An Overview of Analog and Mixed-Mode Circuit Simulation

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SPICE Simulation of Analog Circuits

An analog circuit consists of a source, one or more passive or active devices and a ground. The sources, devices and the ground are connected to one another using wires. Wires carry electric currents. The interconnections of wires create circuit nodes. Each node has a voltage, and each wire entering a node carries a current. A wire connecting two nodes forces the voltages of those two node to be equal. Analog circuits are governed by Kirchhoff’s current and voltage laws (KCL and KVL). Individual analog devices are characterized by v-i equations, which relate the terminal current(s) of a device to the voltages across those terminals or pins. Passive devices like resistors, capacitors and inductors are governed by linear v-i equations (Ohm’s law), while active devices like diodes and transistors are usually characterized by nonlinear v-i equations.

Running a live simulation of an Op-Amp circuit in RF.Spice A/D with circuit animation enabled.

Sources supply your circuit with known voltages or currents. The voltages and currents at all other nodes of your circuit are the unknowns in an analog circuit simulation. Over the past 30 years, the faculty and graduate students at the University of California, Berkeley, have developed extensive computer algorithms for analog circuit simulation. This project, and the resulting computer code, are known as SPICE. RF.Spice A/D uses the Berkeley SPICE simulation engine (with a number of modifications and enhancements) for all circuit simulations. Since SPICE's algorithms have been in widespread use by the industry for a long period of time, you can be confident in the accuracy of the results you get from RF.Spice A/D. We have also incorporated major pieces of the XSPICE program from Georgia Institute of Technology. XSPICE expands the Berkeley SPICE by providing for mixed-mode simulation, a code modeling module that makes it easier to add new models, and inter-process communications. Further, we have modified the DC transfer function to allow for more general parameter sweeps, and we have added Monte Carlo simulations and parameterized AC and transient simulations.

SPICE algorithms incorporate the behavior a variety of devices including resistors, capacitors, inductors, mutual inductors, independent voltage and current sources, four types of dependent sources, switches, uniform distributed RC lines, and the five most common semiconductor devices: diodes, BJTs, JFETs, MOSFETS, and MESFETS. RF.Spice A/D uses the SPICE algorithms and offers the same circuit capabilities. The BSIM3v3.2 model is another piece that we have added to the simulation engine. This was developed by the bsim3 development team at UC Berkeley. SPICE offers default models of the semiconductor devices; however, the desired parameter values need to be specified. Default model parameters are available when you create a new model in the Device Editor. You can also specify additional device models if the defaults are not adequate.

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Running a transient test of the same Op-Amp circuit as above in RF.Spice A/D.

Analog Circuit Analysis Types

SPICE provides several different types of analysis, which can be categorized into three primary types: DC Bias (Operating Point) Analysis, AC Frequency Sweep, and Transient Analysis. DC and Transient analyses are iterative processes which terminate when both of the following conditions are satisfied:

  • Nonlinear branch currents converge to within a tolerance of 0.1% or 1 picoamp, whichever is larger.
  • Node voltages converge to within a tolerance of 0.1% or 1 microvolt, whichever is larger.

Failure to converge in DC analysis usually results from incorrectly specifying circuit connections, initial values, or model parameters. Regenerative switching circuits or circuits with positive feedback will often fail to converge unless the OFF option is used for some of the devices in the feedback path. AC frequency sweep analysis performs a nodal admittance solution of your circuit using the "Small-Signal Approximation". Under small-signal assumptions, all the nonlinear active devices of your circuit are replaced with locally linearized behavioral models established about their computed DC bias points.

The following table lists RF.Spice's available analog circuit simulation and test types (click on each type to learn more about it):

Analysis Domain Output Data
Live Simulation Nonlinear Time-Domain Analysis Node Voltages, Branch Currents, Device Powers
Transient Test Nonlinear Time-Domain Analysis Time-Domain Node Voltages, Branch Currents, Device Powers, and their Frequency Spectra
DC bias Test Operating Point Analysis DC Node Voltages, Branch Currents, Device Powers
DC Sweep Test Operating Point Analysis DC Node Voltages, Branch Currents, Device Powers
DC Sensitivity Test Operating Point Analysis Derivatives of DC Node Voltages, Branch Currents, Device Powers
Device Output Parameters Test Operating Point Analysis Table of Operating Point Details of Primitive Devices
Model Output Parameters Test Operating Point Analysis Table of Operating Point Details of Process Models
AC Sweep Test Small-Signal Frequency-Domain Analysis Frequency-Domain Node Voltages, Branch Currents, Device Powers
AC Sensitivity Test Small-Signal Frequency-Domain Analysis Derivatives of Frequency-Domain Node Voltages, Branch Currents, Device Powers
Distortion Test Small-Signal Frequency-Domain Analysis Fundamental and Higher Harmonic Contents
Transfer Function Test Small-Signal Frequency-Domain Analysis Gain, Input Impedance, Output Impedance
Pole-Zero Test Small-Signal Frequency-Domain Analysis List of Poles and Zeros
Noise Analysis Test Small-Signal Frequency-Domain Analysis Frequency-Domain Voltages, Currents and Powers of Noise Sources
Network Analysis Test Small-Signal Frequency-Domain Analysis Z, Y, H and S Parameters

Defining Analog & Mixed-Mode Inputs & Outputs

RF.Spice A/D allows both analog and digital stimuli for live simulations. For analog stimuli, you can use the voltage and current sources or the Function Generator or Power Supply in the Virtual Instruments panel. For a digital stimulus, you can use a digital input, a digital source or a digital clock. In a mixed-mode circuit, you may define both analog and digital stimuli. Note that when you place a voltage or current source in a circuit, Workshop automatically adds a corresponding Function Generator for you.

A simple resistive circuit with a voltmeter and ammeter.

The transient properties of a voltage source or a current source can be set in the Transient tab of their property dialogs. This allows you to define a variety waveforms including constant DC, sinusoidal, square wave, exponential, and piecewise linear. A piecewise linear waveform is defined as a table of values vs. time. You can edit this waveform table right form the dialog by clicking the button labeled Edit Table and entering the (time, value) pairs row by row in a text editor. Or you can also import these data from an external text file by clicking the button labeled Load From File. To run an AC-type test of your circuit, you need to define additional parameters in the Small-Signal AC and Distortion tab of the source's property dialog. You have to check the Use box and enter values for the Amplitude and Phase of your harmonic source. You can also define a Small-Signal Offset for your source from this dialog. This is particularly useful for settings the bias of active devices.

The voltmeter, ammeter and markers all can be used to probe voltages and currents for analog simulations. You can also probe the circuit using the virtual instruments such as the oscilloscope, voltmeter, ammeter, wattmeter, etc. You can use a digital output or digital probe to observe the state of your digital circuit. Mixed-mode circuits may involve both analog and digital probes. Note that virtual voltmeters and ammeters are added to the virtual instrument panel automatically every time you place a voltmeter or ammeter in your schematic.

When you run one of RF.Spice's many test types on your circuit, you will have access to all the computed voltages and currents at all the nodes and branches of your circuit. However, before you run the test, you have to explicitly indicate which quantities you want to compute. This is done in the "Output Results Options" section of the Test Panel, where you can designate one or more preset graph or table plots.

The DC and Transient tab of Voltage Source dialog showing a sinusoidal waveform.
The DC and Transient tab of Voltage Source dialog showing a square wave signal.
The DC and Transient tab of Voltage Source dialog showing a piecewise linear waveform.
The Small-Signal AC and Distortion tab of Voltage Source dialog showing the AC signal settings.

Setting the Simulation Parameters

RF.Spice A/D operates based on a large number of numerical simulation parameters with default values. In most cases, you do note have to worry about setting or changing these values. However, in certain cases, the simulation of your circuit may not converge or hit a singularity. Then you might be able to improve the simulation speed by adjusting certain parameters. You do this by accessing the Simulate > SPICE Simulation Options... menu. The Simulation Options dialog is divided up into three tabs: Convergence, Mixed-Mode, and Miscellaneous.

The SPICE Simulation Options dialog.

Temperature options

Temperature options can be found under the Miscellaneous Options tab. To set the operating temperature for the simulation, use temp. Tnom is used to set the temperature at which the model parameters were measured at. Don’t use this unless you create custom models and you know what temperature you measured their parameters at.

Convergence options

There are a lot of options available to help with simulation convergence. This reflects the difficulty in getting some circuit simulations to converge.

For operating point convergence, some particularly useful options are itl1, rshunt, reltol, gmin, gminsteps, vntol and abstol. Itl1 is the iteration limit for the operating point and increasing it from 40 to a higher value will help with convergence. Rshunt is the resistance from every node to ground. It defaults to a very high value (1e12). Lower values may help convergence, but lower values also could alter circuit behavior. Reltol, the relative error tolerance, can be increased to help convergence, but at the price of accuracy. If it's too high, then you may get incorrect results as is the case with a simple feedback op-amp circuit with the opamp from the devices menu. Gmin is the minimum conductance. It's used internally across junctions in transistors and diodes. If it's too small, the simulator can fail to converge. If you set it to high, however, it can affect the simulation results. Set it as high as possible without it affecting the simulation results.

Gminsteps is used in the gmin stepping part of the operating point solution. It tries to solve the circuit with various values of gmin. The higher the value of gminsteps, the better the chance for convergence.

Vntol and abstol go along with reltol in determining the error tolerance. They are useful when the node voltage or branch current approach zero and the relative tolerance becomes too constraining.

Itl6 is also helpful for operating point convergence.

Normally, itl6 should be left at zero. However, if the operating point won't converge even after using the options described above and other techniques given in the section on Operating Point Success, then Itl6 can be used. Itl6 activates the source stepping algorithm and sets the maximum number of total iterations for the source stepping algorithm. It is the same as srcsteps.

For dc sweep convergence, itl2 is useful. Increase it to help the simulator converge.

For transient convergence, a number of other options are relevant. itl4 controls the number of iterations in the transient solver at a given timepoint. Increase it to a larger value, e.g. 200, to help with convergence but this may not be enough.

Trtol and chgtol are not tolerance parameters. Trtol is related to truncation error in the Local Truncation Error timestep control algorithm that is used with the transient simulation. It is related to the timestep during the transient simulation. Chgtol is also used in the LTE algorithm. It's purpose is to prevent the transient from locking up. Both of these should not be used unless you are a spice expert.

Minbreak is the minimum break between timepoints while running a transient simulation. For very sharp transitions, minbreak should be reduced. Breakpoints are inserted by the piecewise linear sources and pulse sources and indicate changes in inputs to the circuit.

Pivtol and Pivrel are used in the numerical pivoting algorithm. Pivtol is the minimum acceptable value for a pivot and pivrel is the minimum ratio between a pivot entry and the largest entry in a column of the conductance matrix. Only expert spice users should use these.

Method switches between integration algorithms for calculating capacitor voltages and inductor currents for transient solutions. The possible values are gear for the gear method and trap for the trapezoidal method. The default is trap. Maxord goes along with the gear method and is the maximum order for the multiorder integration method. Higher values of maxord are more accurate but may slow down the simulation.

Mixed-Mode Circuit Options

Some options relate to the analog to digital and digital to analog converters that B2.Spice automatically inserts between analog and digital portions of the circuit. The parameters bb_daOutlow, bb_daOutHigh, bb_adInLow, and bb_adInHigh are for low and high output voltages for D/A converters and for low and high threshold voltages at the input of A/D converters. Bb_daOutX is the indeterminate voltage at the output of the d-to-a converter.

Maxopalter is used in operating point analysis for the maximum alternations between analog and event solutions. Maxevtiter is the maximum number of iterations for event solution. Noopalter prevents the simulator from alternating between analog and event solutions. Ramptime is the transient analysis supply ramping time, but it only seems to be used with xspice models for the capacitor and inductor (rather than the regular spice capacitor and inductor.)

ConvLimit enables convergence assistance for xspice models. ConvStep controls the size of the step between iterations on xspice model inputs. ConvAbsStep controls the maximum absolute step between iterations on xspice model inputs.

Miscellaneous Options

Defl and defw are the default mosfet length and width. You can override these in specific mosfets. They default to 100 microns if left blank. Defad and defas are the default drain and source mosfet areas.

Oldlimit uses old Spice2 mosfet limiting, and badmos3 uses the old mos model. I don't know why you would want to use either of these.

Bypass allows the device calculations to bypass updating values for unchanging elements. I presume that this would speed up the simulation.

Trytocompact is used with transmission lines.

Keepopinfo is not used by RF.Spice A/D.

For more information on these options, see Inside Spice by Ron Kielkowski published by McGraw Hill.

Signal Coverage Options

The first three buttons control how often the graph view is updated while simulation results are received by the workshop. Obviously, updating the graph more often will slow down the operation of the program.

The option to replace old simulation results prevents the workshop from getting overly cluttered from many graphs and text simulation results.

Checking the "Run simulation with interactive communications" box will allow you to view the simulation results while they are being generated, and this will also allow you to pause and stop the simulation in progress. Leaving it unchecked will run the simulations in batch mode, which cannot be stopped or paused, and will not display a graph until the simulation is complete. Batch simulations are generally faster.

Generate extra signals will generate signals for device currents in the circuit (e.g., inductor currents) and also internal device node voltages, e.g. if a diode has a resistance, then the voltage between the actual diode and its internal resistor.

Generate deep results generates signals inside of subcircuits. At a minimum, subcircuit node voltages and branch currents will be enabled. If Generate Extra Signals is enabled, then the extra signals inside of subcircuits will be generated also.

Generate all Top-level Currents will generate all possible current signals for each node.

The Spice program specification allows you to modify the spice simulation engine file path.

The simulation data directory specifies in which directory temporary data files are written to.

A Summary of SPICE Simulation Parameters

Option Name Option Description Default Value
gmin Minimum conductance 1e-12
reltol Relative error tolerance 0.001
itl1 DC iteration limit 40
itl2 DC transfer curve iteration limit 20
itl4 Upper transient iteration limit 10
itl6 Number of source steps X
gminsteps Number of Gmin steps X
shunt Shunt resistance from analog nodes to ground (XSPICE) X
abstol Absolute error tolerance 1pA
vntol Voltage error tolerance 1uV
Method Integration method trap
maxord Maximum integration order X
max_slope Maximum slope for exponential function X
smooth_domain Smoothing domain - used for unit step and ramp X
solve_delta_x Solves system for Delta-X, which is near zero, rather than for X False
bb_daOutLow Low output voltage for implicit D to A converters (XSPICE) 0V
bb_daOutHigh High output voltage for implicit D to A converters (XSPICE) 5V
bb_daOutX Indeterminate output voltage for implicit D to A converters (XSPICE) 2.5V
bb_adInLow Low Input voltage threshold for implicit A to D converters (XSPICE) 0.5V
bb_adInHigh High Input voltage threshold for implicit A to D converters (XSPICE) 4.5V
bb_daTLH Low-To-High delay for implicit D to A converters (Xspice) 1ns
bb_daTHL High-To-Low delay for implicit D to A converters (Xspice) 1ns
bb_adTLH Low-To-High delay for implicit A to D converters (Xspice) 1ns
bb_adTHL High-To-Low delay for implicit A to D converters (Xspice) 1ns
maxopalter Maximum analog/event alternations in DCOP (XSPICE) X
maxevtiter Maximum event iterations at analysis point (XSPICE) X
noopalter Do not do analog/event alternation in DCOP (XSPICE) X
ramptime Transient analysis supply ramping time (XSPICE) X
convlimit Enable convergence assistance on code model (XSPICE) X
convstep Fractional step allowed by code model inputs between iterations (XSPICE) X
convabsstep Absolute step allowed by code model inputs between iterations (XSPICE) X
autopartial Use auto-partial computation for all models (XSPICE) False
temp Operating temperature 27°C
defl Default MOSFET length 100um
defw Default MOSFET width 100um
defad Default MOSFET area of drain 0m^2
defas Default MOSFET area of source 0m^2
tnom Nominal temperature 27°C
Noopiter Go directly to gmin stepping False
minbreak Minimum time between breakpoints X
trtol Truncation error overestimation factor X
chgtol Charge error tolerance 0.01pC
pivtol Minimum acceptable pivot X
pivrel Minimum acceptable ratio of pivot X
oldlimit Use SPICE 2 MOSFET limiting False
bypass Allow bypass of unchanging elements X
trytocompact Try compaction for LTRA lines False
badmos3 Use old mos3 model (discontinuous with respect to kappa) False

In the above table, an "X" for the default value means unspecified. The SPICE engine will either ignores this parameter or generate a value for it internally.

Additional Considerations for a Successful Simulation

Setting Initial Conditions

Setting the initial guess for DC operating point

To set the initial guess at a node, place a Marker component (it's available in the devices menu) in the circuit and attach it to the node. Then double-click on the Marker, which will open a dialog box. Set the initial guess to the appropriate value, and then select OK to close the dialog box. Then select Simulation Setup from the Simulation menu, and activate the checkbox next to Initial Guess (NodeSet).

Setting Transient Initial Conditions

To set the transient initial conditions at a node, place a Marker component (it's available in the devices menu) in the circuit and attach it to the node. Then double-click on the Marker, which will open a dialog box. Set the initial condition to the appropriate value, and then select OK to close the dialog box. Then select Simulation Setup from the Simulation menu, and activate the checkbox next to Initial Conditions.

Operating Point Success

All simulations run an operating point analysis at least once during the simulation. If the operating point analysis fails, then the simulation will fail. If you are having problems with any one simulation, then the best approach for identifying the problem is to run an operating analysis by itself.

When you run a simulation, the simulation engine attempts to find a stable operating point for the circuit. In some cases, it fails. Possible reasons include circuit topology, bistable or unstable circuits, far-off initial node guesses. In addition, highly non-linear circuits are difficult for the simulator to handle and may require tweaking options. Error messages include "Iteration limit reached" and "singular matrix" or "gmin stepping failed". Look in log window after simulation is complete to find error messages.

Common problems/solutions include:

Check your node connections. Use Show Node Numbers from the View menu to check node numbers. Nodes with the same number are connected. Also, you can check the spice netlist for node numbers of devices. Often problems result from missed connections or unintentional connections.

Check for illegal configurations: Voltage sources in parallel, inductors in parallel, voltage source in parallel with inductor. In this case, add small resistor in series with vs or inductor.

current sources in series, capacitors in series, current source in series with capacitor. In this case, add large resistor in parallel with the capacitor or current source.

Check for typos: A common error is confusing zero with the letter "O" for parameter values.

If you can't find a simple error in the circuit, then try one or more of following:

increase itl1 (e.g. 200)

Use shunt resistances: Reduce RSHUNT (e.g. 1e9) low enough to have an impact on the convergence but not so low that it affects the behavior of the circuit.

increase error tolerance: This is the option reltol.

increase minimum conductance: Option Gmin.

NODESET: Set the initial guess at different troublesome nodes in the circuit. Do this by checking the box in simulation set-up for Nodeset, and by adding markers in the circuit at nodes, and setting the initial guess of the markers.

Active Devices Off: When Spice starts a simulation, it assumes, by default, that all active devices are in their active regions. Sometimes setting some active devices to OFF will help in circuits with hysteresis or with multiple feedback paths.

Transient Success

Common error message: Timestep too small. This occurs when the simulator gives up after reducing the timestep below the minimum time step.

There are several options you can try to improve the results.

Simulation Option: Increase itl4 (transient iterations) , e.g. 200. This allows more tries before giving up.

Simulation Option : ITL5: This sets the maximum number of total iterations. It defaults to 5000. Try increasing it.

Make your circuit more realistic. Most importantly, add junction capacitances on transistors. These are in model parameters for transistors.

Decrease shunt resistance: Decrease the value of the shunt resistance using Set Simulation Options.

Simulation Option : RELTOL: This is the relative error tolerance. The default is 0.1%. Try increasing it by a factor of 10, to 1.0 %. This is very useful in solving convergence problems due to the small time step of the transient algorithm.

Increase the absolute current tolerance

Special Notes for the Op Amp

The op-amp model used for the 5-pin opamp is highly non-linear and it will give the wrong results in some cases with default tolerances. To fix this, decrease the relative tolerance option to a very low value (e.g. 1e-7). In general, if the results are looking fishy, decrease the relative error tolerance.


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