A layout system for mixed analog/digital standard cell LSI's is described. The system includes interactive floorplan and placement features and automatic global and channel router. In mixed analog/digital circuits, crosstalk noise causes chip performance degradation. Thus, the proposed global routing algorithm routes analog nets in areas that are free of digital nets as much as possible. The number of line crossovers, especially for analog nets, is minimized by both global and detailed routers, because these crossovers are the dominant factors in the crosstalk noise. Double width lines can be used to avoid unexpected voltage drops caused by parasitic resistances. A postprocess automatically puts up shield lines for very noise sensitive wirings to improve the S/N ratio. Experimental results show that the proposed algorithms are effective in reducing the number of crossovers and redundant vias.