without TD= parameter, TARG matches from t=0 — can hit wrong edge.
• AC measurements use 65k point ceiling — exceeding this silently reduces resolution.
• WHEN/AT measurements return the crossing time (.tran) or frequency (.ac) in the result's at field; the headline values scalar is the constant target LEVEL, not the crossing point.

General Pitfalls

• Node "0" vs "00": Different nodes. Ground is 0 (or GND).
• Impedance ratios: Beyond ~1e16 cause numerical issues (64-bit doubles).
• Parameter sweep: .step param <name> <start> <stop> <increment>
• Parameter list: .step param <name> list <v1> <v2> ...

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LTspice-Specific

Parameters and Expressions

.param Rval=10k
.param fc={1/(2*pi*R1*C1)}
.func myfn(x) {x*2}

• Component values referencing params MUST use braces: R1 in out {Rval}
• .param using other params MUST use braces: .param x={y*2}
• .func body uses braces: .func myfn(x) {x*2}
• B source expressions: do NOT wrap the expression itself in curly braces — parameters inside B source expressions DO use braces: B1 out 0 V=V(in)*{Rval}

Behavioral Sources (B sources)

Four types:

B1 out 0 V=<expression>                      ; voltage source
B2 out 0 I=<expression> [Rpar=x] [Cpar=x]    ; current source
B3 out 0 R=<expression>                       ; resistor (undocumented)
B4 out 0 P=<expression> [VprXover=x]          ; power sink (undocumented)

Conditional: IF(cond, true, false) — NOT ternary ?: (that's ngspice). B source expressions must be single-line in schematics (netlists can use + continuation).

Operator precedence:

1. ~, ! (boolean NOT)
2. ** (exponentiation) — ^ is XOR except in Laplace expressions
3. *, /
4. +, -
5. ==, >=, <=, >, < (comparisons → boolean)
6. ^ (XOR), | (OR), & (AND)

Boolean: >0.5 is True, ≤0.5 is False.

Math functions:

• Trig: sin, cos, tan, asin, acos, atan, atan2(y,x), hypot(y,x)
• Hyperbolic: sinh, cosh, tanh, asinh, acosh, atanh
• Exp/log: exp, ln, log (base e), log10
• Power: sqrt, pow(x,y), pwr(x,y) (sign-preserving), pwrs(x,y), square
• Rounding: round, int, floor, ceil
• Limits: min, max, limit(x,lo,hi), uplim(x,pos,z), dnlim(x,neg,z)
• Logic: buf, inv
• Lookup: table(x,x1,y1,x2,y2,...) — monotonic x required

Time-domain functions:

• ddt(x) — time derivative
• idt(x[,ic[,assert]]) — integral; assert≠0 resets
• sdt(x) — alternate integral
• delay(x,y) — delay by y seconds
• uramp(x) — ramp: x if x>0, else 0
• u(x), stp(x) — unit step (undocumented)

Random: rand(x) (sharp), random(x) (smooth), white(x) (noise ±0.5)

Special variables: time, pi, boltz (1.38e-23), planck (6.63e-34), echarge (1.60e-19), kelvin (-273.15), Gmin (1e-12)

Laplace filter:

B1 out 0 V=V(in) Laplace=1/(1+s/{2*pi*fc})

In Laplace expressions, ^ means exponentiation (not XOR). Response must roll off at high frequencies.

Gotchas:

• ^ is XOR in normal expressions, exponentiation only in Laplace. Use ** for power.
• R=<expr> behavioral resistor: value must never reach zero (causes convergence failure).
• NoJacob flag exists but "greatly increases risk of convergence problems" — avoid.

Monte Carlo

LTspice has no built-in .mc directive — use .step + mc():

.step param run 1 100 1
R1 in out {mc(10k, 0.1)}         ; uniform dist, 10k +/-10%

mc(nominal, tolerance) — uniform between nom*(1-tol) and nom*(1+tol).

Convergence

.options gmin=1e-10               ; min conductance on diode/transistor junctions
.options abstol=1e-10             ; absolute current tolerance (default 1e-12)
.options reltol=0.003             ; relative tolerance (never exceed 0.003)
.options cshunt=1e-15             ; capacitance from every node to ground
.options method=gear              ; alternate integration method

Circuit design tips:

• p/n junctions should have some series resistance and parallel capacitance.
• Avoid strict ideal voltage sources — add realistic parasitics.
• Impedance ratios beyond 1e16 cause numerical issues.
• Be suspicious of circuits needing cshunt — may indicate unrealistic models.

Hidden defaults (LTspice-specific):

• Gfarad — default parallel conductance on capacitors (1e-12). Disable: .options Gfarad=0
• DampInductors — default parallel resistance on inductors (ON). Disable: .options DampInductors=0
• Gfloat — shunt conductance on floating nodes (1e-12 default)
• Inductor coupling factor K cannot reach exactly 1.0 — max is 1-1n

.options Flags (LTspice-specific)

Flag	Effect
List	Dump flattened netlist to error log
DampInductors=0|1	Toggle parallel inductor damping
Thev_Induc=0|1	Toggle 1mOhm series inductor resistance
Gfarad=<value>	Capacitor default parallel conductance
Gfloat=<value>	Floating-node shunt conductance
TopologyCheck=2	Beta circuit matrix optimizations
baudrate=<rate>	Enable eye diagram plotting

Subcircuits

.subckt myfilter in out params: R=10k C=100n
R1 in out {R}
C1 out 0 {C}
.ends myfilter

• .include <path> — include file contents verbatim.
• .lib <path> — same as .include in LTspice (no section argument needed).
• Model aliasing: .model 3904 ako: 2N3904 — inherit and override parameters.
• Model stepping: .step param STM list 3904 2222 with Q1: {STM}.

Design workflow: .cir first, .asc last

Design and iterate over .cir netlists — plain text, no placement overhead, fast to edit and simulate. Only build .asc schematics after the circuit design is finalized or when the user needs a visual schematic for review. The .asc tools are for presentation, not design iteration.

.asc Schematics

.asc files are structured text representing the schematic graphically. While technically readable, hand-editing is error-prone — use the server's schematic tools (create_schematic, add_component, connect, apply_schematic_ops, ...) or LTspice's GUI. These are available in both the full and agentic profiles — geometry-aware editing (orthogonal routing, pin-collision and junction checks) that hand-writing the file can't match. Ack-only mutations (move/remove a component, set an attribute, add or remove a net label, remove a wire) are apply_schematic_ops ops rather than standalone tools, so batch them in one transaction.

• Component attributes: Value, Value2, SpiceLine, SpiceLine2.
• Export to netlist for direct text editing when needed.
• Bus notation: Data[0:7] creates 8 nets (cosmetic — netlister flattens to individual nets).

Common symbol pin offsets (at R0)

Symbol	Pins (name: x,y)	Size (WxH)
nmos	D:(48,0) G:(0,80) S:(48,96)	48x96
pmos	D:(48,0) G:(0,80) S:(48,96)	48x96
voltage	+:(0,16) -:(0,96)	64x80
current	+:(0,0) -:(0,80)	64x80
res	A:(16,16) B:(16,96)	32x80
cap	A:(16,0) B:(16,64)	32x64

Rotations transform pin (x,y) as: R90→(-y,x), R180→(-x,-y), R270→(y,-x), M0→(-x,y), M180→(x,-y). Use symbol_info for exact positions.

MOSFET orientation conventions

Rotation	Gate side	D/S vertical	Typical use
R0	Left	D top, S bottom	NMOS (drain up)
M0	Right	D top, S bottom	NMOS mirrored (symmetric diff pair)
M180	Left	D bottom, S top	PMOS (source to VDD at top)
R180	Right	D bottom, S top	PMOS mirrored (gate faces right)

Choose orientation based on where the gate connects:

• Gate wire must NOT cross through the component's own body. Pick the rotation that puts the gate on the side facing the signal source.
• Example: if M3's gate connects to M5 on the right → use M0 (gate right), not R0 (gate left).
• For diff pairs: M1 at R0 (gate left, toward Vinp), M2 at M0 (gate right, toward Vinn).
• For PMOS current mirrors: M4a at R180 (gate right, toward center), M4b at M180 (gate left, toward center) — gates face each other.
• Use symbol_info with the intended rotation to verify pin directions before placing.

Schematic layout best practices

Component placement:

• Tier alignment: Matched/mirrored transistors (diff pairs, current mirrors, bias mirrors) MUST share the same y-coordinate. Plan horizontal tiers: VDD rail → PMOS loads → diff pair → tail/bias → VSS.
• Drain/source alignment on each branch: Within a vertical branch (e.g., PMOS load stacked above NMOS input), position components so the drain pin of the upper device is on the same x-column as the drain pin of the lower device. This eliminates horizontal jogs between stacked transistors.
• Pin-to-rail alignment: Place voltage/current sources so their pins land directly on the rail they connect to — no wire through the source body. For a VDD source, position it so the + pin y-coordinate equals the VDD rail y-coordinate. Use symbol_info to compute the exact placement origin from the desired pin position (e.g., for voltage + at y=128, place origin at y=128-16=112).
• Minimum 128 units vertical spacing between pin levels of adjacent tiers (e.g., between PMOS drain y and NMOS drain y). This leaves room for horizontal buses and net labels between tiers. With MOSFET bbox height of 96, plan tier origins ~192 units apart.
• Bias circuit alignment: Bias devices (e.g., M5/Ibias) should share the y-level of their functional counterpart (e.g., M3 tail current source).
• Plan the full layout before placing: Decide VDD rail y, tier y-coordinates, and bus y-coordinates first. Verify that buses fit between bounding boxes of adjacent tiers. Use symbol_info to check bbox extents at the intended rotation.

Wiring:

• All wires must be orthogonal — strictly horizontal or vertical. Never route diagonal wires. Use waypoints in connect for L-shaped or multi-segment routes.
• Horizontal buses must route OUTSIDE all component bounding boxes. Use symbol_info to check bbox extents. For PMOS M180 with bbox top at y=160, a gate bus at y=176 is INSIDE the bbox — route at y=144 (between VDD rail and bbox top) instead. Plan bus y-coordinates BEFORE placing components.
• Vertical wires must not pass through component bodies to reach a bus. When connecting a drain to a horizontal bus, jog the wire horizontally outside the bbox first, then route vertically to the bus. Example for PMOS M180 diode connection: route drain (400,256) → right to (448,256) → up to (448,144) → along bus to label, NOT straight up through the body at x=400.
• Leave room for buses between tiers. The minimum 128-unit tier spacing must account for bounding box height plus bus clearance. For PMOS M180 (bbox height 96), if VDD rail is at y=128 and PMOS origins at y=288: bbox occupies y=192–288, bus fits at y=144–160 (between rail and bbox top).
• Heed connect warnings and errors: the tool refuses diagonal wires, pin collisions, and wire junction overlaps. Non-blocking warnings (long runs, bbox crossings) should still be addressed.

Ground and net labels:

• Local ground flags: Place a ground (0) label directly at each grounded pin via an apply_schematic_ops add_net_label op. Never route wires to a distant ground flag.
• One ground per pin: Each component's ground connection gets its own add_net_label op at the pin's coordinates — do not share ground flags between components.
• Do not use connect with net:0 when multiple ground labels exist — the tool errors on ambiguous net references. Place ground flags directly at pin coordinates with an add_net_label op (net="0", pin="M3.S") — no wire needed when the flag is on the pin.
• Named nets (VDD, outp, etc.): Use a single label per unique net name. Connect components to it via connect with net:NAME or waypoints.

Sources:

• Voltage source polarity: + pin is at the top (smaller y), - at bottom. For VDD sources, + connects to the supply rail, - to ground.
• Current source direction: Current flows from + (top) to - (bottom) externally. Place with + on the higher-voltage rail.

Models:

• Model names must not collide with type keywords: Use NMOS_3V3 not NMOS for .model names when the symbol Value is also a MOSFET type.

Other LTspice Quirks

• Unicode mu: LTspice replaces u with Unicode mu (µ) in saved files. Can corrupt netlists on copy/paste.
• startup keyword: LTspice-only in .tran. Ramps sources from zero. Not portable.
• A-devices (mixed-signal primitives like SRflop, Counter, OTA): LTspice-proprietary.
• *!LTspice: <directive>: Treated as a directive, not a comment — despite * prefix.
• Area multipliers: Undocumented m=<value> works on R, Q, J in addition to documented devices.
• Capacitor multiplier: x<number> instead of m=<number> (e.g., x2).