DC analysis finds the DC operating-point or DC transfer curves of the circuit. To generate transfer curves, you need to specify a parameter and a sweep range. The swept parameter can be circuit temperature, a device instance parameter, a device model parameter, a netlist parameter, or a subcircuit parameter for a particular subcircuit instance.
After the analysis is complete, the modified parameter returns to its original value.
The primary difference between Spectre AMS Designer and Spectre DC analysis is that digital simulation and analog/digital synchronization has to be considered in AMS. For DC convergence on analog/digital simulation iteration:
- Analog/Digital simulation at t=0 starts independently from the initial state
- Analog and digital exchange analog/digital signals at each iteration
- DC analysis converges when there are no new analog/digital signals (no new analog-to-digital and digital-to-analog events)
Spectre AMS Designer Simulator supports the following DC analysis parameters:
- State-file Parameters -
readns,write,writefinal
- Output Parameters -
save,nestlvl,print,oppoint
- Convergence Parameters -
homotopy,maxiters,maxsteps
- Annotation Parameters -
annotate
You can run multiple DC analyses at different sweep points. Then, at every sweep point, analog/digital simulation and synchronization iteration is executed. The iteration ends when no new analog-to-digital and digital-to-analog events are detected and the DC analysis converges, or the maximum number of iterations is reached at the sweep point. The table below shows the default value of the maximum iteration number for different solvers.
|
Solver |
setenv AMS_DC_MAX_ITER (default value) |
|---|---|
|
Spectre |
100 |
|
APS |
100 |
|
Fast APS (++aps) |
100 |
|
XPS MS |
1 |
fastdc Option
To speed up DC simulations, you can use the fastdc option. The solution accuracy depends on the value specified for fastdc. The available values are 0, 1, 2, 3, and 4 (default).
|
Value |
Description |
|---|---|
|
|
The most accurate DC result and closest to true DC solutions with the slowest performance. It should be used for the circuits that require accurate DC solution, such as analog circuits. |
|
|
The conservative DC result with slow performance. It should be used for the circuits that are sensitive to DC solution, such as analog dominated circuits. |
|
|
The reasonable DC result with moderate performance. It should be used for the circuits that are lightly sensitive to DC solution, such as mixed-signal circuits. |
|
|
The liberate DC results with fast performance. It should be used for the circuits that are less sensitive to DC solution, such as digital dominated circuits. |
+fastdc=4 |
The least accurate DC result with the fastest performance. It should be used for the circuits that are not sensitive to DC solution, such as digital and memory circuits. |
You can also use +msdc=ms. It is the fastest DC algorithm. It is valuable to speed up for large design power up test bench.
dcopt Option
Thedcoptoption speeds up the DC simulation in Spectre and APS for post-layout circuits that consist of large number of parasitic resistors and capacitors, which require significant time in DC simulation. It can also be used to solve the non-convergence issues in DC simulations of any other circuit.
dcopt command-line option may not be as accurate as the true DC solution.