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68 DESIGN007 MAGAZINE I AUGUST 2021 if you divide the plane into separate, electri- cally isolated areas using a split plane. Just because different voltage amounts are "isolated" doesn't mean you're out of the woods. When more than one voltage is assigned to a power plane layer, your CAM tool does not check whether the voltage of your via matches the section of the board it's going into. If you have a 12V via going into a 3V section of the board, it will fail. Mixing digital and analog signals can also create headaches. An analog ground is noisy, and it fluctuates, while digital grounds are qui- eter and steadier. Connecting digital and ana- log planes together probably won't result in a board failure that trips the smoke detector, but there will be performance anomalies from the noisy analog ground interfering with signals on digital pins. If you are relying on manual oversight to dis- cover such issues before production, it is easy to lose track of individual connections—espe- cially on a complex board design with hun- dreds of vias. How Do You Best Manage Mixed-Signal Output? When you need to assign one or more power nets to a layer on a simpler design, applying plane splits that segregate areas containing each voltage is a simple and effi- cient way to distribute power. Warning! is method relies on the human eye to ensure accuracy and gets risky for more complex board designs. We think a better method is to use polygon area fills to make connections. With polygon area fills, when you name every net connection on each layer, they become visible. Vias of different nets will not connect. e enhanced visibility makes it so your CAM tool can perform error checking on your separate area fills and automatically recognize mismatched voltages. How Do You Prevent Mixed-Signal Problems? Digital and analog signals don't play well together. Here are some actions you can take to prevent problems with mixed-signal output. • Keep the power ground and control ground separate for each power supply stage. • To keep your digital and analog grounds separate, build in a small impedance path. is will limit power circuit interference and help protect your control signals. • Partition your PCB with separate digital and analog areas. – Make sure digital and analog components are assigned to their respective areas – Never route digital signals through analog territory and vice versa – Straddle analog-to-digital converters along the border of the two territories – Use a single, solid ground plane – If you must route a signal trace from one area to the other, place it entirely over the PCB's ground plane