In SPICE-based simulators, specific problems arise with certain circuit topologies. One issue is the absence of a DC path to ground at some node. This may happen when two capacitors are connected in series with no other connection at the common node, or when code models are cascaded. The result is an ill-conditioned or nearly singular matrix that prevents the simulation from completing. Configuring with XSPICE introduces the rshunt option to help eliminate this problem. The option inserts resistors to ground at all the analog nodes in the circuit. In general, the value of rshunt is set to some high resistance (e.g. (1000\mathsf{M}\Omega) or greater) so that the operation of the circuit is essentially unaffected but the matrix problems are corrected. If a `no DC path to ground' or a `matrix is nearly singular' error message is encountered, add the following .option card to the circuit deck:

.option rshunt = 1.0e12

Usually a value of (1\mathsf{ T}\Omega) is sufficient to correct the problem. In bad cases one can try lowering the value to (10\mathsf{ G}\Omega) or even (1\mathsf{G}\Omega).

A different matrix conditioning problem occurs if an inductor is placed in parallel to a voltage source. The AC simulation will fail, because it is preceded by an OP analysis. Option NOOPAC (15.1.3) will help if the circuit is linear. However, if the circuit is non-linear the OP analysis is essential. In such a case, adding a small resistor (e.g. (0.1\mathsf{m}\Omega)) in series to the inductor will help to obtain convergence.

.option rseries = 1.0e-4

adds a series resistor to each inductor in the circuit. Be careful when using behavioral inductors (see 3.2.12), as the result may become unpredictable.