PCB Grounding

Grounding is fundamental to the operation of many circuits. Good or bad grounding techniques can make or break your design. There are several grounding techniques which are always good practices to incorporate into any design. In order to present some important issues of how to have a correct GND structure, below are presented some recommendations.

Ø Use copper, and lots of it. The more copper you have in your ground path, the lower the impedance. This is highly desirable for many electrical reasons. Use polygon fills and planes where possible.

Ø Always dedicate one of your planes to ground on multi-layer boards. Make it the layer closest to the top layer.

Ø Run separate ground paths for critical parts of your circuit, back to the main filter capacitor(s). This is known as "star" grounding, because the ground tracks all run out from a central point, often looking like a star. In fact, try and do this as matter of course, even if your components aren’t critical. Separate ground lines keep current and noise from one component from affecting other components.

Ø If using a ground plane, utilise "split" plane techniques to give effective star grounding.

Ø "stitch" required points straight through to your ground plane, don’t use any more track length than you need.

Ø Use multiple vias to decrease your trace impedance to ground.

Decoupling & Bypassing

Active components and points in your circuit which draw significant switching current should always be decoupled (digital designs) and bypassed (analog designs). This is to "smooth" out your power rail going to a particular device. Decoupling/Bypassing is using a capacitor between PWR and GND as physically and electrically close to the desired component or point in your circuit as possible. A typical decoupling/bypass capacitor value is 100nF, although other values such as 1uF, 10nF and 1nF are often used to bypass different frequencies. You can even have two or three different value capacitors in parallel.

When bypassing, the user cannot replace multiple capacitors with one single capacitor, it defeats the entire purpose of operation! It is not uncommon for a large design to have hundreds of bypass capacitors. As a general rule, you should use at least one bypass capacitor per IC or other switching component if possible. Common values of bypass capacitors are 100nF for general purpose use, 10nF or 1nF for higher frequencies, and 1uF or 10uF for low frequencies.

Special low Equivalent Series Resistance (ESR) capacitors are sometimes used on critical designs such as switch mode power supplies.